Regional specialization of the radial glial cells of the adult frog spinal cord.

The amphibian spinal cord is characterized by the presence of radially oriented astrocytic glial cells. These cells have their somata located in the grey matter of the spinal cord and radial processes that extend from the soma through the grey and white matters to the pial surface of the cord. Here we show that these radial glial cells are the predominant cell type labelled by horseradish peroxidase (HRP) when the marker is applied to the surface of the cord. The morphology of the HRP-labelled processes of an individual cell is different as they pass through the grey and white matter regions of the cord. By indirect immunofluorescence on frozen sections we show that the binding of an antibody raised against mammalian glial fibrillary acidic protein (GFAP) is preferentially localized in those areas of the glial process that traverse the white matter of the spinal cord. By transmission electron microscopy we confirm that there are no astrocyte cell bodies either at the pial surface or throughout the white matter region of the cord. These results demonstrate that all the astrocytes in the adult frog spinal cord can be selectively labelled through the application of HRP to the surface of the cord, and that the processes of these labelled cells display regional morphological and biochemical specializations depending on their location in the cord. We propose that these astrocytes may play an important role in setting up the grey-white matter arrangement of the amphibian spinal cord and that a single astrocyte of the frog spinal cord may combine the properties and functions of both grey and white matter mammalian astrocytes.