Immunohistochemical and lectin-labelling studies of the distribution and development of microglia in the spinal cord of postnatal rats.

The present study describes the development and differentiation of microglial cells in the spinal cord of postnatal rats ranging from 1 day to 3 weeks of age. Using the monoclonal antibody OX-42, three different morphological forms of immunoreactive cells (SP, TLP, and AP) were identified based on their staining intensities, cell shapes and configurations of their cytoplasmic processes. Round or oval cells with short thick processes (SP) and cells with thick or thin long processes (TLP) were common in younger rats (1 day-1 week), while cells with attenuated processes (AP) preponderated in older animals (2-3 weeks). The immunoreactivity of microglial cells was gradually reduced as the cells differentiated progressively from the SP through the TLP type into the AP form. Similar results were obtained using the monoclonal antibody OX-18 and the isolectin Griffonia simplicifolia. None of the cells were stained with the antibody OX-6. A quantitative study showed a rapid increase in the cell density of OX-42 positive cells in both the gray and white matter from 1 day to 2 weeks of age, but this appeared to decrease thereafter. The increase in the cell density was attributed to the active proliferation of the cells as shown by the detection of many bromodeoxyuridine-labelled cells in the same region. Its reduction in 3-week-old rats was most probably due to the apparent expansion of the spinal cord as a result of the growth of its fibre size and other structural elements.