TANGENTIAL MIGRATION OF AMEBOID MICROGLIA IN THE DEVELOPING QUAIL RETINA - MECHANISM OF MIGRATION AND MIGRATORY BEHAVIOR

Long distance migration of microglial precursors within the central ne rvous system is essential for microglial colonization of the nervous p arenchyma. We studied morphological features of ameboid microglial cel ls migrating tangentially in the developing quail retina to shed light on the mechanism of migration and migratory behavior of microglial pr ecursors. Many microglial precursors remained attached on retinal shee ts containing the inner limiting membrane covered by a carpet of Mulle r cell endfeet. This demonstrates that most ameboid microglial cells m igrate tangentially on Muller cell endfeet. Many of these cells showed a central-to-peripheral polarized morphology, with extensive lamellip odia spreading through grooves flanked by Muller cell radial processes , to which they were frequently anchored. Low protuberances from the v itreal face of microglial precursors were firmly attached to the subja cent basal lamina, which was accessible through gaps in the carpet of Muller cell endfeet. These results suggest a mechanism of migration in volving polarized extension of lamellipodia at the leading edge of the cell, strong cell-to-substrate attachment, translocation of the cell body forward, and retraction of the rear of the cell. Other ameboid ce lls were multipolar, with lamellipodial projections radiating in all d irections from the cell body, suggesting that microglial precursors ex plore the surrounding environment to orient their movement. Central-to -peripheral migration of microglial precursors in the retina does not follow a straight path; instead, these cells perform forward, backward , and sideways movements, as suggested by the occurrence of (a) V-shap ed bipolar ameboid cells with their vertex pointing toward either the center or the periphery of the retina, and (b) threadlike processes pr ojecting from either the periphery-facing edge or the center-facing ed ge of ameboid microglial cells. (C) 1998 Wiley-Liss, Inc.