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The response of ornithine decarboxylase activity to hormones in the embryonic left ovary was measured throughout the stages of development. During the early stage of ovarian development (9th day of incubation), the ornithine decarboxylase activity (in terms of pmol CO(2)/30min per mg of protein) was high (766); it decreased from the 10th to the 12th day (575-239), increased slightly from the 13th to the 15th day (306) and finally fell to a low value (192-20) from the 18th day of development to birth. Administration of an optimal dose of oestrogen to the 9-10-day embryo stimulated the ovarian ornithine decarboxylase activity by 48-53%. If the same dose of oestrogen was administered to the 15-18-day embryo, the ovarian enzyme activity was suppressed by 32-43%. This biphasic response to oestrogen for enzyme induction is characteristic of the developing ovary and is not observed in other genital organs of the chick. In the early developmental stage (9-10th day) testosterone has no effect on ovarian ornithine decarboxylase activity, but in the late stage testosterone inhibits the activity by 41%. Organ culture techniques have been used to test the ovarian response to lutropin (luteinizing hormone). Lutropin stimulated ornithine decarboxylase activity by approx. 99-155% in the ovary of the early embryonic stage (10-13th day), and by 175-200% in the ovary of the late embryonic stage (15-18th day). The alteration in enzyme activity in the ovary as assayed in vitro during development is not due to the effect of the size of the endogenous ornithine pool. The relationship of ornithine decarboxylase activity to the morphological and biochemical changes in the developing ovary is discussed.

Studies on sex-organ development. Effect of hormones on ornithine decarboxylase activity in the developing chick ovary