BLOOD-BRAIN-BARRIER OF A REPTILE, ANOLIS-CAROLINENSIS - FREEZE-FRACTURE STUDY

The blood-brain barrier of the central nervous system of the American chameleon, Anolis carolinensis, is based on the presence of 'tight' or occluding intercellular junctions between endothelial cells that line brain capillaries. These junctions are impermeable to systemically injected dyes or protein tracer molecules such as trypan blue and horseradish peroxidase (HRP). Platinum replicas of freeze-fractured lizard brain cortex show the inter-endothelial junctions to consist of a highly complex anastomosing network of intramembrane ridges and complementary grooves. The ridges are situated on PF fracture faces of endothelial cell plasma membrane and their mirror images, intramembrane grooves, are located on the EF fracture faces. The complex of ridges and grooves forms a continuous belt-like structure that circumscribes the apposed endothelial cells near the luminal surface of the endothelium. The zonula occludentes usually consists of 6-10 ridges and grooves arranged in tandem, parallel to the luminal surface of the cell, and, in fortuitous replicas, can be seen to extend for considerable distances without interruption. Based on the structural complexity as illustrated in this report, and on their impermeability to exogenous protein tracer, the junctions can be regarded as 'very tight'. They therefore constitute a highly impermeable seal between adjacent endothelial cells of brain capillaries. © 1979.