Development of an astrocytic response to lesions of the spinal cord in the North American opossum: an immunohistochemical study using anti-glial fibrillary acidic protein.

We have shown previously that rubral axons grow around a lesion of their pathway in developing opossums and that a critical period exists for that plasticity. The critical period begins when rubral axons first reach the level of the lesion and ends sometime between postnatal days (PD) 26 and 30. The aim of the present study was to examine the development of an astrocytic response to lesioning the spinal cord to determine if there is a temporal correlation between the development of such a response and the end of the critical period. The astrocytic response was examined immunohistochemically, 2 and 4 weeks after hemisecting the thoracic spinal cord, using an antibody to glial fibrillary acidic protein (GFAP). A response was first seen at PD21 in the 2-week series. The response was relatively mild, however, and limited to the white matter. When the lesion was made at PD26, the response was still restricted to the white matter, but hypertrophied astrocytes were found at the gray/white matter junction and cystic cavities were present. When the lesion was made at PD41, the response had spread to the gray matter and it occupied a larger area rostral and caudal to the lesion than at earlier ages. The animals allowed to survive 4 weeks after lesioning were subjected to a second operation 4-5 days before sacrifice so that Fast Blue could be injected bilaterally two to three segments caudal to the lesion. When the hemisection was made at PD15, a response was present in the ventral and ventrolateral funiculi, but not in that part of the lateral funiculus that contains rubrospinal axons.(ABSTRACT TRUNCATED AT 250 WORDS)