The development of the septal region in the rat. I. Neurogenesis examined with 3H-thymidine autoradiography.

Neurogenesis in the rat septal region was examined with 3H-thymidine autoradiography. The rats in the prenatal groups were the offspring of pregnant females given two injections of 3H-thymidine on consecutive days in an overlapping series: embryonic day (E) 13 + E14, E14 + E15,. . . E21 + E22. The rats in the postnatal groups were injected in a nonoverlapping series: the day of birth and postnatal day (P) 1, P2 + P3, P3 + P4. On 60 days of age, the percentage of labelled cells and the proportion of cells added during each day of formation were determined at several anatomical levels within the midline nuclear group (nucleus of the diagonal band, medial and triangular septal nuclei), the lateral septal nucleus, and the ventrolateral nuclear group (nucleus accumbens, bed nuclei of the stria terminalis and the anterior commissure). The neurons within each nuclear group form in significantly different waves, those of the midline group forming between E13-E17, the lateral septal nucleus between E15-E19, the bed nuclei of the stria terminalis and anterior commissure between E14-E18, the nucleus accumbens between E17-P2. All nuclei and nuclear groups show characteristic gradients of formation. Both the midline nuclear group and the bed nucleus of the stria terminalis (including the commissural bed nucleus) have their earliest forming neurons lying near the crossing of the anterior commissure; younger nuerons are located both rostrally and caudally with the youngest neurons lying in the most rostral extension of the diagonal band nucleus and the strial bed nucleus. The lateral septal nucleus forms along a strong mediolateral gradient throughout its length after neurogenesis is almost complete in the midline nuclear group. Throughout the length of the nucleus accumbens, the oldest neurons are located ventrally while progressively younger cells are found dorsally beneath the inferior horn of the lateral ventricle.