Analysis of Spermatogenesis Using an Eel Model

Chiemi Miura and Takeshi Miura*

Research Group for Reproductive Physiology, South Ehime Fisheries Research Center, Ehime University, 1289-1, Funakoshi, Ainan, Ehime 798-4292, Japan

Abstract

Spermatogenesis is an indispensable process for the continuity of life. The process of spermatogenesis is very complex; it begins with spermatogonial renewal, then proceeds to proliferation of spermatogonia towards meiosis, two meiotic reduction divisions and spermiogenesis, during which the haploid spermatid develops into a spermatozoa. After spermiogenesis, non-functional sperm pass the process of sperm maturation and then become mature spermatozoa, fully capable of vigorous motility and fertilization. These processes are mainly controlled by sex steroid hormones. Spermatogonial renewal is controlled by estrogen; estradiol-17β (E2) through the expression of platelet-derived endothelial cell growth factor (PD-ECGF). The proliferation of spermatogonia toward meiosis is initiated by androgen; 11-ketotestosterone (11-KT) produced by FSH stimulation. 11-KT prevents the expression of anti-Müllerian hormone (AMH), which functions to inhibit proliferation of spermatogonia and induce expression of activin B, which functions in the induction of spermatogonial proliferation. Meiosis is induced by progestin; 17α,20β-dihydroxy-4-pregnen-3-one (DHP) through the action of trypsin. DHP also regulates the sperm maturation through the regulation of seminal plasma pH.

Keywords

fish, teleost, testis, Germ cell, in vitro culture, meiosis, gene transfer, androgen, oogenesis, reactive oxygen species


1. Introduction

Germ cells provide the continuity of life between generations. In many animals (Gilbert 1985), there is an established germ line that separates from the somatic cells early in the developmental stage, these germ cells migrate into the future gonads from other places through the embryonic tissues. In the developing gonads, germ cells will become either oogonia in ovaries or spermatogonia in testes. In the gonads they will be exposed to various hormones and cellular interactions following which, gametogenesis starts. Gametogenesis, spermatogenesis and oogenesis is a process by which diploid or haploid precursor cells undergo cell division and differentiation to form mature haploid gametes. These gametes; ovum and spermatozoon, are fertilized and then a new generation is started. We have been investigating spermatogenesis using fish models for 20 years. In this monograph, we describe the process and control mechanisms of spermatogenesis based on our investigations.

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2. Spermatogenesis

Spermatogenesis, the formation of sperm, is a complex developmental process that begins with the mitotic proliferation of spermatogonia and proceeds through extensive morphological changes that convert the haploid spermatid into a mature, functional spermatozoon. Morphologically and physiologically, the process of spermatogenesis can be divided into the following stages: proliferation of spermatogonia, two meiotic divisions, spermiogenesis, spermiation and sperm maturation.

2-1. Proliferation of spermatogonia

Spermatogenesis starts with the mitotic proliferation of type A spermatogonia, which are spermatogenetic stem cells. The type A spermatogonium is a relatively large cell (approximately 10 μm in diameter in various species) and has a clear, large homogeneous nucleus containing one or two nucleoli. Type A spermatogonia occur independently, with each cell almost completely surrounded by Sertoli cells. In some species, early type B spermatogonia can be distinguished from type A spermatogonia (Schulz et al. 2010). Although early type B spermatogonia are morphologically similar to type A spermatogonia, they tend to form a cyst of two or four germ cells surrounded by Sertoli cells. It is unclear whether early type B spermatogonia represent a renewal of the stem cells or a further progress into spermatogenesis. In the Japanese eel (Anguilla japonica), type A and early type B spermatogonia are undeveloped and resting spermatogonia (Miura et al. 1991c). These types of spermatogonia proliferate rapidly by mitosis and, as a result, appear in the seminiferous lobules or tubules. The morphology of late type B spermatogonia differs from that of their earlier undeveloped spermatogonial counterparts by the fact that their nucleus is denser and more heterogeneous and their mitochondria are smaller. After the proliferation of spermatogonia, the germ cells enter meiosis (Miura et al. 1991c).

The mitotic divisions of spermatogonial stem cells preceding meiosis are species-specific. In teleosts, a spermatogonial stem cell of medaka (Oryzias latipes) will yield spermatocytes following 9 or 10 mitotic divisions (Ando et al. 2000), and those of the zebra fish (Danio rerio) after 5 or 6 (Ewing 1972) and the Japanese eel after 10 divisions (Miura et al. 1997). However, it is not clear whether the number of mitotic divisions is an inherent property of the type A spermatogonial stem cell, environmentally controlled or both.

2-2. Meiosis in fish spermatogenesis

Following mitotic proliferation, type B spermatogonia differentiate into primary spermatocytes. These cells enter the first meiotic prophase and then proceed with the first meiotic division to produce secondary spermatocytes. These, in turn, undergo a second meiotic division to produce haploid spermatids, cells with only a single set of chromosomes.

In primary spermatocytes, the leptotene stages are distinguished from the final spermatogonia by their larger and more homogeneous nuclei. In some species (Schulz et al. 2010), however, it is difficult to distinguish the leptotene spermatocytes from late type B spermatogonia, due to their morphological similarities. During the zygotene stage of prophase, spermatocytes can be identified by locating the synaptonemal complex in their nuclei using an electron microscope. Because it is very short, the secondary spermatocyte is difficult to observe. After two meiotic divisions, the germ cells develop into spermatids having small, round and heterogeneous nuclei.

2-3. Spermiogenesis

During spermatogenesis, the round spermatids transform into spermatozoa. This process is characterized by remarkable morphological changes associated with the formation of the sperm head and its condensed nucleus, with the midpiece, and with the flagellum. The structure of spermatozoa varies considerably in complexity among teleost species (Miura 1999). For example, carp (Cypinus carpio), sculpin (Aottus hangiongenesis), and tilapia (Oreochromis niloticus) spermatozoa have spherical heads with a flagellum attached to one side. Salmonid spermatozoa have a slightly elongated and cylinder-like head. On the other hand, the spermatozoa of guppy have an elongated and an extremely well-developed midpiece. Furthermore, eel spermatozoa possess a crescent-shaped nucleus, their flagellum has a 9 + 0 axonemal structure (generally, the axonemal structure of the flagellum is 9 + 2) and a single large and spherical mitochondrion with developed tubular cristae is attached to the caput end at one side of the head. An acrosome is absent in the spermatozoa of most teleosts but it is found in acipenserid fish, lamprey and shark.

2-4. Spermiation

In mammals, "spermiation" indicates that embedded bundles of spermatozoa are released from the enveloping Sertoli cell and are swept into the efferent duct. In most teleosts (except in Poeciliidae), however, spermatozoa are not associated with Sertoli cells. By comparison, spermiation of teleosts indicates the release of spermatozoa from the seminal cysts into the lobular lumen or efferent duct. From the point of view of fisheries science, however, "sperm release" or "ejaculation," which occurs after milt hydration and sperm migration down the sperm duct, is more readily observed than spermiation. Therefore, the term spermiation is often used interchangeably with these other terms in fish.

2-5. Sperm Maturation

Although the spermatozoa in the testis have already completed spermatogenesis, in some species they are still incapable of fertilizing eggs. In salmonids, the spermatozoa in the testis and sperm duct are immotile. If sperm from the sperm duct are diluted with fresh water, they become motile, whereas testicular sperm, if diluted with fresh water, remain immotile. Thus, spermatozoa acquire their ability to become motile during their passage through the sperm duct.

The acquisition of the motile ability of sperm is different from the initiation of motility. The development from nonfunctional gametes to mature spermatozoa fully capable of vigorous motility and fertilization is referred to as "sperm maturation". Sperm maturation involves physiological, not morphological, changes. In salmonids, sperm maturation (acquisition of sperm motility) is induced by the high pH of the seminal plasma (approximately pH 8.0) in the sperm duct and involves the elevation of intrasperm cAMP levels. Maturation of eel spermatozoa is also induced by the high pH of the seminal plasma and/or HCO3.

This spermatogenesis is controlled by numerous hormones and unknown factors (Steinberger 1971; Hansson et al. 1976; Callard et al. 1978; Billard et al. 1982; Cooke et al. 1998). Specifically, sex steroid hormones, estrogen, androgen and progestin play significant roles in the control of spermatogenesis (Miura 1999).

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3. Regulation of spermatogenesis in eel in vivo or in vitro

Among species of teleosts, there are various reproductive styles and gametogenetic patterns. Teleosts constitute the largest group (approximately 23,700 species) of living vertebrates (∼48,200 species) (Nelson, 1994). The Japanese eel, Anguilla japonica, one of such species, has a special spermatogenetic pattern. In the Japanese eel, insufficient gonadotropin in the pituitary is attributed to the immature testes containing only non-proliferated type A and early type B spermatogonia under culture conditions (Nagahama and Yamamoto 1973). However, a single injection of human chorionic gonadotropin (hCG) can induce all stages of spermatogenesis from the proliferation of spermatogonia to spermiogenesis in vivo (Miura et al. 1991a) and this induction is achieved via gonadotropin stimulation of Leydig cells to produce 11-ketotestosterone (11-KT) (Miura et al. 1991b) (Fig. 1). Germ cell development is almost synchronous throughout the testis and the proliferation of spermatogonia, meiosis and spermiogenesis occur at definite times: 3, 12 and 15 days after hCG injection, respectively (Miura et al. 1991c) (Fig. 2).


