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Cell death in the posterior necrotic zone (PNZ) of the chick wing-bud: a stereoscan and ultrastructural survey of autolysis and cell fragmentation.

The participation of lysosomes and cell profile changes were studied during mesenchymal cell death in the PNZ of forelimbs of the 4 1/2-day chick embryo. Lysosome participation was studied cytochemically using as substrate either beta-glycerophosphate or p-nitrophenylphosphate. Acid phosphatase (AP) is localized within the Golgi cisternae of the prospective dying cells, and small AP-positive autophagic vacuoles appear when degeneration commences. As degeneration by enzymic digestion proceeds, these vacuoles increase in size and appear to become autolytic since part of the AP activity seems to 'leak' out of the autophagic vacuoles. Next, the cells fragment and the fragments are then enclosed in heterophagic vacuoles of macrophages where digestion is completed. The SEM showed that these intracellular changes in degenerating cells are accompanied by changes in the cell surface structure. First, the degenerating cells lose the stellate appearance of healthy mesenchymal cells and become rounded and pitted, then constrictions appear and finally the cytoplasm breaks up into many small pieces. This final fragmentation may be an active process rather than a mere consequence of vacuolation.