Postnatal mitotic division of endothelial cells in the brain

Ultrastructural details of capillary endothelial cells undergoing mitosis in the central nervous system are reported. These mitotic endothelial cells were seen in the brain of 10-day-old mice treated with 6-aminonicotinamide, an antagonist of niacin. Electron microscopy revealed these cells to be characterized by segregated chromosomal materials and the absence of a nuclear membrane. They possessed well-formed tight junctions with adjacent endothelial cells. Numerous microtubules radiated from a centriole toward the chromosomal material. The basal lamina of these dividing endothelial cells was not always well defined and, in some portions was discontinuous. We believe this to be the first ultrastructural demonstration of mitotic endothelial cells in the postnatal brain.