DIFFERENTIATION OF HYPOTHALAMIC NUCLEI DURING ONTOGENETIC DEVELOPMENT IN RAT

The ontogenetic development of the hypothalamic nuclei of the rat was examined from the 16th post-coital day to adult age in both sexes, which were determined separately. The following conclusions have been drawn: 1. The hypothalamus is visible already before the 16th day of gestation. 2. The walls of the third ventricle are first composed of primitive cell layers, from which the migration of neurons proceeds on the 15th or 16th day. At this stage the differentiation of the wall of the third ventricle occurs: the germinal-, mantle-and marginal layers appear. 3. The differentiation of the nuclei starts, with some exceptions, before the 19th day of gestation. 4. Nn. suprachiasmaticus, supraopticus, periventricularis anterior, arcuatus and one part of paraventricularis differentiate from the lateral border of the germinal layer. Nn. ventromedialis, dorsomedialis, hypothalamicus anterior, praeoptici, praemamillaris ventralis, praemamillaris dorsalis, mamillaris medialis, mamillaris lateralis and the greater part of paraventricullaris differentiate from the mantle layer. Lateral nuclei, of which only nucleus tuberomamillarius has been described, differentiate from the marginal layer. 5. Some nuclei which belong to the hypothalamo-hypophyseal area have a peak in their differentiation during the critical period when the function of the hypothalamo-hypophyseal axis also starts. 6. It is stated that the development of the nuclei in the rat hypothalamus compared to that of the other mammals is similar only to that of the mouse. It takes place quite slowly before and after birth. 7. Sexual differences are not to be seen in the development of the nuclei. © 1969 Springer-Verlag.