Apoptotic cell death in the limb and its relationship to pattern formation.

Detection of cell death throughout embryogenesis demonstrates its importance in the normal form and function of the organism. We have examined cellular death during normal limb development by use of markers that display the morphology of cell death, the presence of phagocytic cells, and lysosomal activity. In addition in situ labeling confirms fragmentation of DNA in the mammalian limb. By these criteria, cell death in the developing limb can be categorized as type 1 or apoptotic cell death. However, the signal(s) responsible for cellular destruction and activation of phagocytosis by neighboring cells or recruited macrophages remain to be identified. The decision for cellular fate during development and regulation of it once the decision is made are key questions. To address the specific question of what determines that one cell will die while its neighbor survives, we have used compounds, such as retinoic acid (RA), that have been shown to alter the pattern of normal development. We and others have shown that RA does indeed alter the pattern of cell death to the extent of inducing malformations in the limb. The mouse mutant Hammertoe (Hm) provides an abnormal system in which the pattern of cell death is specifically altered in the interdigital regions of the limb. Our preliminary data suggest that RA can also introduce cell death between digits 2, 3, 4, and 5 of the Hm mutant where there was no cell death to begin with. Our observations of the effect of RA on mutant limbs suggest that a direct relationship between RA and cell death does exist and that this interaction may be required for correct pattern formation. The alteration in the pattern of cell death in the mutant mouse is of great interest, since it would provide a rare example of specific correction of a birth defect by direct intercession against the physiological effect of the mutation.