Cyto-differentiation and portal vascular development in the mouse adenohypophysis.

Pituitaries of fetal and postnatal (15 days p.c.-28 days p.n.) and adult (male) mice were studied by light and electron microscopy to correlate the developmental pattern of the hypothalamo-hypophysial vascular system with the time of onset of function of the adenohypophysis. The superior and anterior regions of the adenohypophysis become vascularized at 17 days p.c., when portal vessels extend from oral primary plexus to the pars distalis for the first time. Adenohypophysial vascularity and the number of portal vessels steadily increase to reach the adult pattern at 5 days p.n. At 1 day p.n. deep capillary loops appear in the caudal regions of the oral primary plexus; a capillary (tangential) plexus underlies the ependymal lining of the third ventricle by 6 days p.n. Superficial capillary loops were not observed until the third postnatal week. Granulation of secretory cells commences at 16 days p.c., predominantly in the upper and anterior adenohypophysis; at 17 days thyrotropes, gonadotropes and corticotropes are recognizable and by morphological criteria appear actively secretory on days 17-18 p.c., although few appear active at 19 days p.c. and 1 day p.n. Somatotropes are first seen at 18 days p.c., predominantly in the central and lateral regions of the pars distalis. Active secretory cells increase in number over the period 2-10 days p.n., but after 11 days p.n. thyrotropes and corticotropes seem to become progressively less active; fewer gonadotropes are seen after 15 days p.n., and these apparently become progressively less active from day 19. Most somatotropes appear active until 28 days p.n. The observations suggest that hypothalamic control of adenohypophysial function may exist in the mouse from 17 days p.c.