Neurogenesis in the rat neostriatum

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Abstract
Neurogenesis in the rat neostriatum was examined with [3H]thymidine autoradiography. For the animals in the prenatal groups, the initial [3H]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 [3H]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.