Motility and ramification of human fetal microglia in culture: an investigation using time-lapse video microscopy and image analysis.

Microglia are mononuclear phagocytes of the central nervous system and are considered to derive from circulating bone marrow progenitors that colonize the developing human nervous system in the second trimester. They first appear as ameboid forms and progressively differentiate to process-bearing "ramified" forms with maturation. Signals driving this transformation are known to be partly derived from astrocytes. In this investigation we have used cocultures of astrocytes and microglia to demonstrate the relationship between motility and morphology of microglia associated with signals derived from astrocytes. Analysis of progressive cultures using time-lapse video microscopy clearly demonstrates the dynamic nature of microglia. We observe that ameboid microglial cells progressively ramify when cocultured with astrocytes, mirroring the "differentiation" of microglia in situ during development. We further demonstrate that individual cells undergo morphological transformations from "ramified" to "bipolar" to "tripolar" and "ameboid" states in accordance with local environmental cues associated with astrocytes in subconfluent cultures. Remarkably, cells are still capable of migration at velocities of 20-35 microm/h in a fully ramified state overlying confluent astrocytes, as determined by image analysis of motility. This is in keeping with the capacity of microglia for a rapid response to inflammatory cues in the CNS. We also demonstrate selective expression of the chemokines MIP-1alpha and MCP-1 by confluent human fetal astrocytes in cocultures and propose a role for these chemotactic cytokines as regulators of microglial motility and differentiation. The interchangeable morphological continuum of microglia supports the view that these cells represent a single heterogeneous population of resident mononuclear phagocytes capable of marked plasticity.