Sequential appearance of anionic domains in the developing blood-brain barrier.

The distribution of anionic sites in the walls of mouse brain micro-blood vessels (MBVs) during development and maturation of the blood-brain barrier (BBB) was studied by electron microscopy. Cationic colloidal gold (CCG) and Lowicryl K4M-embedded brain samples obtained from mouse fetuses (13th and 19th days) and from 1-, 5-, 12- and 24-day-old and adult mice were used. The labeling of anionic sites with CCG was more intense on the abluminal than on the luminal front of the endothelial cells (ECs) in fetuses and in newborn mice. Only a few anionic sites appear on the luminal front of the ECs of proliferating blood vessels invading the neural tissue in 13-day-old fetuses. They become slightly, although steadily, more abundant during further stages of development, and their number rapidly increases between the 12th and 24th day of life at which time they attain the density typical for mature animals. The maturation of the basement membrane (BM), which occurs during the myelinization period (12th-24th day of life), also coincides with an increasing concentration of anionic sites. These observations suggest that the gradual appearance of anionic sites on both fronts of the endothelium, as well as in the developing and maturing BM, represents one of the mechanisms responsible for differentiation of cerebral microvasculature into BBB-type MBVs.