Development of the capillary system in the neurohypophysis of the rat.

The development of the vascular system in the neurohypophysis of the rat shows three periods of structural differentiation. The first period lasts until the 17th fetal day. During this period the superficial and the border plexuses are formed, and rapid vascularization of the neural lobe anlage takes place. The end of this period is marked by the formation of the perivascular space. The second period lasts from the 18th fetal day until the 10th day after birth. Its initial phase is characterized by the appearance of the first endothelial pores. During this period the formation of the internal capillaries proceeds, together with the maturation of the previously formed vascular elements. The third period lasts from the 10th postnatal day until the end of the first month after birth. At the end of this period the vascular net of the neural lobe has attained its fully developed state. The results of this study, together with data from previous investigations, lead to the conclusion that at the 18th fetal day the neural lobe displays signs of functional activity, and that the maturation of the vascular network in the neural lobe occurs more rapidly than the maturation of its other structural components.