Cell death during development.

There are many ways to measure apoptosis and other forms of programmed cell death in development. Once nonmammalian embryos have passed the midblastula transition, or much earlier in mammalian embryos, apoptosis is similar to that seen in adult organisms, and is used to sculpt the animal, fuse bilateral tissues, and establish the structure of the nervous system and the immune system. Embryos present unique problems in that, in naturally occurring cell deaths, few cells are involved and they are frequently in very restricted regions. Thus, identification of apoptotic or other dying cells is more effectively achieved by microscopy-based techniques than by electrophoretic or cell-sorting techniques. Since embryos have many mitotic cells and are frequently more difficult to fix than adult tissues, it is best to confirm interpretations by the use of two or more independent techniques. Although natural embryonic deaths are frequently programmed and require protein synthesis, activation of a cell death pathway is often post-translational and assays for transcriptional or translational changes-as opposed to changes in aggregation of death-related molecules or proteolytic activation of enzymes-is likely to be uninformative. Also, embryos can frequently exploit partially redundant pathways, such that the phenotype of a knockout or upregulated death-related gene is often rather modest, even though the adult may develop response or regulation problems. For these reasons, the study of cell death in embryos is fascinating but researchers should be cautious in their analyses.