Fig. 1. Induction of spermatogenesis by hCG injection into the Japanese eel. Under culture conditions, male Japanese eels have immature testes (small arrows) containing only non-proliferated type A and early type B spermatogonia. A single injection of human chorionic gonadotropin (hCG) can induce testicular development (arrowheads) for 18 days. These eels have a lot of spermatozoa in their testes. Electron micrographs are reprinted with permission from Zoological Science, 8, Miura et al., Induction of spermatogenesis in male Japanese eel, Anguilla japonica, by a single injection of human chorionic gonadotropin, 63–73, Fig. 2, © 1991, Zoological Society of Japan.

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Fig. 2. Timetable of spermatogenesis of hCG injected eel testis. Germ cell development is almost synchronous throughout the testis and the proliferation of spermatogonia, meiosis and spermiogenesis occur at definite times: 3, 12 and 15 days after hCG injection, respectively. Reprinted with permission from Reproduction, 142, Miura et al., Gh is produced by the testis of Japanese eel and stimulates proliferation of spermatogonia, 869–877, Fig. 1, © 2011, Society for Reproduction and Fertility, and reprinted with permission from Kaiyo to Seibutsu, 24, Miura and Miura, The challenge of artificially producing sperm and egg from immature gametes in vitro, 114–119, Fig. 1, © 2002, Seibutsukenkyusha.

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Furthermore, the Japanese eel is the only animal in which complete spermatogenesis has been induced by hormonal treatment in vitro using an organ culture system (Fig. 3) (Miura et al. 1991a), and a germ cell/somatic cell co-culture system (Fig. 4) (Miura et al. 1996). Therefore, these eel culture systems could be the best system for analysis of the control mechanisms of spermatogenesis because their direct action on spermatogenesis can be investigated (Fig. 5). Furthermore, to analyze functions of genes encoding eel spermatogenesis related substances (eSRSs), we have developed a new assay system using gene transfer techniques combined with co-culture of the eel germ-somatic cells. The electroporation method provides a good tool in the search for factors regulating spermatogenesis (Fig. 6). Thus, the eel testis provides an excellent system for studying the regulation of spermatogenesis. Using the culture system, we analyzed the control mechanisms of gametogenesis.


Fig. 3. The eel testicular organ culture system. Freshly removed immature eel testes were cut into small pieces, which were placed on floats of elder pith covered with a nitrocellulose membrane in 24-well plastic tissue-culture dishes (upper left). By using this system, 11-KT can induce the entire process of spermatogenesis for 36 days. Each symbol indicates: GA, type A spermatogonia; GB, type B spermatogonia; SC, spermatocytes; ST, spermatid; SZ, spermatozoa. Bar, 10 μm. Reprinted with permission from Handbook of Animal Cell Technology (Edited by Japanese Association for Animal Cell Technology), Miura and Miura, 287–289, Fig. 15.6, © 2000, Asakura Publishing Co., Ltd., reprinted with permission from Kaiyo to Seibutsu, 24, Miura and Miura, The challenge of artificially producing sperm and egg from immature gametes in vitro, 114–119, Fig. 2, © 2002, Seibutsukenkyusha, and reprinted with permission from Zoological Science, 18, Miura and Miura, Japanese eel: a model for analysis of spermatogenesis, 1055–1063, Fig. 2, © 2001, Zoological Society of Japan.

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Fig. 4. The schema of the method of germ-somatic cells coculture system. Immature eel testes were enzymatically dissociated and the cell suspension was filtered through meshes and centrifuged in Nycodenz gradients. Separated cell suspension was centrifuged to make pellets and they were cultured with or without 11-KT. After 30 days culture, many spermatozoa (white arrows) having one or two flagella were observed around the pellet of germ cells and somatic cells. Reprinted with permission from Kaiyo to Seibutsu, 24, Miura and Miura, The challenge of artificially producing sperm and egg from immature gametes in vitro, 114–119, Fig. 4, © 2002, Seibutsukenkyusha.

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Fig. 5. Using the organ culture system, germ-somatic cells coculture (pellet culture) system and cell culture system, we can investigate unknown factors directly added to the medium.

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Fig. 6. Expression of GFP gene in germ-somatic cell pellets after electroporation. Transient expression of transfected genes was examined two days after the electroporation of GFP cDNA into germ-somatic cell pellets. A train of eight square pulses (60 V; duration 50 msec; interval 950 msec), resulted in widespread expression of GFP fluorescence in many round germ cells and somatic cells. CMV, cytomegarovirus promoter; GFP, green fluorescent protein. Reprinted with permission of John Wiley & Sons, Inc. from Molecular Reproduction and Development, 74, Miura et al., Transfer of spermatogenesis-related cDNAs into eel testis germ-somatic cell coculture pellets by electroporation: methods for analysis of gene function, 420–427, Fig. 1, © 2007, Wiley-Liss, Inc., a Wiley Company.

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4. The endocrine control of fish spermatogenesis

4-1. Gonadotropins and spermatogenesis

Eel spermatogenesis is also endocrinologically controlled, in the same way as in other vertebrates. It is well established that in vertebrates, gonadotropins (GTHs), follicle-stimulating hormone (FSH) and luteinizing hormone (LH) are the primary hormones regulating spermatogenesis (Nagahama 1987). FSH and LH are members of the pituitary glycoprotein family, including thyroid-stimulating hormone. These hormones are heterodimers, each consisting of a common α and a hormone-specific β subunit (Pierce and Parsons 1981). In mammals, LH and FSH have different roles in spermatogenesis, respectively; LH regulates sex steroid production in Leydig cells and FSH regulates Sertoli cell activities, such as the structural, nutritional and regulatory support of germ cell development (Huhtaniemi and Themmen 2005). On the other hand, in fish it has also been established that two types of GTHs, FSH and LH, exist (Swanson et al. 1991; Van der Kraak et al. 1992; Planas and Swanson 1995; Yoshiura et al. 1999). However, the definitive function of each GTH has not been established. Recently, in the development of molecular biological techniques, it has become possible to analyze the differences in the roles of FSH and LH in fish (Kamei et al. 2006; Kazeto et al. 2008; Hayakawa et al. 2008). In some salmonids, it has been reported that FSH but not LH is secreted from the pituitary of immature fish, while LH release is higher during the period of sperm maturation (Swanson et al. 1989; Prat et al. 1996). In addition, it seems that in coho salmon, FSH acts at early stages of spermatogenesis because FSH is able to stimulate steroid hormone production, similarly to LH. However, FSH-stimulated production of steroid hormones decreases towards the period of sperm maturation (Planas and Swanson 1995). Furthermore, in the Japanese eel, FSH may act on early stages of spermatogenesis, considering that the fshβ subunit mRNA is expressed in the pituitary of immature fish while the lhβ subunit mRNA is not expressed until much later in the period of sperm release (Yoshiura et al. 1999). Because of the differences in their expression pattern, the roles of FSH and LH in spermatogenesis were estimated in fish (Schulz et al. 2010).

As mentioned above, in vitro systems provide direct evidence of hormonal action in biological events. Therefore, we investigated and resolved the roles of FSH in early spermatogenesis using eel testicular culture systems (Ohta et al. 2007). To understand the FSH function in spermatogenesis, it is necessary to know the expression pattern of its receptor. In western blot analysis using purified plasma membrane fraction of immature eel testis, eel FSH receptor (eFshr) exhibited two forms, each with different molecular mass: one of 41 kDa and another 72 kDa. Using extracted protein from whole testis, however, eFshr exhibited only a band of 41 kDa. This result suggests that the 72 kDa form is full length eel Fshr from the deduced amino acid sequence of efshr cDNA and the 41 kDa form is extracellular domain of eFshr, whose full-length receptor was cut during the process of extracting plasma membrane or testicular proteins. Moreover, to evaluate how expression of eFshr protein changes during spermatogenesis, we performed western blot analysis on hCG-treated eel testis. Eel Fshr was expressed before the initiation of spermatogenesis and continuously expressed during all stages. It is therefore possible that FSH acts on all stages of spermatogenesis (Ohta et al. 2007).

To determine the distribution of Fshr in the testis, we performed immunohistochemistry using an anti-eFshr antibody. The antibodies stained Leydig cells, which produce steroid hormones and Sertoli cells surrounding type A or early type B spermatogonia during spermatogenesis (Ohta et al. 2007). In some teleosts, FSH protein increases at an early stage of spermatogenesis (Swanson et al. 1989; Planas and Swanson 1995; Prat et al. 1996), and in eel, fshβ subunit mRNA is expressed in the pituitary of immature fish (Yoshiura et al. 1999). These results suggest that FSH acts on early stages of spermatogenesis via Leydig and/or Sertoli cells.

To understand whether FSH acts on spermatogenesis, we investigated the effects of FSH on in vitro spermatogenesis using recombinant eel FSH (r-eFSH) produced from a yeast expression system. Adding r-eFSH to the culture medium induced the complete process of spermatogenesis from the proliferation of spermatogonia to spermiogenesis. In the Japanese eel, it has been reported that FSH induces the secretion of 11-KT in immature testis (Kamei et al. 2005). Therefore, it is possible that the role of FSH is to induce 11-KT secretion, which in turn will stimulate spermatogenesis.

Using trilostane that specifically inhibits 3β-HSD activity, we investigated whether FSH acts on spermatogenesis via the production and secretion of 11-KT in testicular organ culture. Adding r-eFSH and trilostane to the culture medium reduced the percentage of cysts of late type B spermatogonia compared to treatment with only r-eFSH and the progress of spermatogenesis was inhibited. These results indicate that FSH is related to the regulation of spermatogenesis by triggering the secretion of 11-KT (Ohta et al. 2007).

