Neurogenesis in the rat neostriatum.

Neurogenesis in the rat neostriatum was examined with [(3)H]thymidine autoradiography. For the animals in the prenatal groups, the initial [(3)H]thymidine exposures were separated by 24 h; they were the offspring of pregnant females given two injections on consecutive embryonic (E) days (E13-E14, E14-E15, … E21-E22). For the animals in the postnatal (P) groups, the initial [(3)H]thymidine exposures were separated by 48 h, each group receiving four consecutive injections (P0-P3, P2-P5, P4-P7). On P60, the percentage of labeled cells and the proportion of cells originating during either 24 or 48 h periods were quantified at several anatomical levels for both the large and medium-sized neurons. Neurogenesis of the large neurons occurs mainly between E13 and E16 in a strong caudal-to-rostral gradient. The medium-sized neurons throughout the neostriatum are generated in a prominent ventrolateral-to-dorsomedial gradient so that ventrolateral cells originate mainly between E14 and E18, dorsomedial cells between E18 and E21-22 (fewer than 10% originate between P0 and P4). Medium-sized neurons also show two other gradients. First, there is a superficial-to-deep gradient in the anterior part of the caudoputamen, while more posterior levels have a deep-to-superficial gradient. Second, anterior parts have a caudal-to-rostral gradient while posterior parts have a gradient in the opposite direction. This shift in neurogenetic gradients along both superficial-deep and rostrocaudal directions is developmental evidence that an anterior 'caudate' can be separated from a posterior 'putamen' in the rat. Finally, neurogenetic gradients in the medium-sized caudoputamen neurons can be linked to the patterns of their anatomical interconnections with the substantia nigra.