Neurogenesis in the olfactory tubercle and islands of Calleja in the rat

Abstract

Neurogenesis in the rat olfactory tubercle and islands of Calleja was examined with [3H]thymidine autoradiography. Animals in the prenatal groups were the offspring of pregnant females given an injection of [3H]thymidine on two consecutive gestational days. Ten groups of embryos (E) were exposed to [3H]thymidine on E12–E13, E13–E14 4 E21–E22, respectively. Three groups of postnatal animals (P) were given four consecutive injections of [3H]thymidine on P0–P3, P2–P5, and P4–P7, respectively. 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. Three populations of neurons were studied: 1.(1) large cells in layer III2.(2) small to medium-sized cells in layers II and III, and in the striatal bridges3.(3) granule cells in the islands of Calleja. Neurogenesis is sequential between these three populations with No. 1 oldest and No. 3 youngest. The large neurons in layer III originate mainly between E13 and E16 in a strong lateral-to-medial gradient. Neurons in population No. 2 are generated between E15 and E20, also in a lateral-to-medial gradient; neurogenesis is simultaneous along the superficialdeep plane. Granule cells in the smaller islands of Calleja are generated between E17 and E22 in combined lateral-to-medial and superficial-to-deep gradients. Neurons in the large island of Calleja are generated mainly between E19 and E22 in a strong rostral-to-caudal gradient. Neurogenesis is reduced to < 10% in both populations No. 2 and No. 3, occurring mainly between P0 and P4. The neurogenetic patterns in populations No. 2 and No. 3 are similar to those in the striatum, while neurogenesis in population No. 1 fits into the pattern of the globus pallidus and substantia innominata. These developmental patterns indicate that the olfactory tubercle is a mixed striato-pallidal system rather than an olfactory cortical area. The lateral-to-medial neurogenetic gradient shown in each neuronal population in the olfactory tubercle correlates with both differential anatomical projections and differential neurochemical characteristics along the lateral-medial plane.