In males, androgens including 11-KT are synthesized by Leydig cells in the testis (Payne and Youngblood 1995; Dufau et al. 1997). In coho salmon, Fshr is localized to Sertoli cells at all stages of spermatogenesis, while Lhr was only found in Leydig cells in spermiating fish (Miwa et al. 1994). Nevertheless, FSH promoted the synthesis of androgen in immature and mature testis similar to LH (Swanson et al. 1989; Planas and Swanson 1995). It is therefore possible that Leydig cells express Fshr or that paracrine factors secreted by Sertoli cells upon FSH stimulation promote Leydig cell's androgen production (Lejeune et al. 1996). Fshr is expressed in Leydig and Sertoli cells surrounding type A and early type B spermatogonia in the Japanese eel (Ohta et al. 2007). This suggests that FSH directly acts on Leydig cells via Fshr activation and promotes the synthesis of 11-KT.

Although Fshr localizes to Sertoli cells from fishes to mammals, including the Japanese eel, the clear functions of FSH in Sertoli cells via Fshr activation have not been established. In mice, the absence of functional follicle-stimulating hormone beta-subunit (Fshbeta) or Fshr genes leads to reduced testis size but the males are still fertile (Kumar et al. 1997; Dierich et al. 1998; Abel et al. 2000; Krishnamurthy et al. 2000). Moreover, in the Japanese eel, all stages of spermatogenesis are induced by 11-KT alone in vitro (Miura et al. 1991a; Miura et al. 1996). This suggests that FSH also supports testicular development and maintenance of sperm production through the action of Sertoli cells. However, the functions of FSH via Sertoli cells are not clear.

Thus, FSH stimulates the Leydig cells and regulates spermatogenesis via the production and secretion of steroid hormones.

4-2. The regulation of spermatogonial stem cell renewal

As mentioned above, the first step of spermatogenesis is spermatogonial mitotic proliferation. Spermatogonial mitosis can be categorized in slow spermatogonial renewal and rapid proliferation of differentiated spermatogonia toward meiosis (Clermont 1972). Both kinds of spermatogonial mitosis are regulated by different mechanisms by steroid hormones. We indicated that spermatogonial renewal is regulated by estrogen, estradiol-17β and spermatogonial proliferation toward meiosis is regulated by androgen, 11-KT in fish (Miura T et al. 1999).

It is widely accepted that estrogen is a female hormone in all animals. However, it has been reported that estrogen exists in some male vertebrates (Schlinger and Arnold 1992; Fasano and Pieratoni 1993; Betka and Callard 1998), and that its receptors are expressed in the male reproductive organs (Callard and Callard 1987; Ciocca and Roig 1995). Estradiol-17β (E2), a natural estrogen in vertebrates, was found in Japanese eel serum and its receptor was expressed in the Sertoli cells (the only non-germinal elements within the seminiferous epithelium of the testes) during the whole process of spermatogenesis. Thus, estrogen and its receptor are present in the eel testes. Using eel in vivo and in vitro experimental systems, we investigated the relationship between spermatogenesis and E2 (Miura T et al. 1999). E2 and tamoxifen (an antagonist of estrogen) were given intraperitoneally to eels using a silastic capsule. After 24 days of implantation, the fish were sacrificed and their spermatogenesis was analyzed. E2 implantation significantly increased and tamoxifen implantation significantly decreased germ cell DNA synthesis compared with control. The effect of E2 on spermatogenesis was confirmed by in vitro experiment; E2 treatment induced DNA synthesis and mitotic division in germ cells in in vitro testicular organ culture. Even though E2 treatment in vivo and in vitro induced spermatogonial mitosis, the germ cells did not progress further into meiosis. As mentioned above, spermatogonial mitosis can be categorized by spermatogonial renewal and spermatogonial proliferation toward meiosis. FSH or 11-KT induce spermatogonial proliferation and the further stage of spermatogenesis and finally produced spermatozoa in vitro. Therefore, spermatogonial mitosis induced by E2 may not be directed toward the formation of spermatozoa but for spermatogonial renewal. In Japanese huchen (Hucho perryi), E2 also promoted spermatogonial renewal in vitro (Amer et al. 2001). These findings clearly indicate that estrogen is an indispensable "male hormone", and plays an important role in spermatogonial renewal.

In in vitro testicular organ culture, supplementation of 10, 100 and 1000 pg/ml of E2 to the culture medium stimulated DNA replication and mitosis of the primary stage of spermatogonia. The range of the effective E2 concentration, 10–1000 pg/ml, conforms to the levels found in the male serum. This shows that E2 is effective at much lower concentrations than 11-KT, since 10 ng/ml of 11-KT are needed to induce full spermatogenesis (Miura et al. 1991a). Thus it is indicated that in the Japanese eel, low concentrations of E2 (10 pg/ml) act on the primary stages of spermatogonia through Sertoli cells, by stimulating and maintaining their proliferation prior to the progress of a further stage of spermatogenesis.

Generally, E2 induces the target gene expression through its receptor and the factor translated from this gene affects the biological process. We used cDNA cloning to identify a factor that regulates spermatogonial renewal after estrogen stimulation and subsequently clarified its functions. To identify factors that are regulated by E2 stimulation, we carried out gene expression cloning in the Japanese eel (Miura et al. 2003). As a result of this experiment, we found one previously unidentified cDNA clone that was up-regulated by E2 stimulation and named it eel spermatogenesis-related substance (eSRS) 34 cDNA. The transcription and translation of eSRS34 were detected in testes at every experimental stage; such expression parallels spermatogonial renewal, which occurs continuously throughout spermatogenesis (Miura et al. 2003). E2 constantly exists in serum during spermatogenesis (Miura T et al. 1999). Furthermore, eSRS34 protein was expressed in Sertoli cells, in which the receptor for E2 is expressed. Taken together, these findings suggest that eSRS34 fulfills the criteria defined for a key factor regulating spermatogonial renewal regulated by E2. A homology search of the predicted amino acid sequence showed that eSRS34 shares comparatively high similarity with platelet-derived endothelial growth factor (PD-ECGF). Furthermore, eSRS34 contains seven conserved cysteine residues as well as the recognition motif for thymidine and pyrimidine nucleotide phosphorylase, which are key features of PD-ECGF (Miyazono et al. 1991; Furukawa et al. 1992). The function of eSRS34 was examined using eel in vitro systems. Recombinant eSRS34 produced by Baculovirus system induced spermatogonial renewal in testicular organ culture. Furthermore, the addition of a specific anti-eSRS34 antibody prevented only spermatogonial renewal induced by E2 stimulation in a germ cells/somatic cells co-culture system (Miura et al. 2003). These results indicate that eSRS34 is a "spermatogonial renewal factor" in fish (Fig. 7).


Fig. 7. A schematic diagram summarizing the possible control mechanisms of spermatogenesis in the Japanese eel. FSH, follicle-stimulating hormone; LH, luteinizing hormone; 17α,20β-DHP, 17α,20β-dihydroxy-4-pregnen-3-one; PD-ECGF, platelet-derived endothelial cell growth factor; AMH, anti-Müllerian hormone; CAll, carbonic anhydrase; eSRS, spermatogenesis related substances.

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5. The regulation of spermatogonial proliferation toward meiosis

5-1. The role of 11-KT in spermatogenesis

The second step of spermatogenesis; spermatogonial proliferation toward meiosis, is initiated by the secretion of FSH and the main action of FSH on spermatogenesis is the production and secretion of 11-KT, via stimulation of the Leydig cells, as mentioned above.

11-KT was first identified by Idler et al. (Idler et al. 1961) as a major androgenic steroid in the male sockeye salmon (Oncorhynchus nerka). In various teleost fishes, 11-KT has been shown to be synthesized in the testis following GTH stimulation and high levels were detected in the serum during spermatogenesis (Billard et al. 1982). As mentioned above, under aquaculture conditions, the male Japanese eel has immature testes containing only type A and early type B spermatogonia; primary spermatogonia, which are premitotic. A single injection of hCG can induce all the stages of eel spermatogenesis in vivo. This injection also causes an increase in serum levels of 11-KT (Miura et al. 1991c).

Although numerous in vivo studies (Sakai 2002; Hong et al. 2004) have suggested the important role of androgens in vertebrate spermatogenesis (Steinberger 1971; Callard et al. 1978; Billard et al. 1982), there have been, to our knowledge, no in vitro studies to directly show the involvement of androgens in this process. A testicular organ culture system has been developed using eel testes, which have only undeveloped spermatogonia. Organ cultures provide a simplified experimental system in which the direct effects of various factors upon the testes can be investigated. Using an eel testicular organ culture system which we have newly developed, we investigated the role of 11-KT on spermatogenesis (Miura et al. 1991a).

Testes removed from eels were cultured for 15 days in a medium with or without various concentrations of 11-KT (0.01, 0.1, 1, 10 and 100 ng/ml). The appearance of proliferated spermatogonia (late-type B spermatogonia) in cysts was used as the criterion for mitosis. A supplement of 11-KT into the culture medium was effective for the initiation of spermatogenesis. Concentrations of 10 and 100 ng/ml were almost equally effective; mitosis occurred in 50–60% of cysts. The concentration of 10 ng/ml corresponds to that in the plasma of maturing male eels receiving a single injection of hCG (Miura et al. 1991c). In contrast, the lower two concentrations had no effect.

11-KT is the most effective androgen for the initiation of spermatogenesis. Seven different androgens (11-KT, 11β-hydroxytestosterone (11β-HT), testosterone (T), 5β-dihidrotestosterone, dehidroepiandrosterone, androsterone and androstendione) were investigated for their ability to induce the proliferation of spermatogonia in vitro. Testicular fragments were cultured in a medium containing one of these steroids at a dose of 10 ng ml for 15 days. The active mitosis occurred within the cultivated testes only when 11-KT was added to the medium. Although a slight stimulation was observed with 11β-HT and T, this may have been from the conversion of these steroids to 11-KT by endogenous enzymes (Miura et al. 1991a). 11-KT can induce all stages of spermatogenesis, from spermatogonial proliferation to spermiogenesis. Sequential changes in germ cells were investigated in cultures for periods up to 36 days in the presence of 11-KT at 10 ng/ml. Nine days after the start of culture, spermatogonia began mitotic proliferation. Zygotene spermatocytes of the meiotic prophase occurred in testicular fragments cultured for 18 days. Afterwards, spermatids and spermatozoa were initially observed. After 36 days, all stages of germ cells, including 8.2% of spermatozoa, were present. The action of 11-KT for spermatogenesis is not limited to the Japanese eel; it has been also recognized in goldfish (Carassius auratus) (Kobayashi et al. 1991) and Japanese huchen (Hucho perryi) (Amer et al. 2001).

