6.2 Cellular source of steroid hormones
6.3 C18 estrogenic steroids
6.4 C19 and C21 steroids
6.5 Sex recognition and pheromones
6.6 Post-ovulatory follicle
The ovary in most teleost fishes is a hollow sac-like organ into which extend numerous ovigerous folds lined by germinal epithelium. The germ cells, the endodermally derived oogonia, multiply mitotically and get transformed into nonyolky primary oocytes whose, nuclei are arrested at the prophase of the first meiotic division until maturation. These processes can take place even in the absence of the pituitary (Barr, 1968; Hoar, 1969). Primary oocytes, covered generally by two layers of follicle cells, an outer thecal layer and an inner granulosa layer, undergo vitellogenesis when yolk is deposited in the. ooplasm. During maturation, the first polar body is given cut and the second meiotic division is arrested at metaphase. The eggs are spawned by the fish at this stage and the second polar body is released only after fertilization. In some fishes, ovulation and- spawning occur almost at the same time, whereas in ethers (rainbow trout and milkfish) ovulated oocytes are retained in the ovarian or peritoneal cavity and spawning takes place much, later.
Application of improved histochemical techniques and electron microscopy has shown promise of localizing the cellular sources of steroid hormone's in fish ovary (Lofts and Bern, 1972; Guraya, 1976a, 1979; Hoar and Nagahama, 1978). Cells involved in steroidogenesis have smooth endoplasmic reticulum and mitochondria have tubular cristae (Christensen, 1975). They also contain the key enzymes involved in the biosynthesis of steroids such as hydroxysteroid dehydrogenases (3ß-HSD), required in the oxidation of steroids and glucose 6-phosphate dehydrogenase responsible for the NADPH-mediated hydroxylation of steroids. 3ß-HSD in the special thecal and granulosa cells has been shown in a number of teleost fishes (see de Vlaming, 1974; Lance and Callard, 1978). Ovarian steroidogenesis has been attributed to the granulosa cells in Cyprinus carpio; Mugil capito and Sarotherodon (Tilapia) aurea (Blanc-Livni, 1971) and in Sarotherodon (Tilapia) nilotica (Yaron, 1971). In rainbow trout, Salmo gairdneri, the granulosa and interstitial cells of the ovary are involved in estradiol production (Lambert et al., 1978; van den Hurk and Peute, 1979). According to Hoar and Nagahama (1978) ovarian hormones are mostly synthesized by the special thecal cells which are homologous to the Leydig cells in males. Ovarian, homogenates from rainbow trout when incubated with tritiated pregnenolone or androstenedione during different, stages of the ovarian cycle show that the latter is used as substrate for conversion to estrone and estradiol during endogenous previtellogenesis, whereas the former is used during exogenous vitellogenesis (Lambert and van Bohemen, 1979).
That the teleost ovary is capable of producing C18 estrogenic steroids has been determined by experiments involving bilateral ovariectomy and replacement therapy experiments (Dodd, 1960; de Vlaming, 1974; Lance and Callard, 1978) and more directly, by extraction and identification of estrogenic steroids from the ovary (see Bern and Chieffi, 1968 for extensive bibliography; Hoar, 1969; Nandi, 1967; Ozon, 1972b; Reinboth, 1972; Colombo, del Conte and Clemenze, 1972; Sandor, 1979). The estrogens are also responsible for the secondary sexual characters and for sexual behaviour. Several investigators have shown that the teleost ovarian tissue has the capacity to transform labelled precursors into estrogenic steroids such as estradiol-17ß, estrone and estriol (see Ozon, 1972b; de Vlaming, 1974; Lance and Callard, 1978; van Bohemen and Lambert, 1978; Lambert, 1978; Lambert and van Bohemen, 1979; Sandor, 1979). Plasma levels of estrone and/or estradiol have been determined in channel catfish, Ictalurus punctatus (Eleftheriou, Böehlke and Tiemeier, 1966), Salmo gairdneri (Schreck, Lackey and Hopwood, 1973; Lambert et al., 1978; Fostier et al., 1978; Billard et al., 1978) goldfish, Carassius auratus (Schreck and Hopwood, 1974), common carp, Cyprinus carpio (Billard et al., 1978), plaice, Pleuronectes platessa (Wingfield and Grimm, 1977), and Sarotherodon (Tilapia) aurea (Yaron et al., 1977; Terkatin-Shimony and Yaron, 1978) and catfish, Heteropneustes fossilis (Sundararaj, Goswami and Lamba, 1979; Sundararaj, Lamba and Goswami, 1980).
