Permeability of the pineal gland of the viscacha (Lagostomus maximus maximus) to the lanthanum hydroxide.

Lanthanum hydroxide together with a fixative solution was injected into the third ventricle of the viscacha. The tracer penetrated freely into the pineal gland demonstrating an extensive network of intercommunicating intercellular spaces. Numerous gap junctions were observed indicating that they are the predominant interparenchymal contacts. The capillaries were characterized by non-fenestrated endothelial cells lining a moderate-sized perivascular space. Precipitates of lanthanum hydroxide were found in the basal membranes of the perivascular and interendothelial spaces, but not in the vascular lumina. The tracer stopped at tight junctions; these constitute a barrier between the pinealocytes and the blood compartment. Active micropinocytosis observed in the capillary walls suggest a transendothelial transport.