Light and electron microscopic study of an avian pretectal nucleus, the lentiform nucleus of the mesencephalon, magnocellular division.

Using several light microscopic methods we have identified the lentiform nucleus of the mesencephalon, magnocellular division, by its position in the pretectum, its cellular composition, and its complement of retinal afferents and have distinguished it from neighboring structures. At the light microscopic level large neurons (approximately 30 X 21 microns) and small neurons (approximately 13 X 9 microns), which are more numerous, are seen interspersed among myelinated axons. The large neurons are generally ovoid and contain an eccentrically located nucleus and large clumps of Nissl-stained material. In the electron microscope the most notable feature of these neurons is the presence of ribosome rosettes and many parallel arrays of rough endoplasmic reticulum (RER). On the basis of cytological and ultrastructural features, we conclude that only one class of large neuron is present. Although in the light microscope the small neurons appear to be similar, at the ultrastructural level three neuron types have been distinguished: (1) ovoid shape with cytoplasm densely packed with organelles especially RER, (2) round shape with very little cytoplasm with few organelles, and (3) triangular shape with a pale cytoplasmic matrix with some RER. Subsurface membrane configurations are often seen in the somata of all neuron types. In addition, axon terminals, some containing flat vesicles, and other less frequent ones containing round vesicles are seen terminating on the somata of all neuronal cell types. In the neuropil, three types of presynaptic profiles can be identified. Two of these profiles are axodendritic and the third is dendrodendritic. The type R profile, which is often as large as 4 micron 2, is the most numerous, contains large round synaptic vesicles, and is often seen synapsing on several dendritic profiles. The type F profile contains flat vesicles and a relatively dense cytoplasm, and is smaller in area than type R. The third profile, which contains small clusters of pleiomorphic vesicles and apparently is dendritic, forms synapses with dendritic profiles. We propose that the type R axon terminals originate mostly from the retina and the type F mostly from the visual telencephalon.