5-2. IGF-1 and spermatogenesis

As mentioned above, 11-KT is an inducing steroid of spermatogenesis in fish. However, it is believed that the action of 11-KT is mediated by other factors produced by Sertoli cells, in which the androgen receptor exists (Ikeuchi et al. 2001). It is possible that some of these factors are growth factors, such as insulin-like growth factor-I (IGF-I). IGFs are known to be mediators of growth hormone action in vertebrates. In the rainbow trout (Oncorhynchus mykiss) testis, IGF-I is expressed in spermatogonia and/or Sertoli cells and it binds to type 1 IGF receptors (LeGac et al. 1996). Furthermore, IGF-I stimulates DNA synthesis in spermatogonia (Loir 1994; Loir and LeGac 1994; LeGac et al. 1996). Although IGF-I is also necessary for the regulation of eel spermatogenesis, its role is to support the action of 11-KT. More specifically, in the Japanese eel, 11-KT is necessary for the induction of spermatogenesis, whereas IGF-I is necessary for the continuation of the process (Nader et al. 1999).

5-3. The regulation of initiation of spermatogenesis by two growth factors; AMH and activin B

How does 11-KT initiate spermatogonial proliferation in fish? In the Japanese eel, two members of the transforming growth factor-like (TGF-β) super family, anti-Müllerian hormone (AMH) (Miura et al. 2002) and activin B (Miura et al. 1995a), have important roles during the initiation of spermatogenesis induced by 11-KT.

Activin B is a dimeric growth factor belonging to the TGF-β super family, and is composed of two activin-βB subunits. In the Japanese eel, activin B was found in the testis at the initiation of spermatogenesis after hCG stimulation, with its expression site restricted to Sertoli cells. Both transcription and translation of eel activin B were induced by 11-KT stimulation in vitro. Furthermore, activin B induced proliferation of spermatogonia but its treatment could not induce meiosis and further spermatogenesis (Miura et al. 1995a, b). It has been reported that these activin B, IGF including IGF-binding protein and numerous other growth factors regulate the early stage of spermatogenesis in teleosts and mammals (Watanabe and Onitake 1995; Zhao et al. 1996; Li et al. 1997; Kim et al. 1998). Further investigation is needed to fully understand the relationship between activin B and spermatogenesis in fish.

In the Japanese eel, it has become clear that a "spermatogenesis-preventing substance" is present in immature testis and spermatogenesis is initiated by the suppression of its expression (Miura et al. 2002). Under freshwater cultivation conditions, male Japanese eels have immature testes containing only non-proliferated spermatogonia (Miura et al. 1991a). It is possible that factors that suppress the progress of spermatogenesis are expressed in the testis when the fish are in fresh water. In other words, eel spermatogenesis may be initiated by the downregulation of the genes encoding suppressive factors. On the basis of this hypothesis, we used gene expression cloning to isolate cDNA clones that show suppressed expression after hCG treatment. As a result of this cDNA cloning, we succeeded in identifying eel spermatogenesis related substance 21 (eSRS21) cDNA. eSRS21 shares amino acid sequences similarity with mammalian AMH at approximately 40%. Thus, we called eSRS21 eel AMH (eAMH). eAMH was expressed in Sertoli cells of immature testes before the initiation of spermatogenesis, but disappeared after gonadotropin stimulation. The initiation of spermatogonial proliferation corresponds with the disappearance of eAMH expression. Expression of eAMH mRNA was also suppressed in vitro by 11-KT, which is a spermatogenesis-inducing steroid. To examine the function of eAMH in spermatogenesis, recombinant eAMH produced by a CHO cell expression system was added to a testicular organ culture system (Miura et al. 2002). Spermatogonial proliferation induced by 11-KT in vitro was suppressed by recombinant eAMH. Furthermore, the addition of a specific anti-eAMH antibody induced spermatogonial proliferation in a germ cell/somatic cell co-culture system. These indicate that eAMH prevents the initiation of spermatogenesis and therefore, suppression of eAMH expression is necessary to initiate spermatogenesis. The discovery of the spermatogenesis preventing substance suggests that spermatogonial proliferation toward meiosis is directly regulated by the rivalry of a stimulating factor such as activin B and a preventing factor, such as eAMH (Fig. 7).

Recently, the medaka's AMH receptor; amhrII gene was identified, which represents the first characterization of this receptor in nonmammalian vertebrates (Kluver et al. 2007). In fish, the Wolffian duct is associated with the pronephros regardless of sex and the sperm duct and oviduct are derived from the coelomic epithelium (Suzuki and Shibata 2004), analogous to the amniote Müllerian duct. In medaka, amh and amhrII are expressed in the somatic cells of the developing gonads identically between the sexes during larval and juvenile stages. These results suggest that the medaka amh and amhrII genes are involved in gonadal development, including the regulation of germ cells in both sexes (Morinaga et al. 2007).

As mentioned above, in freshwater conditions eel spermatogenesis arrests at an immature stage prior to the initiation of spermatogonial proliferation. This immature stage seems to be maintained by the expression of spermatogenesis-preventing substance in eel Sertoli cells in freshwater conditions. When eels down-migrate to the ocean, spermatogenesis resumes and progresses to maturation (Larsen and Dufour 1993). This indicates that during eel migration, spermatogenesis is resumed through the suppression of AMH, spermatogenesis-preventing substance, expression caused by an increase in 11-KT, which is induced by gonadotropin stimulation.

To further elucidate the key genes involved in spermatogonial proliferation toward meiosis, we performed screening of stage-specific genes during eel spermatogenesis using cDNA subtraction and differential display methods (Miura et al. 1998; Miura C et al. 1999). The key genes coding factors that showed unique expression during spermatogenesis were considered. As a result of these experiments, 28 independent cDNA clones showing unique expression patterns during spermatogenesis were obtained (Miura and Miura 2001). Among these, 16 clones are up- or down-regulated by 11-KT, the spermatogenesis inducing hormone. The progression of eel spermatogenesis toward meiosis may be regulated by some of these factors.

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6. Initiation of meiosis

6-1. The entry of spermatogonia into meiosis

In the cultivated male Japanese eel, type A and early type B spermatogonia, which are primary cells, are the only germ cells present in the testis. Exogenous gonadotropin like hCG injection can induce complete spermatogenesis, from proliferation of spermatogonia to spermiogenesis. In some cases, however, hCG injection fails to induce complete spermatogenesis (Miura et al. 1997). Testicular morphological observations revealed that hCG-injected eels could be classified into three types, based on their testicular conditions. In Type 1 eels, complete spermatogenesis, from proliferation of spermatogonia to spermiogenesis, was successfully induced. In type 2 eels, spermatogenesis was also induced by hCG injection but there were no spermatocyte or spermatids in their testis. Type 3 eels had thready testis, which did not develop any germ cells during the experimental period. These results suggest that despite elevations of plasma 11-KT levels, hCG injections were not successful in inducing the completion of spermatogenesis in type 2 and 3 eels. In most spermatogonia of type 2 eels, meiosis of 23 to 26 late-type B spermatogonia was not induced in most cysts. Moreover, cysts with 27 or more spermatogonia were not observed. This suggests that spermatogonial stem cells undergo four or five and occasionally six mitotic divisions before the interruption of spermatogenesis in type 2 eels. It is proposed that those numbers of mitotic divisions are related to a mediator that regulates the entry of spermatogonia of the Japanese eel into meiosis (Miura et al. 1997).

Following mitotic proliferation, late-type B spermatogonia differentiate into primary spermatocytes. The number of spermatogonial generations is genetically determined (Courot et al. 1970; Schulz et al. 2010). For example, 6 generations were found in Sakhalin taimen, 8 in masu salmon (Oncorhynchus masou), 6 in white spotted char (Salvelinus leucomaenis), 8 in goldfish (Ando et al. 2000), 14 in the guppy (Poecilia reticulata) (Billard, 1986) and there are 10 mitotic divisions in the Japanese eel (Miura et al. 1991a, 1997). Although the regulatory mechanisms for the initiation of meiosis are not yet clear, it has been shown that in the Japanese eel there is a regulatory stage around the fifth mitotic division of spermatogonia, prior to the cells entering meiosis (Miura et al. 1997).

