Positional determination of supernumerary blast cell death in the leech embryo.

Segmented animals are divided into a longitudinal array of developmentally homologous subunits known as metameres. The embryonic origin of the segmental body plan has been studied in a variety of organisms, with particular emphasis on the mechanisms underlying the delineation of the individual metameres and their secondary diversification. I have examined the embryonic events which determine the total number of segments in the glossiphoniid leech Helobdella triserialis. The germinal band of the leech consists of chains of segmental founder cells, called blast cells, and is normally reduced to 32 segments by the degeneration of supernumerary blast cells located at its caudal end. By using a novel technique for selective cell ablation, the segmental register of the four ectodermal cell lines can be altered so that lineally identifiable blast cells take part in the formation of ectopic segments. I show here that the survival or death of ectodermal blast cells is determined by position independent of their cell lineage identities, implying that the final number of segments is imposed on the ectoderm by interactions with the other embryonic tissues.