Development of the Diencephalon in the Rat

The development of the nuclei of the hypothalamus was examined in normal and X-irradiated embryos from day 13 (El31 to the day before birth (E22). The diencephalic neuroepithelium was subdivided into three lobes (dorsal, medial, and ventral) and two lobules (superior and inferior). The hypothalamus is derived from the ventral lobe and the inferior lobule. The ventral neuroepithelial lobe generates the neurons of most of the early arising hypothalamic structures, including those of the lateral tier nuclei associated with the medial forebrain bundle, and the heterogeneous intermediate tier nuclei. A specialized neuroepithelial region lining the diamond shaped ventricle produces the early neurohypophysial magnocellular neurons; the neurons of the paraventricular nucleus remain a t this site, whereas the neurons of the supraoptic nucleus could be traced migrating laterally. The neurons of the late arising hypophysiotropic area of the posterior hypothalamus are derived from components of the inferior neuroepithelial lobule: the dorsomedial and ventromedial nuclei apparently from a shared matrix in the main portion of the inferior lobule; the tuberomammillary-arcuate complex from its posteroventral recess. The triple-decked and sequentially produced components of the mammillary system may arise from separate neuroepithelial sites. The autoradiographic results of the previous study (Altman and Bayer, '78a) showed that the structural and functional heterogeneity of the mature hypothalamus is paralleled by cytogenetic heterochronicity; the present embryonic observations indicate that many of the distinguishable components of the hypothalamus arise from a mosaic of heterogeneous neuroepithelial sites. Since the pioneering studies of His (1893, '04) the development of the diencephalon has been investigated by many workers, for its intrinsic interest and as an aid for the rational parcellation of this complex region of the brain. His claimed that he could distinguish between alar and basal plates in the diencephalon, and proposed that the hypothalamus is derived from the basal plate, the rest of the diencephalon from the alar plate. Herrick ('10) subdivided the diencephalon into four horizontal zones (metathalamus, dorsal thalamus, subthalamus and hypothalamus) on the basis of the presence of three sulci in the neuroepithelium of the third ventricle of amphibian embryos, the sulcus dorsalis, medius and ventralis. Herrick's schema was widely acJ. COMP. NEUR. (1978) 182: 973-994. cepted because i t appears applicable to the developmental pattern of the diencephalon in practically all vertebrates, including man (Gilbert, '35; Cooper, '50; Kuhlenbeck, '54; Kahle, '56; Coggeshall, '64; Christ, '69). The hypothalamus has been subdivided on the basis of adult cytoarchitecture and connections (Nauta and Haymaker, '69) into three sagittal zones, the lateral, medial and periventricular. The lateral zone includes the lateral preoptic and hypothalamic areas that are traversed by the medial forebrain bundle; the medial zone includes the medial preoptic, anterior hypothalamic, ventromedial, dorsomedial, and premammillary nuclei; the periventricular zone consists of several nuclei bordering the third ventricle, stretching from the