Interactions between intraspinal Schwann cells and the cellular constituents normally occurring in the spinal cord: an ultrastructural study in the irradiated rat

Relationships between intraspinal Schwann cells and neuroglia, particularly, astrocytes, were studied following X-irradiation of the spinal cord in 3-day-old rats. Initially, this exposure results in a depletion of the neuroglial population. By 10 days post-irradiation (P-I), gaps occur in the glia limitans, although the overlying basal lamina remains intact. Development of and myelination by intraspinal Schwann cells is well underway by 15 days P-I. These Schwann cell-occupied regions have a paucity of astrocyte processes, a finding which persists throughout the study (60 days P-I), and several types of Schwann cell-neuroglial interfaces are observed, including: (1) astrocyte separation of Schwann cells from oligodendrocyte-myelinated regions; (2) intermingling of Schwann cell-myelinated axons and oligodendrocyte-myelinated axons in the absence of astrocyte processes; and (3) ensheathment of unmyelinated axons by astrocyte processes which separate these axons from the Schwann cells. The gaps in the glia limitans widen as the P-I interval increases. At 45 and 60 days P-I, the basal lamina no longer forms a singular, continuous covering over the spinal cord surface, but follows instead a rather tortuous course over the disrupted glia limitans and the intraspinal Schwann cells. Although the mode of initial occurrence of Schwann cells within the spinal cord is not yet understood, the data indicate that the astrocyte population is involved in that process, as well as in limiting the further development of Schwann cells within the substance of the spinal cord.