The red nucleus of the opossum (Didelphis marsupialis virginiana): A light and electron microscopic study

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Abstract
The opossum midbrain was sectioned in frontal, sagittal and horizontal planes and subsequently stained with cresyl violet to determine the cytoarchitecture of the red nucleus. The neurons were visualized in more detail in Golgi preparations, one micron plastic sections and by employing electron microscopy.Distinct parvicellular and magnocellular subdivisions are not apparent in this species. The characteristic neurons can be divided into three types by utilizing the following criteria: size, distribution of Nissl bodies and dendritic patterns. The cytoplasm of the giant neurons (45–70 μ) is filled with Nissl granules and the dendrites are long and tapered, and display an occasional spine. The large‐medium neurons (40–25 μ) can be differentiated from the giant neurons on the basis of size and their less extensive pattern of dendritic branching. Small neurons (10–20 μ) are achromatic or have a uniformly basophilic cytoplasm and may be classified as Golgi type II neurons.The large‐medium size neurons are the most numerous of the three types described and they are distributed throughout the entire extent of the nucleus. The small neurons, although found less frequently than the large‐medium size cells, also are scattered throughout the nucleus. The giant neurons are limited to the caudal medial 0.5 mm of the nucleus.A morphological classification of synaptic terminals is described: (1) small (1–2 μ), (2) elongate (up to 10 μ), (3) boutons en passage and (4) large (2–4 × 5–9 μ). Differences in the population of synaptic vesicles in the four types of synaptic endings are described. In addition, differences in the distribution of the four types of synaptic terminals with respect to the soma, proximal dendrites and dendrites in the neuropil are illustrated.