Ultrastructure of frog cerebral and pial microvessels and their impermeability to lanthanum ions.

Permeabilities of single pial microvessels in the frog are now being estimated. Fine structural information on these vessels will therefore be needed in attempts to correlate structure and permeability. Microvessels from the dorsal part of the telencephalon and from the overlying pial layer in the common frog (Rana temporaria) and their permeability to lanthanum ions were studied in the electron microscope. Extensive structural similarities between the endothelial lining of cerebral and pial microvessels were demonstrated, indicating similarity in functional characteristics. Adjacent endothelial cells in both categories of vessels were joined by elaborate tight junctions. Lanthanum ions did not permeate the endothelium. Diffusion of the ion in the interendothelial clefts was stopped by the junctions. It is concluded that the frog has an endothelial blood-brain barrier which is tight to lanthanum ions.