The microcirculation of the area postrema in the rat

The validity of current theories concerning the function of the area postrema ultimately rests on the nature of its microcirculation. Therefore a detailed study of the vasculature of the postremal region was undertaken in the rat. The findings indicate a singular arteriolar supply from branches of the posterior inferior cerebellar arteries. These vessels are caudal, intracranial branches of the vertebral arteries. Networks of enlarged capillaries are prominent features of sections taken through the area postrema. These enlarged capillary channels are re‐entrant. At the borders of the structure they are joined by short interconnecting vessels to capillaries of smaller caliber typical of the medullary tissue. The bed is considered to be a portal system because two distinctive, serially connected capillary beds are interposed between artery and vein. No arteriovenous shunts or thoroughfare channels are observed in the rat, contrary to reports of this type of vessel in other species. This microcirculatory pattern would seem to be adequate to current theories of postremal function, which attribute neurosecretory or chemosensory functions to this region.