Neurogenetic patterns in the medial limbic cortex of the rat related to anatomical connections with the thalamus and striatum.

[3H]Thymidine autoradiography was used to investigate neurogenesis in all areas of the limbic cortex in the medial wall of the hemisphere. The experimental animals were the offspring of pregnant females injected with [3H]thymidine on 2 consecutive days: Embryonic Day (E)13-E14, E14-E15, ...E21-E22, respectively. On Postnatal Day (P)60, the proportion of neurons originating during 24-h periods were quantified at nine anteroposterior levels. Three types of neurogenetic gradients are found. (i) Deep cells are older than superficial cells: layer VI is generated mainly on E15-E16, layer V on E16-E18, and layers IV-II on E18-E20. (ii) There is a ventral/older to dorsal/younger gradient between the dorsal peduncular, infralimbic, and anterior cingulate areas rostral to the genu of the corpus callosum. A ventral/older to dorsal/younger gradient is also found between superficial cells (layers II-IV) in anterior cingulate (CG3/CG1), posterior cingulate (CG2/CG1), and retrosplenial areas (RSG/RSA). (iii) An anterior/older to posterior/younger gradient is found between areas throughout the medial limbic cortex. Some of these neurogenetic patterns correlate with anatomical interconnections between the supracallosal medial limbic cortex (posterior cingulate and retrosplenial areas) and the anteroventral/anteromedial thalamic nuclei: older thalamic cells have longer axons that terminate in cortical areas containing younger cells, while younger thalamic cells have shorter axons that terminate in cortical areas containing older cells. Projections from the medial limbic cortex to the striatum also correlate with neurogenetic gradients: older cortical source cells in the infralimbic area project to the older striatal cells in the enkephalin-rich patches, while younger cortical source cells in the cingulate areas project to younger striatal cells in the surrounding matrix.