Differences between the clearance of apoptotic cells by professional and non-professional phagocytes

Both professional and non-professional phagocytes [1] participate in cleari ng the massive numbers of cells that undergo apoptosis during animal develo pment [2], but it is not known how they divide this task, Using time-lapse recordings of cells in culture, we show that professional phagocytes (brain macrophages or microglia) are highly motile, ingest apoptotic cells Immedi ately, and digest them quickly, Nonprofessionals such as BHK and lens epith elial cells are sessile, often recognize apoptotic cells as soon as they di e by showing characteristic palpating movements, but delay ingestion until several hours later. By pre-ageing apoptotic cells, we show that this delay is because the apoptotic cells must undergo further changes before non-pro fessionals can ingest them. The difference was also apparent in vivo, using immunofluorescence and electron microscopy of the developing central nervo us system. This arrangement favours prompt clearance by professionals if pr esent in adequate numbers; if they are scarce, however, non-professional by standers will reluctantly clear the apoptotic cells.