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Light and electron microscopic observations on the development of the blood vascular system of the human brain.

The development of the blood vascular system of the human fetal brain was examined by both light and electron microscopy. By light microscopy the brains of human embryos and fetuses ranging in size from 8.5 to 70 mm crown-rump length have been studied in serial sections, usually of the whole embryo or fetus. In the smallest specimens the whole brain was encapsulated by a very dense vascular plexus. From this, perforating offshoots passed to or from the substance of the brain, while other connexions were effected with neighbouring arterial and venous channels. These latter vessels were never more mature than capillaries, and their status only recognizable by their location and ultimately by their connexions with the heart. The neopallial part of the cerebral hemisphere was later than all other parts of the brain in receiving vessels perforating its substances. There is some evidence, however, that the cellular basis of a blood vascular supply was present in this part of the brain before lumina and blood cells appeared. The cerebral cortex of fetuses ranging from 50 to 100 mm crown-rump length was examined. Both the extrinsic and intrinsic vessels of the cortex were never more mature than capillaries; occasionally a capillary was seen to penetrate the cortex from the surrounding pial investment. Within the developing cerebral cortex blind ending solid endothelial sprouts were identified, as well as 'seamless' capillaries.