The mechanisms of cell death and phagocytosis in the early chick lens morphogenesis: A scanning electron microscopy and cytochemical approach

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Abstract
The cell surface characteristics of degenerating cells and phagocytes, as well as the participation of lysosomes in the cell death process associated with the early embryogenesis of chick lens rudiment, were studied by means of scanning electron microscopy and cytochemically using the Gomori‐b̃‐glycerophosphate method for acid phosphatase. The prospective dying columnar epithelial cells lose their apical and basal processes and become rounded. The rounded, isolated, dying cells initially show a rough surface with some cytoplasmic constrictions followed by progressive break‐up into several pitted fragments. Coincident with the loss of the columnar cell shape, acid phosphatase is localized within the Golgi apparatus and autophagic vacuoles which progressively increase in size. In contrast, the isolated dying cells and fragments do not show significant acid phosphatase activity. The role of lysosomes in this degenerative process is discussed. Neighboring epithelial cells phagocytose the dead cell fragments, becoming nonspecialized phagocytes. These consist of columnar epithelial cells and free cells which have migrated from the lens epithelium. Two mechanisms of internalization are observed. The most frequent mechanism takes place in both the columnar epithelial cells and the free cells, and consists of the progressive engulfment of the fragments into craters of the cell surface. The other mechanism is only detected in the free cells and takes place by pseudopod engulfment. We suggest that both phagocytic procedures could be related to the degree of intercellular connection. The presence of phagocytic internalization by crater formation in the epithelial cells could be a mechanism preserving the epithelial stability, which is necessary for a normal morphogenesis. Small microprocesses binding the surface of the phagocyte and the fragment are present prior to the internalization process. In the lens stalk and in the space located between the ectoderm and the lens vesicle, there are some cells displaying migratory characteristics. This fact suggests that an active migration of epithelial cells from the lens stalk could account for the process of detachment of lens vesicle from the ectoderm. The free cells appear to undergo an in situ progressive degeneration.