Rete mirabile medullae: vascular specialization of the avian brain stem.

A circumscribed vascular plexus is located between the conjoint proximal ganglion of cranial nerves IX-X and the lateral surface of the avian brain stem. Gross, light, and electron microscopic analyses reveal that this structure is a rete mirabile. This rete is unique in that it resides in the subarachnoid space where it is bathed by cerebrospinal fluid. Microscopic and ultrastructural studies document that the rete is composed of arterioles with wide luminal diameters, marginal folds project from the endothelial cells into the lumen of the blood vessel. The endothelial cells contain numerous pinocytotic vesicles. The underlying basal lamina is penetrated by myoendothelial junctions. One to three layers of smooth muscle cells comprise the tunica media. Bundles of unmyelinated nerve fibers course through the loose connective tissue encompassing the blood vessels.