Changes in immunoreactive FSH and inhibin in developing chicken embryos and the effects of estradiol and the aromatase inhibitor R76713.
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The interrelationship between immunoreactive inhibin and FSH was measured in chicken embryos from Day 8 to Day 20 of the incubation period. Plasma inhibin was measured by a heterologous RIA and FSH by a recently developed homologous RIA. Male and female inhibin plasma concentrations demonstrated a similar time course: a sharp increase with a peak on Day 13 was followed by a drop towards Day 18, after which inhibin levels stabilized. In male embryos, a similar time course was observed for the FSH concentrations. In sharp contrast, the FSH concentrations in the female embryos remained low and unchanged throughout the incubation period. Accordingly, sex differences were large for FSH and rather small for inhibin, and were noted from Day 10 and Day 12 on, respectively. No obvious inverse relationship was observed between inhibin and FSH. In male embryos, estradiol administration resulted in a dose-related decrease in FSH, accompanied by a less pronounced decrease in inhibin. Treatment with an aromatase inhibitor (R76713) increased FSH in female embryos and inhibin in both sexes. Generally, the experimentally induced changes in FSH were accompanied by comparatively small changes in inhibin. In conclusion, these observations suggest 1) that estradiol is an important inhibitor of pituitary FSH secretion in the chicken embryo, 2) that inhibin is not likely to play a major role in the feedback regulation of the hypothalamo-pituitary-gonadal axis, and 3) that an important fraction of the measured inhibin immunoactivity escapes the control of FSH. In view of the present data, the hypothesis is advanced that the gonads are not the major source of circulating immunoreactive inhibin in the chicken embryo.