Expression of brain-specific hyaluronectin (BHN), a hyaluronate-binding protein, in dog postnatal development

Monoclonal antibodies reacting with the brain-specific form of hyaluronectin, a hyaluronate-binding protein, were used in conjunction with antibodies to the glial fibrillary acidic protein (GFAP), the subunit of astrocyte-specific intermediate filaments, to study the postnatal development of spinal cord and cerebral white matter in the dog. As previously reported, the distributions of brain-specific hyaluronectin (BHN) and GFAP in adult dog spinal cord white matter were similar. Both antigens formed a mesh surrounding individual myelinated axons. Furthermore, the glia limitans on the surface of the spinal cord and the glial septa were stained by both antibodies. In newborn dog spinal cord, hyaluronectin immunoreactivity was confined to the glia limitans on the surface. At this time GFAP-positive fibers formed a dense mesh throughout the myelinated white matter. Staining of spinal cord white matter with BHN antibodies first appeared on day 15 and reached its mature appearance in the fully myelinated spinal cord on day 21. In cerebral white matter BHN immunoreactivity was first observed on day 21. With GFAP antibodies astrocytes were extremely few in the nonmyelinated cerebral white matter of 1- and 3-day-old dogs. GFAP-positive astrocytes in cerebral white matter had markedly increased on day 9 before the onset of myelination on day 15. On day 21, myelination was confined to deep cerebral white matter and myelin sheaths were still very few in subcortical white matter. We conclude that BHN expression by white matter astrocytes is an extremely late event in brain development, first occurring after the onset of myelination.