10.1 Effects of mammalian and piscine gonadotropins
10.2 Effects of androgens
Spermatogonial proliferations proceed in some fishes studied even in the absence of the pituitary gland (Barr, 1968; Lofts, Pickford and Atz, 1966) even though at a retarded rate (Ahsan, 1966b; Sundararaj and Nayyar, 1967). Mammalian gonadotropins such as luteinizing hormone (LH), follicle-stimulating hormone (FSH), human chorionic gonadotropin (hCG) and pregnant mare serum gonadotropin (PMSG), as well as piscine gonadotropins have been tested for testicular stimulation in several fishes (see Pickford and Atz, 1957; Dodd, 1960, 1972; de Vlaming, 1974). Ovine LH induces spermatogenesis in the regressed testes of Blennius sphinx (Blüm, 1972), whereas hCG stimulates testicular hydration in intact Gillichthys mirabilis at both 12° and 27°C (de Vlaming, 1974). Mammalian LH, but not FSH, reinitiates spermatogenesis in the hypophysectomized Fundulus heteroclitus (Pickford and Atz, 1957) and Couesius plumbeus (Ahsan, 1966b). In methallibure-treated Cymatogaster aggregata, treatment with LH reinitiates the arrested spermatogenesis (Wiebe, 1969). In hypophysectomized cat-fish, Heteropneustes fossilis, partially purified salmon gonadotropin (SG-G100) (Sundararaj et al., 1971; Nayyar et al., 1976), LH and hCG but not FSH (Sundararaj and Nayyar, 1967; Sundararaj and Anand, 1972) reinitiate and maintain spermatogenesis. In the intact Sarotherodon (Tilapia) leucosticta, hCG treatment increases plasma androgen levels (Hyder, Shah and Krischner, 1970).In Gillichthys mirabilis SG-G100 prevents warm-temperature-induced testicular regression (de Vlaming, 1974) and in hypophysectomized goldfish, carp gonadotropin maintains spermatogenesis (Billard, Burzawa-Gérard and Breton, 1970). On the other hand, mammalian LH and hCG do not activate spermatogenesis in the hypophysectomized goldfish (Billard and Escaffre, 1973) and hypophysectomized Gillichthys mirabilis (de Vlaming, 1974), respectively. Pickford et al. (1972) reported an interesting observation that mammalian growth hormone accelerates Spermatogonial multiplication, while LH stimulates later stages of spermatogenesis in Fundulus heteroclitus.
Promotion of spermatogenesis by gonadotropins (piscine or mammalian) is mediated through the formation of androgens (Dodd, 1960, 1972; de Vlaming, 1974; Hoar and Nagahama, 1978). Testosterone propionate hastens germ cell maturation in Lebistes reticulatus (Eversole, 1941). In the hypophysectomized Fundulus heteroclitus, androgens stimulate increase in testicular size and spermatogenesis (Lofts, Pickford and Atz, 1966). In the hypophysectomized catfish, Heteropneustes fossilis testosterone, albeit in high doses, not only reinitiates but also maintains complete spermatogenesis (Sundararaj and Nayyar, 1967; Sundararaj et al., 1971; Nayyar et al., 1976). Testosterone treatment does not activate spermatogenesis in Sarotherodon (Tilapia) sp. pretreated with methallibure (Hyder, 1972) but restores secondary sexual characters and spawning behaviour (Dadzie, 1973).
There is very little information in the literature on the phenomenon of Spermiation which involves hydration of the testes and ejection of spermatozoa from the seminiferous tubules in a medium, the milt. The process is mediated by the secretory cells lining the sperm duct as in the salmonids, the seminal vesicles as in catfishes and gobiid fishes and even by the Sertoli cells. The work on endocrine regulation of Spermiation has been reviewed by Clemens and Grant (1964, 1965), Yamamoto and Yamazaki (1967) and Grant, Pang and Griffith (1969). Hypophysectomy blocks while SG-G100 stimulates Spermiation in goldfish, (Yamazaki, 1962; Yamazaki and Donaldson, 1968a) and in the. immature Oncorhynchus gorbuscha (Donaldson et al., 1972a; Funk and Donaldson, 1972). Gonadal hydration and seminal thinning responses have been demonstrated in common carp and rainbow trout after pituitary injections (Clemens and Grant, 1964, 1965). A partially purified gonadal hydration principle has been isolated from the pituitary of the common carp (Clemens et al., 1964). In goldfish, gonadotropins act on Leydig cells to promote androgen secretion which in turn stimulates the Sertoli cells in the spermiation response (Yamazaki and Donaldson, 1969). In the hypophysectomized catfish, Heteropneustes fossilis (Sundararaj et al., 1971), and in the intact grey mullet, Mugil cephalus (Donaldson and Shehadeh, 1972; Shehadeh, Madden and Dohl, 1972), Spermiation can be induced with SG-G100 but not by LH and testosterone.
Sanchez-Rodriguez et al. (1978) have studied variations in plasma gonadotropin (GtH), total androgens as well as other parameters such as spermatocrit In the milt of rainbow trout up to 12 weeks following the onset of Spermiation. The beginning of Spermiation is associated with low sperm count and high levels of GtH. Significant sperm production occurs several weeks later when GtH levels decrease and androgen levels increase. Thus, both GtH and androgens may be involved in Spermiation in the second and more active phase of spermiation.