6-2. The mechanisms of initiation of meiosis in spermatogenesis

Meiosis is a special type of cell division that is restricted to germ cells. Meiosis produces haploid cells and forms the basis of sexual reproduction. Many studies on meiosis are directed toward chromosome dynamics (Shinohara and Shinohara 2004; Gerton and Hawley 2005; Watanabe 2005) or to oocyte maturation, which resumes and completes the prophase of the first meiotic division (Masui 2001; Thomas et al. 2002). However, the mechanism initiating the first meiotic division is not clear. Progestins are sex steroid hormones that are important for reproduction. In mammals, the principal physiological action of progestin is to prepare the reproductive tract for pregnancy and to provide nutritive support for the embryo during gestation (Burris 1998); however, this is an evolutionarily recent function. In all vertebrates, progestin also plays important roles in gametogenesis. Progestins regulate oocyte maturation (Nagahama 1997) by binding to an oocyte plasma membrane receptor, inhibiting oocyte adenylate cyclase, followed by reduced cAMP-dependent protein kinase activity, which induces the activation of maturation promoting factor via Cdc25, eventually triggering the resumption of division I of meiosis (Nagahama 1997; Thomas et al. 2002). In fish spermatogenesis, progestin also plays an important role in spermiation and sperm maturation (Ueda et al. 1985; Miura et al. 1991b, 1992). A major progestin in teleost fish, 17α,20β-dihydroxy-4-pregnen-3-one (DHP), induces sperm hydration (Ueda et al. 1985) and the acquisition of sperm motility in some species (Miura et al. 1991b, 1992). A related progestin, 17α,20α-dihydroxy-4-pregnen-3-one is the spermiation-inducing hormone in amphibia (Kobayashi et al. 1993). Thomas et al.'s (2005, 2006) data on progestin actions on fish gametes suggest the widespread involvement of membrane progestin receptor alpha (mPRα) in oocyte maturation and sperm hyperactivity in spotted sea trout (Cynoscion nebulosus) and Atlantic croaker (Micropogonias undulatus). Thus, progestin is an indispensable hormone for gametogenesis. However, studies on progestins have been directed mainly at functions in late maturational stages in both sexes. In several fish species, DHP is found in blood serum at puberty in males (Baynes and Scott 1985; Amer et al. 2001). In salmonids, there are two peaks of DHP, big and small peak, in the blood level. One big peak is in the spawning season and another small peak is in the progression of spermatogonial proliferation. The big peak of DHP in the spawning season is related to spermiation and sperm maturation, which will be discussed later. The small peak of DHP was known in salmonids (Depeche and Sire 1982; Scott and Sumpter 1989) but its role had not yet been clearly described. Furthermore, we demonstrated that DHP induced spermatogonial DNA synthesis in Japanese huchen (Amer et al. 2001). These findings suggest that progestin has an important role not only in final maturation but also in the early stages of gametogenesis. However, there is no information on the role of DHP in the early stages of spermatogenesis. We show that DHP action in early spermatogenesis became clear using the eel testis tissue/cell culture systems (Miura et al. 2006).

To understand the possibility that DHP also acts on the early stages of spermatogenesis, we quantified testicular DHP in eels during hCG-induced spermatogenesis and the expression of nuclear types of progesterone receptor (PR) in the eel testis. Treatment with hCG induced spermatogenesis and a strong increase in testicular DHP levels in vivo. Furthermore, types 1 and 2 of nuclear PRs were expressed in the immature testis before initiation of spermatogenesis. These findings open the possibility that DHP also acts on the regulation of early spermatogenesis in the Japanese eel. Interestingly, as mentioned above, DHP has been detected in serum during the proliferation of spermatogonia in some salmonids (Baynes and Scott 1985; Amer et al. 2001). Based on these findings, DHP may be involved in regulating early spermatogenesis in salmonids and other teleosts but there is no direct physiological role for this steroid. Since our RT-PCR studies showed that the types 1 and 2 PR showed partially different cellular sites of expression in the testis, DHP may not have just one function. We found that DHP stimulated DNA the replication of spermatogonia in testicular organ and germ cell/somatic cell co-culture, as did 11-KT. What, however, may be the function of DHP in early spermatogenesis and how does it differ from that of 11-KT? To address these questions we carried out germ cell/somatic cell co-culture, using specific DHP antibodies for 6 (short-term culture) and 15 days (long-term culture). During eel spermatogenesis in vitro, spermatogonial proliferation starts 3 days and meiosis 15 days after commencing the culture with 11-KT. Therefore, the 5-bromo-2'-deoxyuridine (BrdU) index of the short-term culture reflects DNA synthesis related to spermatogonial proliferation and that of long-term culture relates mainly to meiosis. In both the short- and long-term culture, DHP and 11-KT induced germ cell DNA synthesis. In the short-term culture, the DNA synthesis induced by 11-KT stimulation was not prevented by anti-DHP treatment. In the long-term culture, however, the DNA synthesis induced by 11-KT stimulation was prevented by anti-DHP treatment. In the long-term culture for 15 days, DHP antibodies were only present during the last 6 days, suggesting that 11-KT can induce two kinds of DNA synthesis in germ cells, one that is not and another, subsequent one that is mediated by DHP. As mentioned above, the germ cells progress from spermatogonial proliferation to meiosis on day 15 after initiation of the culture with 11-KT (Miura et al. 1991a). These findings suggest that DHP acts on a late stage of spermatogonial proliferation and or on meiosis (Miura et al. 2006).

To understand the relationship between DHP and meiosis, we investigated the DHP-induced change of expression of the meiosis-specific markers Dmc1 and Spo11 by using a testicular organ culture. Spo11 is involved in the formation of DNA double-strand breaks during the homologous recombination of the meiotic prophase in yeast (Keeney et al. 1997; Smith and Nicolas 1998) and Dmc1 is an Escherichia coli Rec A-like protein involved only in meiotic recombination in yeast (Bishop et al. 1992; Bishop 1994). The roles of these proteins in meiosis are conserved throughout eukaryotic species (Keeney 2001). Both markers were induced by DHP in a testicular organ culture. Immunocytochemistry showed that the germ cells expressing Spo11 were undifferentiated spermatogonia. Furthermore, synaptonemal complexes, structures specific for the meiotic prophase, were observed in some germ cells of this type in testicular fragments cultured with DHP. Collectively, these data suggest that DHP induced early spermatogonia to enter the meiotic prophase (Miura et al. 2006).

In conclusion, we have demonstrated a new function of progestin by using an eel testis in vitro culture system. Progestin is an essential hormone involved in the regulation of not only final maturation but also of early stages of spermatogenesis, especially the initiation of meiosis in fish. Hence, progestin is an initiator of meiosis in spermatogenesis (Fig. 8).


Fig. 8. Effect of DHP on induction of meiosis in the testis of Japanese eels in vitro. Electron micrographs showing testicular section from fragments cultured in basal medium without hormone (control) or with 10 ng/ml DHP for 6 days. Testicular fragment cultured with DHP showed that germ cell nuclei contained synaptonemal complexes (arrowheads) characteristic of meiotic cells.

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6-3. A common point of meiosis in early oogenesis and spermatogenesis

The control of early oogenesis, from the proliferation of oogonia to the initiation of meiosis and the control mechanisms of the early stages of spermatogenesis are very similar. As mentioned above, we have shown that DHP is an essential factor for the initiation of meiosis in spermatogenetic cells of the Japanese eel (Miura et al. 2006), thus, we investigated the involvement of DHP and E2 in early oogenesis (Miura et al. 2007).

During early oogenesis, oogonia proliferate by mitosis and subsequently develop into primary oocytes that have initiated meiosis. In general, during oogenesis, primary oocytes arrest division at the diplotene stage in the prophase of the first meiotic division and accumulate yolk during meiotic arrest. Thereafter, the oocytes resume the first meiotic division and differentiate into mature eggs through final maturation. In fish, the differentiation of primary oocytes into maturing oocytes that resume meiosis requires several steroid hormones. As in other vertebrates, oogenesis is primarily regulated by pituitary GTHs and ovarian endocrine factors, including estrogens and progestins. The roles of estrogens and progestins have been elucidated largely through correlating seasonal changes in circulating hormone levels with the different stages of the annual ovarian cycle in a variety of fish species (Crim and Idler 1978; Lamba et al. 1983; Kime et al. 1991; Cornish 1998).

Using two species of teleost fish, Japanese huchen (Hucho perryi) and common carp (Cyprinus carpio), we investigated whether sex steroids are involved in early oogenesis in vitro (Fig. 9). Ovarian fragments were cultured to examine the effects of a progestin, DHP and an estrogen, E2. DHP and E2 significantly promoted DNA synthesis in ovarian germ cells, as judged by 5-bromo-2-deoxyuridine (BrdU) incorporation into these cells. Furthermore, to detect the initiation of the first meiotic division of early oogenesis, we assessed ultrastructurally the occurrence of synaptonemal complexes (SCs) and analyzed by immunohistochemistry the expression of a meiosis-specific marker, Spo11. In huchen, a higher percentage of oocytes with SC was seen in DHP-treated ovarian fragments than in control or E2-treated ovarian fragments. Spo11 was expressed in germ cells after DHP treatment of carp ovarian explants. These data suggest that the progression of germ cells through early oogenesis involves two sex steroids: E2, which acts directly on oogonial proliferation and DHP, which acts directly on the initiation of the first meiotic division of oogenesis. Therefore, DHP is also implicated in the regulation of early oogenesis from oogonial proliferation to initiation of the first meiotic division (Fig. 10) (Miura et al. 2007).


Fig. 9. The ovarian epithelium culture technique. To remove oocytes of previtellogenic and vitellogenic stage, ovarian fragments of carp were excised and treated with enzymes. The residual ovarian fragments were precultured for one month. After preculture, ovarian explants were cultured in media with or without 1 ng/ml DHP for 14 days. Small arrows indicate oogonia. Photographs of 1 month culture is originally published in Miura et al., A Progestin and Estrogen Regulate Early Stages of Oogenesis in Fish, Biology of Reproduction 77, 822–828, Fig. 7, 2007.

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Fig. 10. Effect of DHP on common carp ovarian fragments cultured for 14 days. Light micrographs showing ovarian section from fragments cultured in basal medium without hormone (control) or with 1 ng/ml DHP. Cells with arrows are chromatin-nucleous stage oocytes. Electron micrograph of germ cells with synaptonemal complexes (arrowheads) in ovarian fragments cultured with 1 ng/ml DHP. Photographs by light microscope is originally published in Miura et al., A Progestin and Estrogen Regulate Early Stages of Oogenesis in Fish, Biology of Reproduction 77, 822–828, Fig. 10, 2007.

