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

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-beta-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.

Cathepsin D gene expression outlines the areas of physiological cell death during embryonic development

“…Consistent with this finding, lysosomal activation has been formerly reported to occur in the dying ectodermal cells (Garcia-Porrero et al, 1984).…”

Macrophages use different internalization mechanisms to clear apoptotic and necrotic cells

“…In addition, in the present study craters were occasionally observed on the surface of macrophages cocultured with apoptotic cells (Figures 2c and c 0 ). A mechanism of 'sinking-in craters' for internalization of apoptotic cells in early embryogenesis of the chick lens rudiment has been documented by Garcia-Porrero et al 35 This mechanism can be considered as a variation of the zipper model. 33 L929sAhFas cells treated with mTNF underwent necrosis; SEM showed that their surfaces were irregular, pointing to loss of the integrity of the plasma membrane ( Figure 3).…”

Neural crest derivatives in ocular and periocular structures

“…Concurrently with the flux of crest cells reaching the optic cup, a minor but essential progression of mesodermal cells occurs towards the interface between the superficial ectoderm and the lenticular vesicle leading to the differentiation of macrophage-like cells (Hay, 1980;Garcia-Porrero et al, 1984;Cuardos et al, 1991). These cells clean up the debris released by the apoptotic events associated with lens morphogenesis, a prerequisite for the deposition of a primary corneal stroma permissive for neural crest cell migration into the lensectoderm interspace (Hay, 1980).…”