14 - Anteroposterior Pattern Formation in the Leech Embryo

Abstract

The leech embryo develops via a stereotyped sequence of cell divisions, and thus it is feasible to uniquely identify cells of the developing embryo by their lineage history. In glossiphoniid leeches such as Helobdella triserialis and Theromyzon rude, the early embryo is large and readily amenable to experimental manipulation. Considerable progress has been made in understanding the cellular events and to some extent also the molecular events that underlie the initial patterning of the anteroposterior (AP) axis. The large size of the nervous system in late embryos has permitted elegant analysis of AP patterning events associated with terminal cell differentiation. One modification involves the degree to which segmentation or other aspects of AP patterning depend on an invariant pattern of cell lineage. Many features of leech segmentation are closely tied to a segmentally repetitive sequence of cell lineages, and in this regard the leech embryo provides an interesting counterpoint to the other segmented animals currently in use as model systems.