Spheroids and altered axons in the spinal gray matter of the normal cat. An electron-microscopic study.

In our recent ultrastructural studies on synapses of the nucleus dorsalis, central cervical nucleus, and anterior horn of the spinal cord of the normal cat we happened to find spheroids and several types of axonal alterations. These spheroids were up to 39 micrometer in diameter. They were found in myelinated and unmyelinated terminal axons and at the node of Ranvier and showed two different types of internal structure. One type was large and composed of spirally arranged neurofilaments and mitochondria in increased quantity, although the mean population density of mitochondria was not high being 1.4/micrometer2 as compared to the normal value 2.0/micrometer2. Another type was smaller and consisted of small mitochondria and dense bodies which were increased in number: their mean population densities were 4.5/micrometer2 and 1.9/micrometer2, respectively. At present, the rare occurrence of spheroids and atypical axonal alterations makes it difficult to determine the origin of affected axons, although some of them presumably arise from primary afferents.