4.2 Identification of gonadotrophs
4.3 Immunocytological identification of gonadotrophs
4.4 Pituitary and plasma gonadotropin levels
The general organization and morphology of the pituitary gland or hypophysis has been described in a number of teleost fishes including a few cultivated species (see Pickford and Atz, 1957; Ball and Baker, 1969; Sage. and Bern, 1971; Rao, 1972; Holmes and Ball, 1974; de Vlaming, 1974; Baker, Keshavanath and Sundararaj, 1974; Fontaine and Olivereau, 1975; Moitra and Sarkar, 1976a; Jose and Sathyanesan, 1977).
The pituitary gland in fishes, as in all other vertebrates, is composed of the adeno-hypophysis derived from the Rathke's pouch and the neurohypophysis originating from the diencephalon. The adenohypophysis, which regulates gonadal functions in fishes, is the site of synthesis, storage and release into circulation of several peptide and protein hormones. The adenohypophysis is divided into the rostral (pro-adenohypophysis) and the proximal (meso-adenohypophysis) pars distalis, and the pars intermedia (meta-adenohypophysis) (Ball and Baker, 1969; Schreibman, Leatherland and McKeown, 1973).
Heretofore, cell types in the fish pituitary have been identified by histochemical techniques and validated by histophysiological experiments (see Ball and Baker, 1969; Nagahama, 1973; Fontaine and Olivereau, 1975). The gonadotropin, secreting cells - the gonadotrophs - are most often located in the proximal pars distalis and are identified on the basis of topographical location and tinctorial reactions with the periodic acid - Schiff reagent and Herlant'a tetrachrome stain.
Correlations between changes in the number and granulation of the basophils (gonadotrophs) and events in the reproductive cycle have been studied only in a few cultivated species such as the catfish, Heteropneustes fossilis (Sundararaj, 1959, 1960), the common carp, Cyprinus carpio (Leray, 1965; Blanc-Livni and Abraham, 1968), the catfish, Clarias batrachus (Lehri, 1966, 1970), the grey mullet, Mugil cephalus (Blanc-Livni and Abraham, 1968) and mrigal, Cirrhina mrigala (Moitra and Sarkar, 1976b). The basophils are more numerous and heavily granulated during the breeding season than during the sexually quiescent phase.
Doerr-Schott (1976) and Follénius, Doerr-Schott and Dubois (1978) have reviewed the literature on the immunocytology of pituitary cells of teleost fishes. The detection of piscine protein hormones by immunological methods depends upon the availability of purified fish pituitary hormones and the corresponding antisera, which should work on histological sections. Burzawa-Gérard (1971) reported that an extract of carp pituitary powder does not react in the chicken luteinizing hormone (LH) assay system, while purified carp gonadotropin (GtH) reacts only at very high dose levels. Further, antisera against carp GtH show only limited affinity for ovine LH in competitive radioimnunoassay (Breton et al., 1971b). However, cross-reactivity of the piscine GtH with the antibodies to corresponding mammalian hormone Is not a reliable criterion for the detection of the hormone nor is it sufficient proof that substances so detected correspond to the biologically active hormone (see Nicoll, 1975 for discussion).
