Neurogenesis in the rat primary olfactory cortex

Neurogenesis in the rat olfactory peduncle 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 gestation days. Nine groups of embryos were exposed to 3H]thymidine on embryonic days (E) E13-E14, E14-E15,...E21-E22, respectively. One group of postnatal animals was given four consecutive injections of 3H]thymidine on postnatal days (P) P0-P3. On P60, the percentage of labeled cells and the proportion of cells originating during either 24 or 48 hr periods were quantified at seven anatomical levels through both the anterior olfactory nucleus and the transition areas. A caudal (older) to rostral (younger) neurogenetic gradient is found both within and between structures in the olfactory peduncle. Neurons in the dorsal, lateral, and ventral-lateral transition areas are generated mainly between E14 and E19, those in the anterior olfactory nucleus mainly between E15 and E21. Only 3-4% of the neurons in the most anterior pars lateralis and pars dorsalis originate after birth. All parts of the anterior olfactory nucleus show a strong superficial (older) to deep (younger) neurogenetic gradient (the 'outside-in' pattern). In contrast, neurons in the ventral-lateral transition area and in the dorsal transition area originate in a deep to superficial neurogenetic gradient (the 'inside-out' pattern), suggesting that these areas are, in reality, primary olfactory cortex. The lateral transition area is truly 'transitional', showing no neurogenetic gradient along the superficial-deep plane. The medial transition area originates between E15 and E19 in a center (older) to edge (younger) 'sandwich' neurogenetic gradient along the rostrocaudal plane, a pattern apparently unrelated to neurogenetic gradients in other olfactory peduncle structures. These data suggest that characteristic patterns of neurogenesis, namely the 'inside-out' vs the 'outside-in' gradients, permit the assignment of different structures to cortical vs ganglionic cytoarchitectonic components of the olfactory relay system.