Macrophage clearance of cell death during mouse embryogenesis

Macrophages are haematopoietically derived cells. In adult tissues they have been shown to be motile and phagocytic. This investigation examines their function as phagocytes during mouse embryonic development, in particular focusing on their role in the clearance of apoptotic cells in the remodelling footplate and during organogenesis of the metanephric kidney. Using the macrophage specific monoclonal antibody, F4/80, macrophages are revealed in the developing limb from embryonic day (E) 11.5 and in the metanephric kidney from E12.5. In these organ systems, macrophage appearance is tightly correlated with that of programmed cell death, suggesting that the majority, if not all, of the dead cells are engulfed and cleared away by macrophages. By injecting F4/80 into the interdigital space of a living mouse embryo's footplate at E13.5, macrophages can be "tagged" and their movements followed. 24 hours after labelling, tracking experiments reveal that macrophages move out of the interdigit, proximally, as far as the base of the limb. To further examine the role of macrophages in development, two strategies for macrophage depletion have been studied. Macrophages within cultured E11.5 kidney rudiments were killed by exposure to toxic-liposomes. This treatment results in organ cultures that were smaller, appear less differentiated and have fewer tubules. The PU. 1 null mouse provides a further opportunity to investigate development in the absence of macrophages. These mice have no macrophages and yet their limbs appear superficially to develop in an identical fashion to their wild-type litter mates. Transmission electron microscopy of PU. 1 null footplates reveals that mesenchymal fibroblasts are able to compensate and can engulf dying cells in the absence of macrophages; this is one of the first examples of cell (as opposed to gene) redundancy revealed by transgenic mouse strategies, and phagocytic clearance of dying cells in the embryo cannot be the prime role of macrophages