Cytoarchitectonic analysis of the bullfrog (Rana catesbeiana) spinal cord by means of electron microscopy with special reference to distribution of microneurons.

In order to determine cytological nature of the "small cells" found in the dorsal half of the bullfrog spinal cord at the electron microscopic level, the complete mapping of these elements was carried out, using both paraffin and thick sections. As a result, these elements were demonstrated to have four main fine structural characteristics of an unequivocally neuronal nature: (1) synaptic contacts, (2) subsurface cisternae, (3) typical pattern of the rough-surfaced endoplasmic reticulum, and (4) close contacts of astroglial processes with the somal surface. The most prominent features in the bullfrog spinal cord can be summarized as follows: the microneurons occupy the dorsal region as well as the region around the central canal of the gray, and the most dorsal part of the gray in exclusively occupied by these microneurons. They show, furthermore, a tendency to separate grossly into medial and lateral groups. No microneurons are visible in the ventral horn. According to the results obtained from the cytoarchitectonic comparison of the bullfrog spinal cord and the rat spinal cord, it can be said that in the former no differentiated laminar structures can be recognized, while in the latter the gray matter is sufficiently differentiated to permit full percellation.