CHANGES IN IMMUNOREACTIVE FSH AND INHIBIN IN DEVELOPING CHICKEN EMBRYOS AND THE EFFECTS OF ESTRADIOL AND THE AROMATASE INHIBITOR R76713

The interrelationship between immunoreactive inhibin and FSH was measu red 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 recentl y developed homologous RIA. Male and female inhibin plasma concentrati ons demonstrated a similar time course: a sharp increase with a peak o n Day 13 was followed by a drop towards Day 18, after which inhibin le vels stabilized. In male embryos, a similar time course was observed f or the FSH concentrations. In sharp contrast, the FSH concentrations i n the female embryos remained low and unchanged throughout the incubat ion period. Accordingly, sex differences were large for FSH and rather small for inhibin, and were noted from Day 10 and Day 12 on, respecti vely. No obvious inverse relationship was observed between inhibin and FSH. In male embryos, estradiol administration resulted in a dose-rel ated decrease in FSH, accompanied by a less pronounced decrease in inh ibin. Treatment with an aromatase inhibitor (R76713) increased FSH in female embryos and inhibin in both sexes. Generally, the experimentall y induced changes in FSH were accompanied by comparatively small chang es in inhibin. In conclusion, these observations suggest 1) that estra diol is an important inhibitor of pituitary FSH secretion in the chick en embryo, 2) that inhibin is not likely to play a major role in the f eedback regulation of the hypothalamo-pituitary-gonadal axis, and 3) t hat an important fraction of the measured inhibin immunoactivity escap es the control of FSH. In view of the present data, the hypothesis is advanced that the gonads are not the major source of circulating immun oreactive inhibin in the chicken embryo.