The vascular architecture of the developing organum vasculosum of the lamina terminalis (OVLT) in the rat.

The development of the angioarchitecture of the organum vasculosum laminae terminalis (OVLT) is described at the light-microscopical level in connection with the formation of the brain ventricles in rat embryos (gestational day 12 through day 21) injected with India-ink via the umbilical vessels. Initial observations from electron-microscopic investigations (in progress) are also reported. The external surface of the brain vesicle is covered by a capillary network from the earliest stage examined (day 12), serving as the anlage for the vessels of the lamina terminalis and the prospective median eminence. A characteristic vascular feature of the OVLT can be observed first on day 16. At this time the OVLT lies nearly parallel to the cranial base under the preoptic recess in front of the optic chiasma. Vascular connections to the subfornical organ, the subependymal plexus and the retrochiasmatic area are present as early as days 17 and 18. The OVLT does not change its position substantially throughout embryonic life. Preliminary electron-microscopical investigations revealed that capillaries of the OVLT invading the neural tissue are fenestrated already before day 19.