Neurogenesis of the magnocellular basal telencephalic nuclei in the rat

Neurogenesis in the magnocellular basal telencephalic nuclei of the rat was examined with [(3)H]thymidine autoradiography. The experimental animals were the offspring of pregnant females given two injections of [(3)H]thymidine on consecutive embryonic (E) days (E12-E13, E13-E14, … E21-E22). On postnatal day (P) 60, the percentage of labeled cells and the proportion of cells originating during 24 h periods were quantified at several anatomical levels throughout the magnocellular basal telencephalic nuclei. The neurons of the horizontal limb of the diagonal band originate mainly between E13 and E16 in a combined rostral-to-caudal and lateral-to-medial gradient. The neurogenetic gradients in the horizontal limb are continued by generation patterns of cells in the vertical limb of the diagonal band-medial septal complex, the large cells in the polymorph layer of the olfactory tubercle, and the large cells of the anterior amygdaloid area. The substantia innominata originates between E13 and E17 in combined caudal-to-rostral and lateral-to-medial gradients. The globus pallidus originates between E13 and E17 in combined caudal-to-rostral, ventral-to-dorsal and medial-to-lateral gradients. The entopeduncular nucleus originates between E12 and E14 in a 'sandwich' gradient where neurons in the core of the nucleus are older than those in either the anterior or posterior ends. There is an overall superficial (ventral) to deep (dorsal) neurogenetic gradient between the magnocellular basal nuclei present at any given rostrocaudal level. An important finding is that neurogenetic gradients in the individual components of the magnocellular basal nuclei are alike (with the possible exception of the entopeduncular nucleus) indicating they are part of a single system. Finally, evidence is presented that neurogenetic gradients in the magnocellular basal telencephalic neurons can be correlated with their anatomical projections to the cerebral cortex.