Islands of Calleja in the Rat

Abs t rae t -Neurogenes i s 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 E I2 -E I3 , E13-EI4 . . . . E21-E22, respectively. Three groups of postnatal animals (P) were given four consecutive injections of [3H]thymidine on P0-P3, P2-P5, and P4P7, 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) large cells in layer Ill, (2) small to medium-sized cells in layers 11 and Ill, and in the striatal bridges, (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 Ill originate mainly between El3 and E l6 in a strong lateral-to-medial gradient. Neurons in population No. 2 are generated between E l5 and E20, also in a lateral-to-medial gradient; neurogenesis is s imultaneous 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 arc generated mainly between E l9 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 str iatum, while neurogenesis in population No. I 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 neurogcnctic 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.