ONTOGENIC DEVELOPMENT OF DIFFUSIONAL RESTRICTION TO PROTEIN AT THE PIAL SURFACE OF THE RAT-BRAIN - AN ELECTRON-MICROSCOPIC STUDY

Blood-brain, blood-CSF and ventricular CSF-brain barriers to protein, are present very early in brain development. In order to determine whe ther the outer pial surface of the brain also restricts free penetrati on of macromolecules, the dorso-lateral part of the sensorimotor corte x from rats at embryonic day 12 (E12), 14, 16, and 18, the day of birt h (P0), and adult rat, was studied by electron microscopical technique s. Potassium ferrocyanide, Ruthenium Red and immunogold labelling of e ndogenous albumin were used to investigate junctional structures and t he sites of restriction to albumin diffusion. At E12, large fenestrate d sinusoids were present in the pia-arachnoid and the brain surface wa s formed by an incomplete layer of neuroepithelial and presumptive rad ial glial end feet, but capillaries in the pia-arachnoid showed no fen estrations at E14 or later. From E14, we observed the progressive appe arance of distinct junctional structures between the glial end feet wh ich, to our knowledge, have not been described before. Analysis of alb umin distribution from E16 to P0 suggests that the junctions may contr ibute to restriction of diffusion between the subarachnoid space and t he brain extracellular fluid. The restriction to the penetration of pr otein at both the pial and the ependymal surfaces may ensure the isola tion of the neural environment during a critical phase in development of the nervous system. The changes in the structure of the junctions b etween E12 and P0 suggests a transitional series of embryonic junction al types, which eventually give way to the mature junctions of the adu lt. Parallels between the embryonic glial junctions and junctions desc ribed in adult invertebrate brain, suggest some interesting parallels in junctional development in phylogeny and ontogeny.