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Both partially purified chum salmon gonadotropin and 17a-hydroxyprogesterone stimulated in vitro production of testosterone by postvitellogenic follicles of goldfish (Carassius aurafus). Chum salmon gonadotropin further enhanced the conversion of exogenously supplied 17a-hydroxyprogesterone to 17a, 2OP-dihydroxy-4-pregnen-3-one. The increased medium concentrations of 17a, 20&dihydroxy-4-pregnen-3-one were associated with the induction of final oocyte maturation.The capacity of postvitellogenic follicles to produce steroids in response to exogenous 17a-hydroxyprogesterone was examined in females at various stages of final oocyte maturation following the administration of human chorionic gonadotropin in vivo combined with elevation of holding temperature. The maximum production of testosterone in response to 17a-hydroxyprogesterone was obtained in follicles from initial controls. In contrast, 17n, 20P-diOHprog production was very low in initial controls and markedly increased during oocyte maturation (3-6 hr following injection), followed by a significant dxrease in follicles collected at 15 hr. Estradiol-17P production by the follicles was very lo# at any stages of gonadotropin-induced oocyte maturation. These results suggest that gonadotropin-induced shift in the biosynthetic pathway in the follicle from the secretion of predominantly testosterone to 17a, 2OP-dihydroxy-4-pregnen-3-one secretion is a prerequisite step for the induction of oocyte maturation in goldfish.In most vertebrates, full-grown follicle-enclosed oocytes resume meiotic maturation after the preovulatory gonadotropin surge (1). In lower vertebrates, such as amphibians and teleosts, gonadotropin action is mediated by ovarian follicle layers through the production of steroidal mediators, maturation-inducing steroid (MIS) (1, 2, 3). Recently, 17a, 20/3-dihydroxy-4-pregnen-3-one (17cl, 20P-diQHprog) has been identified as the naturally occurring MIS of amago salmon, Oncorhynchus rhodurus (4). 17a, 20P-DiOHprog has also been considered to be a major MIS in several other species of salmonids (3, 5, 6).In goldfish, high plasma levels of 17a, 20B-diQHprog have been reported in females undergoing oocyte maturation and ovulation (7, 8). Previous in vitro studies revealed that 17a, 20P-diQHprog is the most potent steroid for the induction of oocyte maturation in goldfish (5, 9). These results suggest that 17a, 20P-diOHprog is a major MIS responsible for oocyte 555

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The pituitary gland of the milkfish, Chanos chanos, was studied at different stages of sexual maturation and spawning. Consecutive median sagittal sections were treated with a range of stains to demonstrate the different cell types and regions. The milkfish pituitary consists of a neural component, the neurohypophysis, and an epithelial component, the adenohypophysis, which in turn consists of three regions: the rostra1 pars distalis (RPD), proximal pars distalis (PPD), and pars intermedia (PI). However, unlike most teleosts, the pituitary gland of the milkfish is encased in a bony chamber, has dorsal and ventral lobes and extends anteriorly from its point of origin at the base of the brain. PAS (+) basophils are found in all regions of the adenohypophysis, but mostly in the proximal pars distalis. These cells undergo hypertrophy and hyperplasia during sexual maturation, shrinkage and degradation during spawning.