Patterns of cell death during gastrulation in chick and mouse embryos

We have examined the distribution of cells at an early stage of the cell death process in gastrulating chick and mouse embryos, using a DNA nick end-labelling technique to label nuclei that are undergoing DNA fragmentation in situ. In the chick embryo, the incidence of nuclei showing DNA fragmentation was mapped by digitizing the occurrence of these nuclei from sections, and reconstructing the three separate layers of the entire embryo at several stages of gastrulation. In the chick, DNA fragmentation was found in nuclei throughout the embryo, in cells of all three germ layers, but most especially in the epiblast in the rostral germinal crescent and in the lateral marginal zones. This region of greatest cell death formed an arc rostrally and laterally in the epiblast, and was consistent through gastrulation and into the early neurulation stage. While the extensive cell death in the chick embryo may be due to cell redundancy, it is also possible that the pattern of death observed could be related to the compression of the embryo against the barrier of yolk at the periphery of the area pellucida during expansion. In a number of cases in the chick, local regions of elevated cell death were also observed in the primitive streak. This may be associated with the changing cell-cell and cell-matrix interactions experienced by cells traversing the primitive streak. In the gastrulating mouse embryo, by contrast, nuclei undergoing DNA fragmentation showed no consistent regions of elevated incidence, in any of the embryonic layers. DNA fragmentation in these embryos was, however, observed in nuclei of cells in the visceral endoderm and in the epiblast. The lack of any clear pattern of DNA fragmentation in the mouse embryo at this stage of development leaves the roles of the dying cells enigmatic. The death may, however, be lineage-related or be a reflection of a cellular redundancy necessary in a developing system that is undergoing extensive cell rearrangement and cellular adhesive change.