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These findings from male and female gonad tissue and cell culture systems suggest that DHP, a progestin, is an essential factor for the initiation of meiosis; and E2, an estrogen, is essential in gonial proliferation in both spermatogenesis and oogenesis (Fig. 11 or Fig. 12).

Fig. 11. DHP, a progestin, is an essential factor for the initiation of meiosis in both spermatogenesis and oogenesis.

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Fig. 1. A schematic summary of the possible control mechanisms of spermatogenesis and oogenesis in fish. SPS, spermatogenesis-preventing substance; SSRF, spermatogonial stem cell renewal factor. Reprinted with permission from Cybium, 32(2) supplement, Miura and Miura, Progestin is an essential factor for the initiation of the meiosis in spermatogenesis and early oogenesis in fish, 130–132, © 2008, Société Française d'Ichtyologie.

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What are the molecular mechanisms by which DHP initiates meiosis in fish? Although we attempted to clone key factors regulated by DHP stimulation (Ozaki et al. 2006), the mechanism has not yet been clarified.

6-4. The downstream factors of DHP

How does DHP initiate meiosis in spermatogenesis? In the Japanese eel, trypsin, which is a kind of serine protease, is a key factor for the downstream regulation of DHP (Miura et al. 2009). To identify novel factors that are regulated by DHP, we carried out gene expression cloning using eel testicular fragments that were cultured with or without DHP for six days. We screened 25 up-regulated cDNA clones following DHP treatment. Among these, a cDNA clone of trypsinogen was identified (Fig. 13). Trypsinogen is a precursor of trypsin, a member of the serine protease family that is mainly produced in the pancreas as a digestive enzyme. Although it has been reported that serine proteases are present in testicular tissue, their functions have not yet been clarified (Odet et al. 2006; Ogiwara and Takahashi 2007).


Fig. 13. To identify factors that are regulated by DHP, we carried out gene expression cloning using eel testicular fragments that were cultured with (DHP) or without (Cont) 100 ng/mL DHP for 6 days. After cultivation, poly (A)+ RNAs were extracted from these testicular fragments and a subtracted cDNA library was then constructed from +DHP and –DHP cDNA enriched via the RDA procedure (Niwa et al. 1997). One thousand clones from each of these libraries were subsequently screened by differential hybridization, using each enriched cDNA preparation as a probe. We thereby screened 25 non-cross-hybridizing and up-regulated cDNA fragments after DHP treatment. Southern blotting analysis showed 12 typical cDNA fragments, whose names were eSRS36, 37, 38, 39, 40, 41, 42, 43, 44, 56, 29 and 33. Among these, an eSRS56 cDNA fragment corresponding to trypsinogen was identified (GenBank Acc. No. AB519643). Trypsinogen expression in the Japanese eel testis was determined by northern blot analysis of cultured testicular fragments. Pooled testicular fragments from 10 eels were cultured without (C) or with 10 ng/mL of 11-KT, E2 or DHP for 6 days. Lane IC shows the initial control before culturing. Northern blot analysis of EF1, which serves as reference, is also shown. Reprinted with permission from PNAS, 106, Miura et al., Trypsin is a multifunctional factor in spermatogenesis, 20972–20977, Fig. 1, © 2009, National Academy of Sciences.

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To further elucidate the relationship between testicular trypsinogen expression and sex steroids such as DHP, which are associated with the regulation of spermatogenesis, we examined the effects of 11-KT, E2 and DHP upon the trypsinogen mRNA levels in the Japanese eel testis. Eel testicular fragments were cultured for six days with these three steroids and their expression of trypsinogen was examined. Northern blot analysis showed that trypsinogen expression is induced only by DHP stimulation (Fig. 13). To determine the distribution of trypsinogen in the testis, we performed immunohistochemistry using an eel trypsinogen antibody. Positive staining of the Sertoli cells surrounding the late type B spermatogonia and of the spermatids and spermatozoa was observed. These findings suggest that trypsinogen is related to the regulation of initiation of meiosis in early stages of late type B spermatogonial development under DHP stimulation (Miura et al. 2009).

6-5. Trypsin and meiosis

To investigate the relationship between trypsinogen and spermatogenesis, germ cell/somatic cell pellets were cultured with or without an anti-eel trypsinogen antibody and/or DHP or 11-KT for six days. As positive controls, treatment with 11-KT or DHP alone significantly stimulated DNA synthesis in spermatogonial cells. The addition of an anti-eel trypsinogen antibody significantly reduced DHP-induced but not 11-KT-induced spermatogonial DNA synthesis. Since trypsin is a serine protease, we therefore investigated the effects of the serine protease inhibitors, 4-(2-aminoethyl)-benzenesulfonyl fluoride (AEBSF) and phenylmethylsulfonyl fluoride (PMSF) on spermatogonial DNA synthesis stimulated by DHP. Germ cell/somatic cell pellets were cultured with or without these inhibitors and with DHP or 11-KT for six days, but only DHP-induced spermatogonial DNA synthesis was reduced. These findings indicate that trypsin has important role in spermatogonial DNA synthesis under DHP stimulation for serine protease. The direct effects of trypsin on spermatogenesis were monitored using an eel germ cell/somatic cell co-culture system. Germ cells/somatic cell pellets were cultured with various concentration of pig trypsin. At the start of cultivation, all of the germ cells in the pellets were undifferentiated spermatogonia. The addition of trypsin to the culture medium induces DNA replication. Significantly, in the controls without any supplement, the DNA synthesis of germ cells did not change from its initial value. To understand the relationship between trypsin and meiosis, we examined the effects of trypsin on the expression of the meiosis-specific marker Spo11, in a germ cell/somatic cell co-culture system by immunohistochemistry. Prior to cultivation, there was no detectable Spo11 in the germ cells within the pellets. After two days in culture, however, trypsin treatment was found to induce Spo11 expression. These findings indicate that trypsin has an important role in the initiation of meiosis in spermatogenesis.

6-6. Trypsin and spermiogenesis

Trypsin has another important role in spermatogenesis aside from initiating meiosis. Type A spermatogonia were cultured with various concentrations of trypsin (0.1–100 mM) for 15 days and the appearance of the cells was then evaluated. Prior to cultivation, type A spermatogonia remained rounded but after three days in culture with 100 mM of trypsin, these cells adopted a spindle-shaped morphology. After 15 days of treatment with 100 mM of trypsin, a flagelliform structure was detectable on one side of these spindle-shaped germ cells. In cultures without trypsin, no morphological changes were evident in the germ cells. Using flow-cytometry, the nuclear phases of these spindle-shaped germ cells were recorded. The resulting flow-cytometric histograms showed 2C and 4C peaks, i.e., these cells did not undergo a normal meiotic division. The morphology of the elongated germ cells exposed to trypsin was compared with normal eel spermatozoa by histological observation. The flagella of the eel spermatozoa exhibit a 9 + 0 axonemal structure and nine pairs of microtubules in each flagellum are divided into 4 and 5 pairs, respectively, on the sperm head where they extend to the caput end. Both sets of microtubules and the flagella can be detected by immunocytochemistry using α-tubulin antibodies. In sperm-like cells treated with trypsin, an axonemal structure could not be detected in the flagelliform structure using electron microscopy, whereas flagelliform structures and microtubule-like lines at the cell surface were found to be specifically stained by α-tubulin antibodies. These findings suggest that trypsin induces partial spermiogenesis (Miura et al. 2009).

6-7. Trypsin and fertilization

Trypsin also plays an important role in fertilization. The trypsinogen expression profile in eel spermatozoa was analyzed. Using a specific antibody, trypsin was detected in the spermtozoa membranes and its activity was also detected using gelatin zymography. Since it has also been reported that membrane-type serine protease exists in elongated spermatids in mammals (Wong et al. 2001; Scarman et al. 2001), there is a possibility that sperm head trypsin in eels is similar to this serine protease in mammals. We also wished to investigate the possibility that sperm head trypsin is related to fertilization and evaluated the relationship between fertilization and sperm head trypsin in the eel. Ejaculated eel sperm were incubated in artificial seminal plasma supplemented with the specific serine protease inhibitors PMSF or AEBSF or with anti-eel trypsin antibodies, for three hours. Inseminations were then performed using these incubated sperm and normal eel eggs. Four hours after insemination, fertilized eggs were counted and the rate of fertilization was thereby calculated. The results showed that both serine protease inhibitors and also the trypsin antibodies significantly reduced the fertilization rate. In teleost fish, except for some species such as the sturgeon, spermatozoa do not have an acrosome (Grier 1981; Cherr and Clark 1984) and fertilization is facilitated via the egg's micropyle. Hence, an acrosome reaction is not required during the fertilization of fish eggs. However, there is little information currently available regarding the factors or mechanisms that play a role in the fertilization of fish eggs (Yanagimachi et al. 1992; Morisawa 2008). Trypsin or trypsin-like proteases in the sperm head may play a critical role in fertilization in fish.

These results demonstrate that trypsin and/or a trypsin-like protease play an important role in the regulation of three reproductive events in the male eel; that is, the initiation of meiosis, spermiogenesis and fertilization (Fig. 14).


Fig. 14. A schematic summary of the roles of trypsin on spermatogenesis in the Japanese eel.

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7. The regulation of final maturation of male fish

In mammals, the term "spermiation" indicates that spermatozoa are released by the Sertoli cells, which involves the disintegration of a junctional complex between Sertoli and germ cells (O'Donnell et al. 2000). Released spermatozoa can leave the testis suspended in liquid produced by Sertoli cells, via the tubular lumen that is connected to an efferent duct system. In teleosts, however, such a junctional complex between spermatids and Sertoli cells does not exist like other vertebrates (Schulz et al. 2010) and spermiation refers to the opening of spermatogenic cysts, also terminating the close relation between Sertoli and germ cells. Next to this definition sensu strictu, the term spermiation in fish is also used to indicate that spermatozoa hydration has taken place, enabling spermatozoa migration towards the sperm duct, from which it can be released for fertilization.

