Substrate-dependent interactions of leech microglial cells and neurons in culture

The principal aim of the present experiments has been to analyze the properties of microglial cells and their role in nerve regeneration. In the leech, damage to the CNS has been shown to be followed by accumulation of laminin and microglial cells at the site of injury (Masuda-Nakagawa et al., 1990. Proc. R. Soc. Lond. B. 241:201-206; and 1993. Proc. Natl. Acad. Sci. USA 90:4966-4970). Procedures were devised for isolating these small, wandering cells from the CNS of the leech. In culture, they were reliably identified by their sizes, shapes, and phagocytotic activity. Their morphology, motility, and interactions with neurons were influenced by the substrate molecules on which they were plated. On the plant lectin concanavalin A (Con A) microglia had a rounded shape and remained stationary. By contrast on extracts of leech extracellular matrix (ECM) enriched with laminin the cells were mobile and spindle-shaped with long processes. On Con A, neuronal growth cones avoided microglial cells, whereas on ECM extract the presence of a microglial cell did not influence neurite growth. Microglial cells showed immunoreactivity on both substrates when stained with a monoclonal antibody against leech laminin. Together these results suggest that microglial cells are influenced in their properties by molecules in the environment and that they could contribute to neuronal outgrowth at the site of an injury.