The development of the septal region in the rat. II. Morphogenesis in normal and X‐irradiated embryos

Morphogenesis of the septal region was examined in normal rat embryos from embryonic day (El 10 to E22. The greater part of the septal re- gion is postulated to form from two separate anlagen which can be clearly dis- tinguished in the telencephalon by El3 and E14. One lies in the anterior ven- tromedial wall and presumably forms the nucleus of the diagonal band, medial, lateral, and triangular septal nuclei. The other lies in the posterior ven- trolateral ridge and presumably forms the bed nuclei of the stria terminalis and the anterior commissure. On E15, the early differentiating cells in these anlagen fuse in the same region where the anterior commissure will cross on E17. With later embryonic development, differentiating cells of the stria1 bed nucleus accumulate rostra1 and caudal to the fused area. The same pattern is found in the medial and triangular septal nuclei and in the nucleus of the diagonal band. The differentiating cells of the lateral septal nucleus accumu- late dorsal and lateral to the medial and triangular septal nuclei. On El6 and E17, a prominent subependymal zone develops in the anterior septal region and presumably gives rise to the nucleus accumbens. A quantitative analysis was made of three cell zones (neuroepithelium, sub- ependymal zone, differentiating cell zone) at coronal levels through the de- veloping nucleus accumbens and the nucleus of the diagonal band (anterior level) and the medial and lateral septal nuclei (middle and posterior levels). At all levels, the area of the neuroepithelium continually declines, that of the dif- ferentiating cell zone continually increases, and that of the subependymal zone shows a rise and decline. On a proportional basis, both the neuroepithelium and subependymal zone occupy significantly more area anteriorly than posteriorly, while the differentiating cell zone shows the reverse gradient. To accurately locate regions of primitive mitotic and migratory cells within the zones at each level, the number of cells surviving a single exposure to 200 R X-rays in embry- onic brains (E15-E22) were compared with controls. Each zone responded differently to X-ray insult. The radiosensitivity of the neuroepithelium de- creases significantly after E19; the subependymal zone is highly radiosensitive throughout; the differentiating cell zone is radioresistant throughout. The sig- nificance of these findings is discussed in the light of the autoradiographic de- termination of the time of formation of septal neurons (Bayer, '79). Little is known about the embryonic devel- opment of the septal region. It has been only briefly described in a few earlier studies of the developing telencephalon in several verte- brates (Johnston, '13, '23) and in man (Hines, '22; Macchi, '51). This may be because neuro- embryological studies are difficult to inter- pret when they are based on descriptions of normal material alone; cell groups accumulat- ing within the embryonic brain often bear lit- tle resemblance to their adult appearance and location. Since a given nuclear group cannot be seen until after neuron formation has sub- stantially begun and the young neuroblasts leave the germinal zone to differentiate, knowing the time when neuronal populations