During the breeding season, the levels of numerous hormones show remarkable changes in male teleosts, which are initiated by an increase in LH secretion (Prat et al. 1996). LH secretion induces an increase in the production of the testicular steroids, such as 11-KT and DHP or 20β-S. 11-KT injections induced spermiation in goldfish and some salmonids and DHP injections had similar effects in several salmonids and eels. While LH and these sex steroids are clearly involved in regulating spermiation in fish, the mechanisms of action of these hormones on milt hydration, sperm migration to the sperm duct or increase in milt volume, are still unclear. In some teleost species (Miura et al. 1991b, 1992), spermatozoa released from Sertoli cells after completion of spermiogenesis are not yet capable of fertilizing eggs. In salmonids, spermatozoa in the testis and in the sperm duct are immotile. Dilution with fresh water induces motility in spermatozoa collected from the sperm duct while testicular spermatozoa remain immotile after dilution. Thus, spermatozoa acquire motility during their passage through the sperm duct.

Sperm maturation, the phase during which non-functional gametes develop into mature spermatozoa (fully capable of vigorous motility and fertilization) involves physiological but no morphological changes. In salmonids, sperm maturation (the acquisition of sperm motility) has been induced by increasing the seminal plasma pH (approximately to pH 8.0) in the sperm duct, which results in the elevation of intra-sperm cAMP levels (Morisawa and Morisawa 1988; Miura et al. 1992). Similar results have been reported for Japanese eel spermatozoa by Miura et al. (1995b) and Ohta et al. (1997). Sperm maturation is also regulated by the endocrine system. In some teleosts, including the Japanese eel, it has been suggested that DHP regulates sperm maturation (Miura et al. 1991b, 1992). It seems that DHP does not act directly on the sperm; its action is rather mediated through an increase in the seminal plasma pH, which in turn increases the sperm content of cAMP, thereby allowing the acquisition of sperm motility (Miura et al. 1991b, 1992, 1995b). Thus, sex steroid hormones; estrogen, androgen and progestin, are important regulators for the progression of spermatogenesis from spermatogonial stem cell renewal to sperm maturation.

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8. The protection mechanisms of germ cells from chemical-induced stress

Germ cells are highly specialized cells that are responsible for the propagation of DNA which directs the development of future generations. It is essential for organisms to maintain the integrity of germ cell DNA to ensure that the continuation of the species is not compromised. Previous studies have shown that spermatogenic cells have a lower mutation frequency than somatic cells (Walter et al. 1998; Winn et al. 2000). Yet, germ cells are continually affected detrimentally by endogenous and exogenous agents, such as reactive oxygen species (ROS), which can cause DNA damage. Spermatozoa were found to be highly sensitive to ROS-induced damage (Aitken and Clarkson 1987), while spermatogonia are reportedly tolerant to ROS (Aruldhas et al. 2005). Previous studies revealed that in mice exposed to mild heat stress, which can consequently lead to oxidative stress (Paul et al. 2009), numerous apoptotic late-type germ cells were found while apoptotic spermatogonia were rare (Paul et al. 2008). However, the precise reason for this phenomenon remains unclarified. In vertebrates, a variety of antioxidant defense mechanisms have evolved to protect cells and tissues against ROS. Among the well-known antioxidant enzymes protecting cells from ROS are the superoxide dismutases (SOD).

Although the effects of ROS have been extensively studied in mammals, not much is known about its direct impact on vertebrate germ cells. It was previously shown that spermatogonia are highly tolerant to ROS attack while advanced-stage germ cells such as spermatozoa are much more susceptible, however, the precise reason for this variation in ROS tolerance remains unknown (Fig. 15).


Fig. 15. An illustration showing the action of oxidative stress on eel spermatogenesis.

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Using the Japanese eel testicular culture system, which enables a complete spermatogenesis in vitro (Miura et al. 1991a), we report that advanced-stage germ cells undergo intense apoptosis and exhibit strong signal for 8-hydroxy-2'-deoxyguanosine (8-OHdG), an oxidative DNA damage marker, upon exposure to hypoxanthine (Hx)-generated ROS, while spermatogonia remain unaltered. Activity assay of antioxidant enzyme, SOD and western blot analysis using an anti-copper/zinc (Cu/Zn) SOD antibody showed high SOD activity and Cu/Zn SOD protein concentration during early spermatogenesis. Immunohistochemistry showed strong expression of Cu/Zn SOD in spermatogonia but weak expression in advanced-stage germ cells. Zn deficiency reduced the activity of the recombinant eel Cu/Zn SOD protein. Cu/Zn SOD siRNA decreased Cu/Zn SOD expression in spermatogonia and led to increased oxidative damage (Table 1) (Celino et al. 2011).


Table 1. Using the Japanese eel testicular culture system, we found that advanced-stage germ cells undergo intense apoptosis and exhibit a strong signal for 8-hydroxy-2'-deoxyguanosine, an oxidative DNA damage marker, upon exposure to hypoxanthine (Hx)-generated reactive oxygen species (ROS), while spermatogonia remain unaltered. Spermatogonia are highly tolerant to ROS attack while advanced-stage germ cells such as spermatozoa, are much more susceptible. Low ROS, Hx treatment at the low dose (1 μM); High ROS, Hx treatment at the highest dose (100 μM).

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Overall, our study demonstrated that the decrease in levels of SOD activity, expression of Cu/Zn SOD and the levels of Zn as spermatogenesis progresses are the reasons for the vulnerability of advanced stage germ cells to ROS attack. The high levels of Cu/Zn SOD and Zn in spermatogonia may render them less susceptible to ROS attack. Since spermatogonia are on the unprotected side of the testis barrier, they may be prone to more DNA damage from circulating molecular insults than cells on the protected side. As stem cells, spermatogonia need to ensure the integrity of the genes required for development and the continuity of life. Thus, spermatogonia have evolved with a greater need for elevated levels of protective factors against DNA damage (Yamaguchi et al. 2009).

Fish reproduction has been considered to be a reliable indicator of endocrine disruption in aquatic systems by chemical compounds, including arsenic. Of the two forms, As (V) typically dominates both in oxic sea and freshwater (Smedley and Kinniburgh 2002; Duker et al. 2005); consequently, fish are likely to be exposed to As (V). However, few studies described the toxicity of the less toxic form, As (V), on fish reproduction. We have previously demonstrated that in vitro treatment of As (V) inhibited spermatogenesis in the Japanese eel (Yamaguchi et al. 2007). However, the mechanism involved in the direct influence of arsenic on fish spermatogenesis is not yet well clarified. Hence, using the Japanese eel testicular organ culture system we examined the direct effects and toxic mechanisms of arsenic on fish spermatogenesis. We found that arsenic treatment provoked a dose-dependent inhibition of hCG-induced germ cell proliferation, as revealed by BrdU immunohistochemistry. Time-resolved fluorescent immunoassay showed that arsenic suppressed hCG-induced synthesis of 11-KT in testicular fragments that were incubated. A 0.1 mM (7 mg/l) dose of arsenic, which is lower than the World Health Organization drinking water quality guideline of 10 mg/l, most effectively reduced 11-KT production. The hCG-induced synthesis of progesterone from pregnenolone was significantly inhibited by low doses of arsenic (0.1–1 mM), implying an inhibition of 3β-hydroxysteroid dehydrogenase activity. Germ cells undergo apoptosis at the highest dose of arsenic (100 mM). An arsenic concentration-dependent increase in oxidative DNA damage was detected by 8-OHdG immunohistochemistry. A peak in 8-OHdG index was observed in testicular fragments treated with 100 mM arsenic and hCG consistent with the results of TdT-mediated dUTP nick end labeling (TUNEL) assay as a method of detecting apoptotic cells. Thus, these data suggest that low doses of arsenic may inhibit spermatogenesis via steroidogenesis suppression, while high doses of arsenic induce oxidative stress-mediated germ cell apoptosis (Celino et al. 2009). Furthermore, to clarify the direct effects of ROS on germ cells, we studied the effects of hypoxanthine-induced ROS on spermatogenesis. Immunohistochemistry for BrdU showed that Hx treatment at a low dose (1 μM) already inhibits 11-KT-induced germ cell proliferation after six days of culture. TUNEL assay and 8-OHdG immunohistochemistry revealed an intense germ cell apoptosis and high oxidative DNA damage in testicular fragments cultured at the highest dose of the hypoxanthine (100 μM) with 11-KT after three days of culture. Total SOD activity assay showed a decrease in SOD activity in testicular fragments after six days of culture with 11-KT. Thus, these suggest that ROS may directly inhibit spermatogenesis and that decreased SOD activity renders proliferating spermatogonia susceptible to ROS and hence, leading to apoptosis. Our recent studies have shown that high levels of Cu/Zn SOD are present in spermatogonia, which renders it tolerant to oxidative stress compared to advanced germ cells (Celino et al. 2011), confirming these results.

