Boundaries and wounds, glia and glycoconjugates. Cellular and molecular analyses of developmental partitions and adult brain lesions.

During brain development, transient partitions of glia and glycoconjugates (glycoproteins, glycolipids, and glycosaminoglycans) surround forming functional units (e.g., nuclear divisions, whisker-related barrels, and neostriatal striosomes). These partitions, which we think of as boundaries, consist of dense aggregates of glial fibrillary acidic protein (GFAP)-positive radial glia, young astrocytes and their processes, and developmentally regulated glycoconjugates (e.g., J1/tenascin and the 473 proteoglycan) that can be thought of as recognition molecules present on membranes or perhaps within the extracellular matrix. When functional patterns have formed and appear to be stabilized, these boundaries are no longer detectable. Lesions of the developing brain show the existence of a more global astrocytic distribution suggestive of biochemically distinct subsets of astrocytes that reside within boundary versus nonboundary positions. Lesions of the adult brain, in addition to showing gliosis, reveal a reexpression of some of the same macromolecules present in transient brain boundaries during development. It is postulated that developmental boundaries and wounds in the adult brain possess some of the same inhibitory and possibly alluring molecular substrates for neuritic expansion.