Autoradiographic and ultrastructural studies of areas of spinal cord occupied by Schwann cells and Schwann cell myelin.

Schwann cells, peripheral-type myelin and connective tissue elements develop within the dorsal portion of the X-irradiated spinal cord in immature rats. Factors controlling the distribution of these elements within the irradiated site are not fully understood. In the present study [3H]thymidine autoradiography was used to examine proliferative activities of cells in these areas occupied by peripheral nervous system components, and correlative ultrastructural evaluations were made. At 15 and 20 days post-irradiation (P-I), the Schwann cells occupied the dorsolateral portions of the dorsal funiculi, and heavily labeled cells occurred throughout these areas. By 25 days P-I the Schwann cells extended ventrally into the depths of the dorsal funiculi and into the dorsal gray matter, and labeled cells were concentrated in the deeper portions of these areas. Ultrastructurally, the Schwann cells and peripheral-type myelin were more mature in the superficial portions where proliferative activity was diminished. In contrast, much less mature, peripheral-type myelin occurred in the depths where the labeled cells were concentrated. At 30 and 45 days P-I, labeled cells were much less frequent but usually occurred in the depths when observed. Similarly, a dorsal-ventral gradient in maturity of peripheral-type myelin was evident ultrastructurally. By 60 and 90 days P-I, labeling was rare, and mature Schwann cell myelin was present throughout the areas. Astrocytes and their processes were less numerous in regions invaded by Schwann cells, as compared to controls, and studies are in progress to evaluate the relationships between these glial elements and intraspinal peripheral nervous system components.