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9. Conclusion

Thus, fish spermatogenesis is controlled by the sex steroid hormones. Sex steroid hormones; estrogen, androgen and progestin, are important regulators for the progression of spermatogenesis. Mitotic divisions of spermatogonia can be categorized by spermatogonial stem cell renewal and spermatogonial proliferation toward meiosis. Spermatogonial renewal is regulated by E2 and spermatogonial proliferation toward meiosis is promoted by 11-KT. The action of E2 and 11-KT is mediated by other factors produced by Sertoli cells; E2 is mediated by spermatogonial stem-cell renewal factor and 11-KT is mediated by spermatogenesis preventing substance (anti-Mullerian hormone: AMH) and activin B. Meiosis is induced by DHP, which is progestin in teleosts. In oogenesis, DHP also initiates the meiotic prophase. After spermiogenesis, immature spermatozoa undergo sperm maturation. Sperm maturation is also regulated by DHP. DHP acts directly on spermatozoa to activate the carbonic anhydrase that is present in the spermatozoa. This enzymatic activation causes an increase in the seminal plasma pH, enabling spermatozoa to become motile.

By the establishment of eel testicular organ culture and the use of molecular biology techniques, analysis of the control mechanisms of fish spermatogenesis has advanced remarkably. Recently, we tried to establish new methods for analyzing the regulatory mechanisms of spermatogenesis, for example, the method of exogenous gene transfer into testicular cells using electroporation, and the germ cell and Sertoli cell co-culture system. It is highly possible that further investigations using these new methods will lead to a better understanding of the general aspects of spermatogenesis.

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List of Figures

Fig. 1. Induction of spermatogenesis by hCG injection into the Japanese eel. Under culture conditions, male Japanese eels have immature testes (small arrows) containing only non-proliferated type A and early type B spermatogonia. A single injection of human chorionic gonadotropin (hCG) can induce testicular development (arrowheads) for 18 days. These eels have a lot of spermatozoa in their testes. Electron micrographs are reprinted with permission from Zoological Science, 8, Miura et al., Induction of spermatogenesis in male Japanese eel, Anguilla japonica, by a single injection of human chorionic gonadotropin, 63–73, Fig. 2, © 1991, Zoological Society of Japan.

Fig. 2. Timetable of spermatogenesis of hCG injected eel testis. Germ cell development is almost synchronous throughout the testis and the proliferation of spermatogonia, meiosis and spermiogenesis occur at definite times: 3, 12 and 15 days after hCG injection, respectively. Reprinted with permission from Reproduction, 142, Miura et al., Gh is produced by the testis of Japanese eel and stimulates proliferation of spermatogonia, 869–877, Fig. 1, © 2011, Society for Reproduction and Fertility, and reprinted with permission from Kaiyo to Seibutsu, 24, Miura and Miura, The challenge of artificially producing sperm and egg from immature gametes in vitro, 114–119, Fig. 1, © 2002, Seibutsukenkyusha.

Fig. 3. The eel testicular organ culture system. Freshly removed immature eel testes were cut into small pieces, which were placed on floats of elder pith covered with a nitrocellulose membrane in 24-well plastic tissue-culture dishes (upper left). By using this system, 11-KT can induce the entire process of spermatogenesis for 36 days. Each symbol indicates: GA, type A spermatogonia; GB, type B spermatogonia; SC, spermatocytes; ST, spermatid; SZ, spermatozoa. Bar, 10 μm. Reprinted with permission from Handbook of Animal Cell Technology (Edited by Japanese Association for Animal Cell Technology), Miura and Miura, 287–289, Fig. 15.6, © 2000, Asakura Publishing Co., Ltd., reprinted with permission from Kaiyo to Seibutsu, 24, Miura and Miura, The challenge of artificially producing sperm and egg from immature gametes in vitro, 114–119, Fig. 2, © 2002, Seibutsukenkyusha, and reprinted with permission from Zoological Science, 18, Miura and Miura, Japanese eel: a model for analysis of spermatogenesis, 1055–1063, Fig. 2, © 2001, Zoological Society of Japan.

Fig. 4. The schema of the method of germ-somatic cells coculture system. Immature eel testes were enzymatically dissociated and the cell suspension was filtered through meshes and centrifuged in Nycodenz gradients. Separated cell suspension was centrifuged to make pellets and they were cultured with or without 11-KT. After 30 days culture, many spermatozoa (white arrows) having one or two flagella were observed around the pellet of germ cells and somatic cells. Reprinted with permission from Kaiyo to Seibutsu, 24, Miura and Miura, The challenge of artificially producing sperm and egg from immature gametes in vitro, 114–119, Fig. 4, © 2002, Seibutsukenkyusha.

Fig. 5. Using the organ culture system, germ-somatic cells coculture (pellet culture) system and cell culture system, we can investigate unknown factors directly added to the medium.

Fig. 6. Expression of GFP gene in germ-somatic cell pellets after electroporation. Transient expression of transfected genes was examined two days after the electroporation of GFP cDNA into germ-somatic cell pellets. A train of eight square pulses (60 V; duration 50 msec; interval 950 msec), resulted in widespread expression of GFP fluorescence in many round germ cells and somatic cells. CMV, cytomegarovirus promoter; GFP, green fluorescent protein. Reprinted with permission of John Wiley & Sons, Inc. from Molecular Reproduction and Development, 74, Miura et al., Transfer of spermatogenesis-related cDNAs into eel testis germ-somatic cell coculture pellets by electroporation: methods for analysis of gene function, 420–427, Fig. 1, © 2007, Wiley-Liss, Inc., a Wiley Company.

Fig. 7. A schematic diagram summarizing the possible control mechanisms of spermatogenesis in the Japanese eel. FSH, follicle-stimulating hormone; LH, luteinizing hormone; 17α,20β-DHP, 17α,20β-dihydroxy-4-pregnen-3-one; PD-ECGF, platelet-derived endothelial cell growth factor; AMH, anti-Müllerian hormone; CAll, carbonic anhydrase; eSRS, spermatogenesis related substances.

Fig. 8. Effect of DHP on induction of meiosis in the testis of Japanese eels in vitro. Electron micrographs showing testicular section from fragments cultured in basal medium without hormone (control) or with 10 ng/ml DHP for 6 days. Testicular fragment cultured with DHP showed that germ cell nuclei contained synaptonemal complexes (arrowheads) characteristic of meiotic cells.

Fig. 9. The ovarian epithelium culture technique. To remove oocytes of previtellogenic and vitellogenic stage, ovarian fragments of carp were excised and treated with enzymes. The residual ovarian fragments were precultured for one month. After preculture, ovarian explants were cultured in media with or without 1 ng/ml DHP for 14 days. Small arrows indicate oogonia. Photographs of 1 month culture is originally published in Miura et al., A Progestin and Estrogen Regulate Early Stages of Oogenesis in Fish, Biology of Reproduction 77, 822–828, Fig. 7, 2007.

Fig. 10. Effect of DHP on common carp ovarian fragments cultured for 14 days. Light micrographs showing ovarian section from fragments cultured in basal medium without hormone (control) or with 1 ng/ml DHP. Cells with arrows are chromatin-nucleous stage oocytes. Electron micrograph of germ cells with synaptonemal complexes (arrowheads) in ovarian fragments cultured with 1 ng/ml DHP. Photographs by light microscope is originally published in Miura et al., A Progestin and Estrogen Regulate Early Stages of Oogenesis in Fish, Biology of Reproduction 77, 822–828, Fig. 10, 2007.

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Fig. 11. DHP, a progestin, is an essential factor for the initiation of meiosis in both spermatogenesis and oogenesis.

Fig. 12. A schematic summary of the possible control mechanisms of spermatogenesis and oogenesis in fish. SPS, spermatogenesis-preventing substance; SSRF, spermatogonial stem cell renewal factor. Reprinted with permission from Cybium, 32(2) supplement, Miura and Miura, Progestin is an essential factor for the initiation of the meiosis in spermatogenesis and early oogenesis in fish, 130–132, © 2008, Société Française d'Ichtyologie.

Fig. 13. To identify factors that are regulated by DHP, we carried out gene expression cloning using eel testicular fragments that were cultured with (DHP) or without (Cont) 100 ng/mL DHP for 6 days. After cultivation, poly (A)+ RNAs were extracted from these testicular fragments and a subtracted cDNA library was then constructed from +DHP and –DHP cDNA enriched via the RDA procedure (Niwa et al. 1997). One thousand clones from each of these libraries were subsequently screened by differential hybridization, using each enriched cDNA preparation as a probe. We thereby screened 25 non-cross-hybridizing and up-regulated cDNA fragments after DHP treatment. Southern blotting analysis showed 12 typical cDNA fragments, whose names were eSRS36, 37, 38, 39, 40, 41, 42, 43, 44, 56, 29 and 33. Among these, an eSRS56 cDNA fragment corresponding to trypsinogen was identified (GenBank Acc. No. AB519643). Trypsinogen expression in the Japanese eel testis was determined by northern blot analysis of cultured testicular fragments. Pooled testicular fragments from 10 eels were cultured without (C) or with 10 ng/mL of 11-KT, E2 or DHP for 6 days. Lane IC shows the initial control before culturing. Northern blot analysis of EF1, which serves as reference, is also shown. Reprinted with permission from PNAS, 106, Miura et al., Trypsin is a multifunctional factor in spermatogenesis, 20972–20977, Fig. 1, © 2009, National Academy of Sciences.

Fig. 14. A schematic summary of the roles of trypsin on spermatogenesis in the Japanese eel.

Fig. 15. An illustration showing the action of oxidative stress on eel spermatogenesis.

Table 1. Using the Japanese eel testicular culture system, we found that advanced-stage germ cells undergo intense apoptosis and exhibit a strong signal for 8-hydroxy-2'-deoxyguanosine, an oxidative DNA damage marker, upon exposure to hypoxanthine (Hx)-generated reactive oxygen species (ROS), while spermatogonia remain unaltered. Spermatogonia are highly tolerant to ROS attack while advanced-stage germ cells such as spermatozoa, are much more susceptible. Low ROS, Hx treatment at the low dose (1 μM); High ROS, Hx treatment at the highest dose (100 μM).

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