The diencephalon of the vervet monkey (Cercopithecus aethiops). Part II: epithalamus, subthalamus and hypothalamus.

The nuclear configuration and topography of the epithalamus, subthalamus and hypothalamus of the vervet monkey (Cercopithecus aethiops) are described and compared with those of other primates, particularly the macaque monkey. The epithalamus does not show any striking structural differences, except some architectonic differentiation in the lateral habenular nucleus. The subthalamus is a phylogenetically stable structure throughout the primate scale; it does not show any significant changes, except that it extends less rostrally and that the nuclei entopeduncularis and peripeduncularis are much smaller and less well defined in the vervet monkey than those in the diencephalon of lower primates. The nucleus subthalamicus and the fields of Forel, though small in size, are comparatively well developed; the zona incerta appears to be differentiated cytoarchitectonically into two parts. The hypothalamus is divided morphologically into four regions--the preoptic, supraoptic, infundibular and mamillary regions. Although the hypothalamus of the vervet monkey is topographically identifiable with those of other primates, there are cyto- and myeloarchitectonic differences to be found in certain hypothalamic nuclei and areas. The preoptic region is small and poorly delimited from the parolfactory region antierorly and the supraoptic region posteriorly. The nucleus paraventricularis is large and well differentiated into secretory and non-secretory portions; the nucleus supraopticus does not show cellular separation into dorsolateral and ventromedial parts as clearly as they are in other primates. The nucleus dorsomedialis is not as well defined as the nucleus ventromedialis like it is in other primates. The nucleus tuberalis lateralis is comparably small, and is not split into several cellular groups as it is in higher primates. The posterior hypothalamic area is morphologically the best definable of the hypothalamic areas. The mamillary region is developmentally advanced, and very well differentiated into medial, lateral and intercalated nuclei.