The teleost ovary is also known to produce several nonestrogenic steroids (see Sandor, 1979). The ovary of a number of teleost fishes has been shown to produce 11-deoxycorticosterone and 11-deoxycortisol, (Colombo et al., 1973; Colombo and Colombo Belvedere, 1976), epipregnanolone (3a -hydroxy-5ß-pregnan-20-one) and pregnanediol (5ß-pregnan-3a -20a -diol) (Ungar et al., 1977). Progesterone has been identified in the ovaries but its presence in the blood has not so far been reported except in the winter flounder, Pseudopleuronectes americanus (Campbell, Walsh and Idler, 1976). It is also now well established that in teleost fishes, gonadotropins induce oocyte maturation through the synthesis of C21 steroids, the source of which in many species is the ovary (Jalabert, 1976; Sundararaj and Goswami, 1977a) or the interrenal as in catfish, Heteropneustes fossilis (Sundararaj and Goswami, 1977b).
Testosterone and 11-oxygenated androgens have been isolated from incubations of C19 and C21 steroid precursors with ovaries obtained from teleost fishes belonging to seven orders (see Kime, 1979; Sandor, 1979). It is not clear whether testosterone is produced as an intermediate in estrogen biosynthesis or it represents an ovarian hormone. However, the 11-oxygenated androgens have some physiological significance. In Mugil capito, levels of 11-ketotestosterone are much higher in seawater fish than in freshwater fish and this has been related to infertility of fish in fresh water (Eckstein and Eylath, 1970). Eckstein and Katz (1971) have identified testosterone, 11-ketotestosterone and 11-deoxycorticosterone as the major conversion products of radioactive precursors from ovarian tissues of Sarotherodon (Tilapia) aurea, and Katz and Eckstein (1974) have identified and isolated the same steroids from the peripheral plasma and reported that levels of 11-ketotestosterone are higher than those of testosterone in the plasma of spawning Sarotherodon (Tilapia) aurea. However, in rainbow trout the plasma levels of 11-ketotestosterone are undetectable or low during most of the reproductive cycle (Simpson and Wright, 1977). In female catfish, Heteropneustes fossilis, plasma levels of testosterone (see also Truscott et al., 1978) are much lower than those of estradiol-17ß during the vitellogenic period, whereas they exceed those of estradiol-17ß during the maintenance phase (Sundararaj, Goswami and Lamba, 1979; Sundararaj, Lamba and Goswami, 1980).
Reinboth (1972, 1979) has reviewed the work on steroidogenic pathways in ambisexual fishes in which a correlation exists between biological events related to sex inversion and peculiarities in steroid biosynthesis. 11-ketotestosterone which is present in the 12-month old guppy, Poecilia reticulata, is absent in 3-month old fish and it has been suggested that this may be the cause for spontaneous sex reversal sometimes seen in old guppies (Lambert and Pot, 1975). 11-oxygenated androgens may be responsible for the long delay in sexual maturation in eel, Anguilla anguilla (Colombo and Colombo Belvedere, 1976). Ovarian 11ß-hydroxylase has been related to hermaphroditism in protogynous Coris julis and the protandric Pagellus acarne (Reinboth, 1972, 1979). However, the role of ovarian 11-oxygenated androgens in regulation of fertility in teleost fishes is far from clear.
One of the most fascinating phenomena in the spawning of fishes is that they identify the opposite sex easily as they start their specific spawning behaviour. The information on sex recognition in fishes is meagre and fragmentary (Liley, 1969). In many fishes, visual cues are important but in many others chemoreception has been proposed as a method of sex recognition. A substance present in the ovarian fluid of the gravid female, Bathygobius soporator, can elicit courtship behaviour in the male (Tavolga, 1956), and in Lebistes reticulatus, the male becomes hyperactive when exposed to a substance released by the female which has been identified as an estrogen (Amouriq, 1965). In goldfish, a substance produced by the kidney of estrogen-treated fish or mature female is recognized by the androgen-treated fish or mature male (Yamazaki and Watanabe, 1979). Such sex pheromones might be species specific and they should be chemically analysed since they may be of potential value in fish culture as well as in basic studies on spawning behaviour of fish.
The post-ovulatory follicle in the majority of the teleost fishes collapses and merges with the stroma. Unlike in mammals, it does not get organized into a corpus luteum-like structure and has no endocrine functions in most species of fishes studied (see Saidapur, 1978). In some fishes such as the rainbow trout, after ovulation, the granulosa, special thecal and interstitial cells can synthesize steroids that may have a function in the maintenance of ovulated eggs (Guraya, 1976a). Often, many of the developing follicles, or the unovulated yolky oocytes that remain in the follicles, become atretic. The granulosa cell layer in such cases hypertrophies and phagocytoses the yolk; such follicles, termed 'corpora atretica' do not have endocrine functions.
The growth of the follicle and subsequent changes that take place leading to yolk formation and deposition (vitellogenesis) are discussed in the next section.