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Steroidogenesis and the role of steroids in the endocrine control of oogenesis and vitellogenesis in the goldfish, Carassius Auratus Khoo, Khay Huat

Abstract

In this thesis I studied the role of ovarian steroids in oogenesis and ovarian development of goldfish (Carassius auratus L.) and examined the interrelationship of pituitary gonadotropin and ovarian steroids in the endocrine control of teleost reproduction. There are four main parts to the investigation: (i) the cytological analysis of vitellogene-sis, (ii) the demonstration of steroidogenic tissues in the ovary and their endocrine control, (iii) the development and functions of the corpus luteum, and (iv) the role of ovarian steroids on oogenesis and vitel-logenesis. The histological and histochemical examination demonstrated that two types of yolk inclusions are formed during vitellogenesis: yolk vesicles, comprised of mucopolysaccharides, and first formed, followed subsequently by yolk granules composed of proteins, phospholipids and neutral lipids. Neither type of yolk develops in the absence of the pituitary; estrogen regulates the formation of yolk vesicles while pregnenolone was found to control deposition of yolk granules. The significance of these two kinds of yolk is considered. This is the first demonstration of two yolk types in goldfish with a separate endocrine control for the formation of each of them. The major steroid synthesizing tissues in goldfish ovaries are the granulosa cells of oocytes and the corpus luteum. The corpus luteum, whether pre- or post-ovulatory probably synthesizes estrogens. In this study, no functional differences were established between pre- and post- ovulatory corpora lutea. Treatment of post-ovulatory goldfish with tritiated thymidine strongly suggests that the corpora luteal cells proliferate to form another generation of oogonia. Estrogens are probably active hormones in this reaction. The implications of these new findings are discussed. Administration of exogenous estrogen increases oogonia formation in post-ovulatory goldfish whereas chronic administration of estrogens or testosterone into gravid fish induces extensive atresia. Treatment of gravid fish with progesterone or corticosteroids induces ovulation. As a working hypothesis it is proposed that steroids are synthesized by the ovaries under gonadotropin stimulation. Pituitary gonadotropin regulates the whole ovarian steroid synthetic process and not any specific reaction in the steroid metabolic pathways. The synthesis of specific steroids during the reproductive cycle is brought about by localized inhibition of steroidogenic enzymes by steroids, most likely estrogens. The potential practical applications of endocrine manipulations in fish reproduction are discussed.

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