An electron microscope study of the differentiating capillaries of the mouse neurohypophysis.

The developing capillaries of the mouse neurohypophysis were studied in the electron microscope to elucidate the fine structural differentiation of the vascular component of the neuro-hemal contact zones in the external median eminence and the neural lobe. In the embryo the growth of the superficial net of the primary plexus of the hypophysial portal system is largely manifested by the presence of proliferation areas located within the capillary plexus covering the surface of the median eminence. Presumptive shallow capillary loops diverge from these ares in the first postnatal week. Differentiation of the capillary wall follows the pattern outlined for continuous capillaries. A few fenestrae appear in the endothelium of immature, superficial vessels at the 17th gestational day, increase in frequency during the following embryonic days, and occur regularly in the postnatal animal. In the neural lobe the internal capillaries proliferative by vascular sprouts emanating from the vessels on the surface of the gland. At the end of embryonic time an extensive net has developed, composed of capillaries with immature characteristics. Proliferation is largely finished by the end of the third postnatal week, when mature capillaries dominate the picture. Formation of attenuated, porous areas is a postnatal process, apart from single fenestrae appearing in the walls of a few immature capillaries in late fetal life. The structural possibilities for an onset of neurohypophysial function in the mouse is discussed.