Publisher Summary This chapter discusses the functional morphology of teleost gonads. Information presented in this chapter indicates that there are several processes of germ cell development, which are closely associated with changes in cellular activities of somatic cell elements. Recent studies on biochemical aspects of vitellogenesis in teleosts have shown that the hepatic and ovarian yolk proteins are similar to those of amphibian species. An in vitro method involving the separation of the follicular components has facilitated investigations of the detailed mechanism of the production of two major follicular steroid hormones. As a result of the usage of this technique, a two cell-type model involving thecal and granulosa cell layers has been proposed for the production of these two steroids for the first time in lower vertebrates. With further refinements this well characterized incubation procedure should provide an excellent system for studying the molecular basis of the mechanism of gonadotropin action on follicular steroidogenesis.

6 The Functional Morphology of Teleost Gonads

Publisher Summary This chapter discusses that the gonads potentiality to produce steroids, and the regulation of the syntheses. The nature, shape, and intensity of a hormonal signal, ready to be received by a target cell, is the result of an intricate series of positive and negative regulations. In the case of hormonal steroids in fish, only some aspects of this complex have been considered. Once a steroid is secreted, several mechanisms may inactivate it before it reaches its target. Little is known of catabolism of sexual steroids in teleosts. Most available data are concerned with the total radioactivity found in tissues after fish are fed labeled steroid. The biological significance of glucuronidation or sulfonation remains to be explored. Although the conjugated steroids are usually considered to be inactive, recent studies attribute a pheromonal role to glucuronides. In other respects, the binding to plasma proteins may lead to a reversible inactivation, although, in mammals, it has been suggested that steroid secretion may be enhanced by the presence of serum steroid-binding proteins. Finally, the conversion of plasma steroids into biologically active metabolites can occur in some target tissues. The actual physiological role of gonadal steroids in fish is discussed with emphasis on gametogenesis.

7 The Gonadal Steroids

Plasma 17α,20β-dihydroxy-4-pregnen-3-one levels during sexual maturation of amago salmon (Oncorhynchus rhodurus): Correlation with plasma gonadotropin and in vitro production by ovarian follicles

Plasma estradiol-17β and progesterone profiles were correlated with morphological changes in ovarian follicles during the preovulatory and postovulatory periods in the white-spotted char, Salvelinus leucomaenis Plasma estradiol levels were highest in September, and were followed by a sharp drop in October; they remained very low throughout the postovulatory period There was a good correlation between plasma estradiol levels and the gonadosomatic index, thus suggesting that estradiol is responsible for the synthesis of vitellogenic proteins Plasma progesterone levels were very low in August, began to rise in September and reached a peak soon after ovulation; progesterone remained high for several days after ovulation A preovulatory rise in plasma progesterone levels was recorded, and this is discussed in relation to the induction of oocyte maturation

Correlation of plasma estradiol-17β and progesterone levels with ultrastructure and histochemistry of ovarian follicles in the white-spotted char, Salvelinus leucomaenis

In the maturation cycle of ovaries of cultured rainbow trout, three periods can be distinguished: (1) a period of ovulation and previtellogenesis (January–May), (2) a period of exogenous vitellogenesis (May–November/December), and (3) a period of maturation of oocytes (November/December–January). Enzyme cytochemical and electron microscopical data indicate that stroma cells (i.e., interstitial cells and special theca cells) and granulosa cells represent sources of steroids. Steroidogenesis in stroma cells is found throughout the annual cycle, reaching a peak activity in January and February. Weak steroidogenic activity is observed in the granulosa cells of exogenous vitellogenic follicles and young postovulatory follicles. Possible functions of steroids secreted by stroma cells and granulosa cells are discussed.

Cyclic changes in the ovary of the rainbow trout, Salmo gairdneri, with special reference to sites of steroidogenesis.

The follicular epithelial layers of the developing ovary of two cichlid species were examined by electron microscopy for evidence of steroid secretion. As each oocyte grew, its follicular cell layers increased in height, eventually becoming somewhat columnar; no development could be detected in follicle cells of non-activated oocytes. Isolated cells close to capillaries in the thecal layer developed large amounts of smooth membrane indicative of steroidogenesis, appearing similar at maturity to testicular Leydig cells. In Cichlasoma nigrofasciatum the mitochondria of differentiated thecal elements contained microtubule-like inclusions. It is suggested that these cells may produce estrogens during vitellogenesis.

Ultrastructural observations on possible sites of steroid biosynthesis in the ovarian follicular epithelium of two species of cichlid fish, Cichlasoma nigrofasciatum and Haplochromis multicolor.

G. Maple

Papers

Ultrastructural observations on possible sites of steroid biosynthesis in the ovarian follicular epithelium of two species of cichlid fish, Cichlasoma nigrofasciatum and Haplochromis multicolor.