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A fine structural study was undertaken on the area called the anterior necrotic zone of leg buds of the chick embryo at stages 22-29. Physiologically dying cells, which were distributed in the mesenchymal area, were characterized by increased electron density of the cytoplasm, appearance of electron lucent vacuoles and condensed chromatin pattern. Many of the dying cells seemed to be fragmented into several pieces of cell debris during the necrotic process. Two kinds of cells were observed to take up the necrotic cells; one type, large cells 15 microns or more in size and irregular in shape, which had lysosomes, extended microvillus-like cytoplasmic projections and pseudopods, took up pieces of necrotic cells by phagocytosis and stored them in acid phosphate-negative and -positive vacuoles. The other type was several microns in diameter with numerous free ribosomes and stored a small number of vacuoles containing cell debris. At stage 29, profiles of necrotic cells lying free in the mesenchymal area, and cells containing one or two vacuoles filled with cell debris decreased in number as well as large cells containing numerous vacuoles, whose contents seemed to be degraded. When pieces of the anterior portion of chick leg buds at stage 24 were organ-cultured for 3 days after glutaraldehyde-fixed mouse red blood cells were injected, the red blood cells were taken up into cells in the mesenchymal area. Formation of large cells containing numerous vacuoles stuffed with red blood cells was rare. These findings may imply that most necrotic cells in the anterior necrotic zone are disposed of by macrophages and that some are endocytosed by mesenchymal cells.

Fine Structural Aspects of the Anterior Necrotic Zone in the Leg Bud of the Chick Embryo