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The entry and metabolism of 3H-cortisol in oocytes were investigated using isolated follicles of the tilapia (Oreochromis mossambicus) in order to examine the mechanisms of incorporation of maternal hormones into oocytes. The composition of 3H-labeled steroids in the oocyte was analyzed by high-performance liquid chromatography. A significant amount of cortisol was converted to cortisone and an unidentified molecule by the follicular layer. The contents of 3H-cortisol and 3H-cortisone in the oocyte reached an equilibrium level within 12 hr, whereas the content of the unidentified metabolite continued to increase for 36 hr. The total content of the incorporated cortisol and its metabolites was proportional to cortisol in the medium over the concentration range of 5 ng/ml to 5 microg/ml. The amounts of cortisone and the unidentified molecule increased proportionally when the concentration of cortisol in the medium was lower than 500 ng/ml, whereas they reached a plateau when the concentration of cortisol exceeded 500 ng/ml. Cortisol entry was reversible, because 90% of cortisol and cortisone in the oocyte was lost within 18 hr when the medium was changed to that without 3H-cortisol. On the other hand, 50% of the unidentified molecule was preserved at the end of the incubation. In conclusion, the entry of cortisol into the oocyte was considered to be nonspecific and due probably to simple diffusion. However, a considerable amount of cortisol (50-70%) was specifically converted to cortisone and another unidentified molecule during passage through the follicular layer.

To examine the biological significance of thyroid hormones contained in eggs, the hormones were depleted from eggs by treating the mother fish with thiourea. Constantly spawning medaka (Oryzias latipes) were reared in fresh water containing 0.03% thiourea. The plasma thyroxine (T4) and triiodothyronine (T3) concentrations in the mother fish were about 8 and 5 ng/ml, respectively, before the treatment, and decreased to 1/5 of the initial levels within 1 day. Initial concentrations of T4 and T3 in eggs just after fertilization were about 4 and 1 ng/g, respectively, and decreased to less than 1/4 when the mother was treated with thiourea for 7 days. Minimum effective concentration of thiourea to decrease the hormone concentrations in mother's plasma and the eggs was 0.003%. During the course of normal development, egg thyroid hormones are maintained at the initial levels during the first 4 days, and continuously decrease toward hatching (10 days after fertilization). There was no difference between the normal eggs (derived from the mother fish without thiourea treatment) and hormone-deficient eggs (hormone concentrations were less than 1/10 of the control) in hatchability, the time of hatching, and survival under starvation. No difference was seen between the two groups in body weight, body length, condition factor, and survival rate 16 days after hatching. These findings indicate that more than 90% of the hormones contained in eggs are not essential for early development in medaka.