Histogenesis at the level of the basal forebrain: the entopeduncular nucleus.

This study shows that the neurons of the entopeduncular nucleus are derived from a longitudinal slab of isochronically generated neurons on day 11 and 15 h of gestation. Many neurons of this longitudinal slab which we have named the basal forebrain cell column, originate from an ependymal matrix closely associated with the ventral diencephalic sulcus and later become associated with the basal forebrain bundle. Other neurons also originate from the ependymal matrix at the site of emergence of the optic recess and keep close relationships with the optic chiasma through the following stages of development to form the retrochiasmatic nucleus. During the second half of day 12 of gestation, the mantle layer of the forebrain shows an early zone of differentiation along its ventrolateral aspect. At this stage, the basal forebrain cell column extends unbroken from the tuberculum posterius to the chiasmatic plate primordium (site of generation of the retrochiasmatic nucleus). At the level of the caudal aspect of the optic stalk however, the basal forebrain cell column divides in two limbs associated to the ventral and dorsal edges of the optic stalk as it emerges from the forebrain. On day 14 of gestation, the neurons of the dorsal limb of the basal forebrain cell column occupy the mantle layer of the neural tube at least as far rostrally as the ventricular elevation in front of the optic stalk in the floor of the foramen of Monro. The neurons derived from the basal forebrain cell column begin breaking up into a series of more definite nuclei at later stages of development. The main finding of this study is the disclosure of the fact that the entopeduncular nucleus as well as other cell groups as dissimilar as the lateral preoptic area, the central, medial and anterior cortical amygdaloid complex and neurons of the dorsal hypothalamic area appear to be embryologically related, as they are all derived from a common longitudinal slab of the matrix of the forebrain.