The pregeniculate nucleus of the monkey (Macaca mulatta). I. A study at the light microscopy level.

A study of the pregeniculate nucleus of the monkey (Macaca mulatta), in Klüver-Barrera-stained, and Golgi-impregnated material confirmed, with reservations, that the nucleus is bilaminar. The inner lamina extends anterior to posterior as a compact band of neuropil within the optic tract, passing from a dorsomedial position to occupy a more dorsolateral position with respect to the lateral geniculate nucleus. The neuropil of this lamina is formed from a plexus of very fine fibers, many having varicosities, which appear to make contact with dendrites. The outer lamina appears to be formed from that part of the thalamic reticular nucleus that is apposed to the lateral geniculate nucleus. Three types of neurons were observed in the nucleus: Type I are found only in the inner lamina; they have small (12--14 micrometers) spheroid somata with thin dendrites having terminal arborizations formed by long protrusions and axons exhibiting varicosities. Type II neurons in contrast are found only in the outer lamina; their cell bodies are large (21 by 33 micrometers to 26 by 37 micrometers) and polygonal, contain much Nissl substance, and exhibit thick, relatively spine-free dendrites. Type III neurons, which are the most common, are found in both laminae; they have medium-sized (13 by 27 micrometers to 19 by 40 micrometers ellipsoid somata containing moderate amounts of Nissl substance and exhibiting well-developed spine-bearing dendrites. Since the axon of the type I neuron impregnates and is found only within the nucleus, this cell is considered to be an intrinsic neuron; on the other hand, types II and III are judged to the principal neurons, as their axons do not impregnate.