A sensitive bioassay for inhibin based on the suppression of FSH release from cultured sheep anterior pituitary cells was used to determine whether inhibin is present in the preovulatory follicle in the domestic hen. Granulosa and thecal/stromal layers were separated from the five largest (F1-F5) yellow yolky follicles in the ovary and incubated in culture medium for 18 h. Inhibin was found predominantly in the media in which granulosa layers had been incubated. There was a progressive increase in the amount of inhibin produced per mg granulosa layer protein during the 5-6 days before ovulation. The ovary was observed to contain a growth factor which stimulated the proliferation of ovine pituitary cells. Thecal/stromal layer-conditioned medium (ThCM) but not granulosa layer-conditioned medium had a dose- and time-dependent mitogenic effect on cultured sheep pituitary cells. The maximal mitogenic effect achieved for ThCM was four to fivefold greater than control media and was significantly higher than the maximal mitogenic effects of epidermal growth factor (250 ng/ml; 1.5 x control) and transforming growth factor-beta (500 ng/ml; 1.2 x control). It is concluded that inhibin is produced by the granulosa layers in the large yellow yolky preovulatory ovarian follicles of the domestic hen. The thecal/stromal layers in these follicles produce a potent mitogenic factor, not produced by the granulosa layers, which stimulates the division of ovine anterior pituitary cells in vitro.
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