Ontogeny of hypothalamo-adenohypophyseal-gonadal (HAG) interrelationships in the chick embryo

In the male chick embryo the components of the hypothalamo-adenohypophyseal-testicular axis initially function independently of each other. It is not until days 12.5-13.5 that the adenohypophysis begins to regulate testosterone synthesis and secretion; on day 13.5 plasma testosterone reaches a maximum embryonic level. This feed forward regulation of the pituitary-testicular unit appears to involve a cause and effect relation between a statistically significant increase in the number of testicular interstitial cell LH receptors on days 12.5 and 13.0 and an increase in plasma LH levels on day 13.5 (up-regulation). Subsequently, events occur that are interpreted as indicative of the feedback phase of this endocrine axis. Plasma LH levels decrease after day 13.5. Also on day 13.5 and all subsequent embryonic days, there is a significant decline in the volume density of testicular LH receptor-positive interstitial cells (IC) associated with an internalization of the LH receptor complexes and a marked decline in plasma testosterone levels (down-regulation). It is strongly suggested that the decline in the number of LH receptor-positive ICs and the internalization of the LH receptor complexes is indicative of a "desensitization" of the ICs followed by a decrease in testosterone synthesis and secretion. Comparable events that occur in the female embryo with respect to the development of the hypothalamo-adenohypophyseal-ovarian axis are also discussed in this minireview.