Investigators using light and electron microscopic as well as immunocytochemical techniques have identified either one or two types of gonadotropic cells (Follénius, Doerr-Schott and Dubois, 1978). Billard, Breton and Dubois (1971) demonstrated in the pituitary of the common carp, Cyprinus carpio, gonadotrophs using antisera to carp GtH and ovine LH, and thyrotrophs by using carp thyrotropin antiserum absorbed by carp gonadotropin. Similarly, McKeown and van Overbeeke (1971) have reported a positive immunoreaction with ovine LH anti-serum but not with FSH antiserum in most of the basophils situated in the rostral and proximal pars distalis in the sockeye salmon. Immunocytological and immunohistochemical studies indicate the presence of one LH-like gonadotropic factor in the piscine pituitary (Follénius, Doerr-Schott and Dubois, 1978). One gonadotropic cell type has been reported in the pituitary glands of Heteropneustes fossilis (Sundararaj, 1959, 1960; Baker, Keshavanath and Sundararaj, 1974), Clarias batrachus (Dixit, 1970), Trachurus sp. and Gasterosteus aculeatus (Leatherland, 1970a), Anoptichthys jordani (Mattheij, 1970), Oncorhynchus keta (Nagahama and Yamamoto, 1970), Cyprinus carpio (Billard, Breton and Dubois, 1971), Oncorhynchus nerka (McKeown and Leatherland, 1973), Carassius auratus (Kaul and Vollrath, 1974), Poecilia latipinna (Batten, Ball and Benjamin, 1975; Peute et al., 1978), Salmo salar (Ekengren, Peute and Fridberg, 1978), and Rhamdia hilarii (Val-Sella and Sesso, 1980), whereas two types of gonadotropic cells have been shown in Anguilla anguilla (Olivereau and Herlant, 1960; Knowles and Vollrath, 1966a, b), Mugil cephalus (Leray and Carlon, 1963; Stahl, 1963; Leray, 1966; Olivereau, 1968), Oncorhynchus nerka (Cook and van Overbeeke, 1972), Leuciscus rutilus (Båge et al., 1974), Monopterus albus (O and Chan, 1974), and Oncorhynchus masou (Ueda and Hiroshima, 1979). Recently, Olivereau (1978) has examined the pituitary glands of rainbow trout, Salmo gairdneri, at several stages of sexual maturation in winter and reported two types of gonadotropic cells; one which predominates in mature trout has large granules and is labelled with antisera to human chorionic gonadotropin and bovine thyrotropin, and the other type has fine granules and reacts poorly with glycoprotein staining and predominates in females with immature ovaries. Peute et al. (1978), who have correlated the gonadotropic content of the pituitary with ultrastructural changes in the gonadotropic cells in the rainbow trout at various stages of its reproductive cycle, reported a drop in hormone content concomitant with the onset of exogenous vitellogenesis in June/July in the female and with the acceleration of spermatogenesis in September in the male.
van Oordt and Ekengren (1978) have applied the double antibody immunofluorescent technique for the identification of functional cell types in the adenohypophysis of rainbow trout. The globular or cisternal forms of basophils related to different physiological states of the gonadotropic cell have been correlated with the gonadal cycle.
Now that Ng and Idler (1978a, b; 1979) and Idler and Ng (1979) have isolated two types of gonadotropins from the pituitary glands of carp, salmon, winter flounder and plaice, antibodies to the two gonadotropins could be used in the immunofluorescent or immunoenzyme studies to identify the gonadotropic cells that are active during vitellogenesis and ovulation.
With the availability of the radioimmunoassay techniques for measurements of plasma levels of gonadotropic hormones, a number of studies (Billard et al., 1978; Peute et al., 1978; van Oordt and Ekengren, 1978) on seasonal variations in gonadotropin levels in the pituitary and plasma have been made in common Carp and rainbow trout (see section 2 on Reproductive cycles and environmental cues and section 5 on Gonadotropic hormones).
O'Connor (1972) has proposed a circadian pattern of pituitary gonadotropin release in the rainbow trout, where the pituitary gonadotropic content, assayed by using 32P uptake in chick testes, decreases significantly shortly after the onset Of the light period (LD 10:14) and the levels remain high during the rest of the 24-h period. Decrease of GtH in the pituitary could mean that the hormone is being released at a faster rate than its synthesis or it could imply a blockade of both synthesis as well as release. Recently, Peter and Hontela (1978) and Hontela and Peter (1978) have reported circadian rhythmic changes in gonadotropin levels in plasma of goldfish.
The pituitary gland produces two gonadotropic hormones according to recent work on a few species. The isolation and secretory patterns of gonadotropin(s) are discussed in the next section.