Macrophages allocate before apoptosis initiation and produce reactive oxygen species during interdigital phagocytosis

During programmed cell death, it is commonly accepted that macrophages are recruited by apoptotic cells to complete cell degradation. Interdigital cell death, a classical model of developmental cell death, contributes to digit individualization in limbs of mammals and other vertebrates. Here we show that macrophages are present in interdigits before significant cell death occurs and remain after apoptosis inhibition. The typical interdigital phagocytic activity was not observed after a partial depletion of macrophages and was markedly reduced by engulfment/phagosome maturation inhibition, as detected by its association with high lysosomal activity. β-galactosidase activity in this region was also coupled with phagocytosis, against its relationship with cellular senescence. Interdigital phagocytosis correlated with high levels of reactive oxygen species (ROS), common in embryo regions carrying abundant cell death, suggesting that macrophages are the major source of ROS. ROS generation was dependent on NADPH oxidases and blood vessel integrity, but not directly associated with lysosomal activity. Therefore, macrophages prepattern regions where abundant cell death is going to occur, and their activation causes high lysosomal activity and the generation of ROS by an oxidative burst-like phenomenon. Summary statement Recruitment of macrophages to the interdigital regions is not linked to apoptosis initiation and they phagocytize by a mechanism involving an oxidative burst-like phenomenon.