Maturation of the hypothalamo-adenohypophyseal-gonadal (HAG) axes in the chick embryo.

In the chick embryo the components of the hypothalamo-adenohypophyseal-gonadal (HAG) axes initially function independently. On day 13 of development in the male embryo, luteinizing hormone (LH) from the adenohypophysis initiates regulation of testosterone (T) synthesis and secretion by the testes, and as a consequence plasma T levels reach a maximum value on day 13.5. The establishment of this feed-forward phase of the pituitary-testicular axis involves an increase in the numerical density (NV) of testicular LH receptor-positive interstitial cells (IC) as a result of elevated plasma LH levels ("up regulation"). The elevated plasma T levels on day 13.5 cause plasma LH concentrations to decrease immediately after day 13.5, initiating the feedback phase of the endocrine axis. The NV of testicular LH receptor-positive IC also decreases at the same time, continuing to decline for the remainder of the embryonic period as LH receptor complexes are internalized and plasma T levels decrease. The decrease in the NV of testicular LH receptor-positive IC and the internalization of the LH receptor complexes reflect a "desensitization" of the IC, in which there is a decrease in the ability of LH to stimulate T synthesis and secretion by the IC ("down regulation"). In the male chick embryo the hypothalamus begins to regulate the adenohypophyseal-testicular unit during the last third of embryonic development. There appears to be a series of similar, albeit not as well verified, events that occur in the female embryo with respect to the development of the adenohypophyseal-ovarian axis.