The morphology of the diencephalon in the Prosimii. II. The Lemuroidea and Lorisoidea. Part I. Thalamus and metathalamus

Abstract

This study (Part I) covers the comparative structure of the thalamus and metathalamus in the prosimian superfamilies, Lorisoidea and Lemuroidea. Several lemuroid specimens, namely Lemus, Cheirogaleus, Microcebus, Avahi and Daubentonia, and lorisoid specimens such as Galago, Perodicticus and Loris have been used in this extensive study. The anterior nuclear group does not show much difference among these species, although attention is focussed on the differentiation of the principal anterior nucleus into anteromedial and anteroventral parts; the latter increases in structural complexity. The anterodorsal nucleus has regressed into a small, flattened cap-line structure covering the dorsal surface of the anteroventral nucleus. The midline thalamic nuclei are more or less the same throughout the entire prosimian primate scale, though the gradual fusion of the nucleus rhomboidalis with the nucleus centralis medialis is observed from Microcebus to Daubentonia. The medial thalamic nuclear region is dominated by the nucleus mediodorsalis which is clearly differentiated into dorsomedial magnocellular and ventrolateral parvocellular protions. A third division consisting of rather large, darkly staining cells have been observed in the caudal region of the nucleus mediodorsalis. The nuclei centrum medianum and parafascicularis are both large in size, but is not completely demarcated from each other. The dorsolateral thalamic nuclei--the nuclei laterales dorsalis, intermedius and posterior, are well developed, but cannot be distinguished from one another. The pulvinar shows a remarkable phylogenetic development; it does not only enlarge, but is differentiated into several distinct parts. The ventrolateral thalamic nuclear group is comparably larger in structural size than the dorsolateral thalamic nuclei. The nuclei ventrales anterior and lateralis are not clearly demarcated from each other, but on both cyto- and myelo-architectonic grounds, are separate entities. The nucleus ventralis posterior is differentiated into lateral, medial and inferior parts. The posterior thalamic nuclei show the beginnings of regression. The nucleus pretectalis remains comparatively large in size and divided into medial and lateral parts. The nucleus posterior thalamicus is either absent or involuted into other nuclei. The nucleus limitans makes its first appearance in the Prosimii. The lateral geniculate body is very variable in size in all respects among the prosimian species, that is, in the dorsoventral shift along the lateral surface of the thalamus, the cellular differentiation of the lateral geniculate nucleus into 5 or 6 layers, the topographical position of the pregeniculate nucleus and the degree of inversion. The medial geniculate body is shown to be clearly subdivided on cyto-architectonic grounds into dorsomedial and ventrolateral parts; a magnocellular protion can be seen on the dorsal region of this nucleus, and appears to be closely related to the nucleus suprageniculatus.