Histogenesis of the substantia nigra, ventral tegmental area of Tsai and interpeduncular nucleus: An autoradiographic study of the mesencephalon in the rat

Abstract

Abstract The origin of large neurons of the substantia nigra, ventral tegmental area of Tsai and interpeduncular nucleus was determined in three‐week‐old rats receiving a single injection of thymidine‐H 3 during gestation. These neurons underwent final mitotic division on embryonic days 11 through 15 with maximal production on days 14 and 15. Glia and the smallest type of granular neurons were produced from day 11 through 22. Cells in the ventral angle of the neuroepithelium were not labeled one hour after injection of thymidine‐H 3 in any of the series. Study of animals receiving a single injection of thymidine‐H 3 during gestation and killed at serial intervals thereafter showed that neurons forming the substantia nigra migrated from the middle third of the basal neuroepithelium and moved between existing cells in radial patterns. Production of neurons specifically for each division of the substantia nigra was not observed either in cross‐section or rostrocaudally, nor was the destination of neurons related to their time of origin. The ventral tegmental area of Tsai and interpeduncular nucleus formed from neurons originating in the medial third of the basal plate and migrating almost to the ventral surface of the mesencephalon. The neurons then divided into two streams to create a pattern of an inverted fountain comparable to the distribution of neurons containing catecholamines. Some of these neurons may have contributed to the substantia nigra. The primordia in all three nuclear areas were first seen on embryonic day 18.