An ultrastructural study of neurons and non-neuronal cells in the myenteric plexus of the rabbit colon.

The myenteric plexus of the rabbit colon showed a degree of structural organization that was unusually high for the peripheral nervous system, providing a basis for the complex integrative activity which is known to occur. It resembled central nervous tissue in several respects: a wide range of neuron types was present; the proportion of glial cells to neurons was about 2:1; and there was a densely packed, avascular neuropil, not penetrated by connective tissue. Most neurons had at least one surface exposed to the extraganglionic space. Clear evidence was obtained for spontaneous neuronal degeneration. Three types of non-neuronal (glial) cells were observed: Type 1, which was most common, contained many 10 nm 'gliofilaments' and resembled enteric glial cells or astrocytes in the central nervous system; Type 2, composing about 5% of the glial cells, had few filaments; Type 3 was seen only rarely, had a small dark nucleus, little cytoplasm, may have been of extraganglionic origin and resembled microglia of the central nervous system. Fibroblast-like cells were also present in extraganglionic sites. Schwann cells could not be identified within the myenteric ganglia.