Initial development of capillaries in the neuroepithelium of the mouse.

The developmental stage of the mouse embryo at which capillaries first occurred and their localisation in the neuroepithelium were investigated on serial sections of 9 and 10 days old embryos embedded in paraffin and Epon. In addition, areas of the neuroepithelium in which capillaries had been observed at the light microscopical level were investigated by electron microscopy carried out on embryos fixed with glutaraldehyde supplemented with tannic acid. In 5-7 microns serial sections of paraffin-embedded embryos, capillaries were initially seen in the CNS anlage at Theiler's Stage 14 (1972). At this stage, capillaries also occurred in the prosencephalon, the rhombencephalon and in the developing spinal cord. In 1 micron serial sections of resin-embedded embryos, capillaries could be identified in the neuroepithelium one stage earlier, i.e. at Stage 13. These very early capillaries were seen in the dorsal part of the lateral wall of the prosencephalon which later forms the diencephalon. At the ultrastructural level, those areas of the neuroepithelium in which leptomeningeal capillaries first started to spread into the neuroepithelium were characterised by the disintegration of the basement membrane of the neuroepithelium and the appearance of tannic acid-positive extracellular structures between the neuroepithelium and the adjacent leptomeningeal capillaries.