KERATAN SULFATE IS A MARKER OF DIFFERENTIATION OF RAMIFIED MICROGLIA

Recently we reported that the keratan sulphate epitope recognised by t he monoclonal antibody 5D4 is expressed by a population of ramified mi croglia in adult rats. As ramified microglia is believed to differenti ate from ameboid microglia during postnatal development, we studied th e rat brain from birth to 90 postnatal days of life with the monoclona l antibody 5D4. Contrary to all the other microglia markers until now described, keratan sulphate is not expressed by ameboid microglia and by macrophages but appears on the surface of microglia only when the c ells are differentiated and show ramified processes. The keratan sulph ate positive cells become evident at different times in different cent ral nervous system areas; the first were localised in the pyriform cor tex and brainstem from the end of the second postnatal week. These obs ervations suggest that keratan sulphate expression on microglia cells is induced by differentiation and by a resting functional state. Moreo ver the 5D4 monoclonal antibody showed a strong diffuse positive stain ing of some cortical, thalamic and white matter areas during the first two postnatal weeks. This staining was transient and it does not seem biologically correlated with the expression of the keratan sulphate o n differentiated microglia.