Telencephalic microvessels of premature beagle pups.

The ultrastructural morphologic and morphometric characteristics of the telencephalic microvasculature of both germinal matrix and border-zone cerebral cortex are analyzed from premature beagle puppies delivered by caesarean section in order to determine whether differences in the matrical vessels could account for their susceptibility to intraventricular hemorrhage. The germinal matrix microvessels are immature, since they contain intraluminal microvilli and junctional complexes (primarily tight junctions) that are not continuous between the lateral endothelial cell walls. The number of junctional complexes per vessel cross-section is less than either cortical vessels from the same premature animals or tissue from either site in full-term animals. Morphometric data on the cross-sectional area of vessels from the two sites indicate that the germinal matrix vessels have a significantly larger cross-sectional area than the cortical vessels. The increase is due almost exclusively to a larger endothelial cell area per cross section. This pattern is an altogether different one from that seen in the term animals. There the germinal matrix vessels exhibit a steadily decreasing luminal area from birth through 72 hr, whereas endothelial cell cross-sectional area is static. Here the luminal cross-sectional area is smaller, but both endothelial cell and total vessel cross-sectional areas are significantly larger. Structural components of the blood-brain barrier undergo active modifications during the late prenatal period that are distinct from the modifications during the immediate postnatal period.