7.1 Role of piscine gonadotropins
7.2 Role of mammalian gonadotropins
7.3 Role of estrogenic steroids in synthesis of egg-yolk precursors
7.4 Chemistry of egg-yolk and its precursors
In oviparous animals, embryos are dependent on the egg-yolk for their nutritional requirements. The process of yolk deposition in oocytes - vitellogenesis - is a seasonal or cyclic phenomenon. Information on hormonal regulation of vitellogenesis in teleost fishes has been reviewed by Reinboth (1972), Donaldson (1973), de Vlaming (1974), Dodd (1975), Fontaine, Y.A. (1975), Fontaine, M. (1976), van Ree (1977a, b) and Crim and Idler (1978).
All stages of vitellogenesis starting with the mobilization of lipid from storage sites, the synthesis in the liver of a female-specific glycolipophosphoprotein - vitellogenin - and its eventual deposition in oocytes are known to be gonadotropin-dependent (Wallace, 1978; Nath and Sundararaj, 1981a, b). Hypophysectomy results in atresia of yolky oocytes (Dodd, 1960; de Vlaming, 1974), whereas replacement therapy with crude extracts of fish pituitary and purified gonadotropins isolated from carp (Burzawa-Gérard, 1971, 1974; Idler and Ng, 1979), salmon (Yamazaki and Donaldson, 1968b; Sundararaj, Anand and Donaldson, 1972; Donaldson et al., 1972b; Ng and Idler, 1978b; Idler and Ng, 1979) as well as plaice and winter flounder (Campbell and Idler, 1976; Ng and Idler, 1978a, 1979) induce vitellogenesis (Peter and Crim, 1979). Ng and Idler (1978a, b; 1979) and Idler and Ng (1979) have isolated two gonadotropic hormones. The one with low carbohydrate content induces vitellogenesis, while the other, which is rich in carbohydrates, induces maturation and ovulation. Recently, Idler and Campbell (1980) have suggested that in rainbow trout the salmon gonadotropic fraction rich in glycoprotein which is absorbed on concanavalin A-Sepharose is responsible for stimulation of estradiol secretion, whereas a gonadotropin with a lower carbohydrate content that is not absorbed on concanavalin A-Sepharose is involved in yolk incorporation (Campbell and Idler, 1976; Campbell, 1978; Ng and Idler, 1978a, b). In the ovaries of gonadotropin-treated fishes, the granulosa and thecal layers surrounding the oocytes are hypertrophied and show enhanced D 5-3ß-hydroxysteroid dehydrogenase activity (de Vlaming, 1974; van Ree, 1977a, b) indicating that gonadotropins induce vitellogenesis through the production of ovarian steroids, possibly estrogens. Information on the production of estrogens in the teleost ovary in response to gonadotropic stimulation has been reviewed in section 6 on the ovary.
In contrast to piscine gonadotropins, mammalian gonadotropins and estrogens bring about only the induction of vitellogenin synthesis but do not promote its incorporation into developing oocytes in the hypophysectomized catfish, Heteropneustes fossilis, (Sundararaj and Anand, 1972; Anand, Nayyar and Sundararaj, 1975; Nath and Sundararaj, 1981b; Sundararaj and Nath, 1981), even though they maintain yolky oocytes in the hypophysectomized gravid catfish, possibly through production of ovarian estrogen (Anand and Sundararaj, 1974b). Thus, mammalian gonadotropins cannot complete the entire process of vitellogenesis in teleost fishes (see Fontaine, Y.A., 1975; Fontaine, M., 1976; van Ree, 1977a, b). The possible role of other protein hormones in vitellogenesis is controversial (see Upadhyay, Breton and Billard, 1978). Very low doses of thyroxine stimulate vitellogenesis in immature goldfish (Hurlburt, 1977) possibly by increasing ovarian sensitivity to gonadotropin.
During vitellogenesis elevated litres of serum components, specially lipids, calcium and phosphoproteins have been recorded in fish (see review by Wallace, 1978). A part of yolk is synthesized by various organelles in the oocyte cytoplasm itself and this is termed as endogenous yolk (Raven, 1961; Norrevang, 1968; Upadhyay, Breton and Billard, 1978), whereas the remainder of the yolk is extra-ovarian in origin hence known as exogenous yolk (Norrevang, 1968; Wallace, 1978). The key product in vitellogenesis is a female-specific glycolipophosphoprotein called vitellogenin which is synthesized in the liver in response to circulating estrogens and transported by blood to the ovary where it is taken up by oocytes, cleaved into the final egg-yolk proteins - lipovitellin and phosvitin - and deposited as yolk granules or platelets (Wallace, 1978).
Estrogens stimulate the liver of oviparous vertebrates to produce protein and lipid precursors of yolk (Wallace, 1978). A number of investigators have used immunochemical and/or electrophoretic methods to identify female-specific serum proteins and to show that the major protein constituents of eggs are present in the serum of female fish undergoing vitellogenesis (Wallace, 1978; Hara and Hirai, 1978; Korsgaard and Petersen, 1979; Hara, Yamauchi and Hirai, 1980; Campbell and Idler, 1980). In recent years, Emmersen and Petersen (1976) in flounder, Platichtys flesus, Hara and Hirai (1978) and Campbell and Idler (1980) in rainbow trout, Salmo gairdneri, Hara, Yamauchi and Hirai (1980) in the Japanese eel. Anguilla japonica, and Nath and Sundararaj (1981a) in catfish, Heteropneustes fossilis, have isolated by gel chromatography, female-specific lipophosphoprotein from the blood of vitellogenic female and of estradiol-treated female or male fish. Plasma levels of vitellogenin have been quantified by radioimmunoassay in the Atlantic salmon, Salmo salar sebago (Idler, Hwang and Crim, 1979) and rainbow trout (Campbell and Idler, 1980).
Several attempts have been made to determine the biochemical nature of vitellogenin and egg-yolk in fishes (see Wallace, 1978). Analysis of yolky eggs indicates that the egg-yolk is composed of lipovitellin and phosvitin, the former is rich in lipid and poor in phosphorus and the latter is rich in phosphorus and poor in lipid (Wallace, 1978; Campbell and Idler, 1980). The amino acid composition of vitellogenin as well as of lipovitellin and phosvitin has been determined in a number of fishes (Schmidt et al., 1965; Ito et al., 1966; Mano and Yoshida, 1969; Hara and Hirai, 1978; Campbell and Idler, 1980). In the Japanese eel, Anguilla japonica, vitellogenin and the related egg protein have similar molecular weights suggesting that in the eel vitellogenin may be incorporated into egg-yolk as a phosvitin-lipovitellin complex without any structural rearrangement (Hara, Yamauchi and Hirai, 1980).
At the end of vitellogenesis the ovary is packed with yolky oocytes which subsequently undergo maturation and ovulation under favourable environmental conditions.