The pregeniculate nucleus of the monkey (Macaca mulatta). II. A study at the electron microscopic level

An electron microscope study of the ultrastructure of the pregeniculate nucleus of the monkey (Macaca mulatta) shows it to contain three neuronal types and four varieties of presynaptic terminals. Type I neurons are found only in the inner lamina, have small rounded profiles with few axosomatic synapses; the cytoplasm is poor in organelles and the nucleus is deeply infolded. Type II neurons were observed infrequently and only in the outer lamina; they have large oblong profiles, exhibiting many axosomatic contacts, and containing abudant cytoplasm rich in organelles, particularly arrays of granular and agranular endoplasmic reticulum. Type III neurons were the most frequently seen and are found in both laminae; their profiles are elliptical and exhibit only a few axosomatic synapses. The cytoplasm surrounding the infolded nucleus is moderately rich in organelles with agranular endoplasmic reticulum predominating. These three neuronal types were found to correlate well with types of neurons found in material stained with cresylecht violet or impregnated by the Golgi method. Four presynaptic terminal types were discerned: a small cup‐shaped profile containing spheroid vesicles and found predominantly in the outer lamina, a larger elliptical profile containing flattened spheroid vesicles, a large ramifying profile also containing round vesicles and largely restricted to the inner lamina, and a rounded profile containing larger flattened vesicles. Three days after eye enucleation, darkened degenerating profiles containing vesicles and forming asymmetric synapses were observed in the inner lamina, while the third terminal type described above could no longer be seen. The first three are usually associated with asymmetric synaptic densities, whereas in the case of the last type, the postsynaptic synapse is symmetrical. The profile of this last terminal type was also sometimes observed to be both postsynaptic as well as presynaptic to other profiles; however, it was never observed to contain ribosomes. Such a pre‐ and postsynaptic terminal always forms part of the in series or triadic configuration of terminals occasionally observed in the pregeniculate nucleus.