Differentiation of the O and p cell lines in the embryo of the leech. I. Sequential commitment of blast cell sublineages.

The o blast cells are a group of embryonic precursors found in the ectodermal cell layer of the leech germinal band. At the time of their birth these blast cells have the potential to follow either an O or a P developmental pathway and normally become committed to the O pathway as a result of positional cues encountered during the course of their differentiation. The present study characterizes the normal pattern of o blast cell differentiation, including a description of the first six cell divisions in the stereotyped lineage by which the o blast cell gives rise to its clonal descendants. Injection of fluorescent lineage tracers reveals that this clone consists of a precisely defined set of uniquely identifiable neuronal, epidermal, and nephridial descendants and that each of the first three o blast cell divisions brings about a reproducible segregation of these descendant cell fates. Previous work has shown that the o blast cell's descendant clone becomes committed to the O pathway in a stepwise sequence of at least three discrete events which occur, for the most part, many cell divisions prior to histotypic differentiation. The present findings suggest that (i) each of those commitment events is associated with a particular blast cell division, and (ii) each commitment event independently determines the fate of a different blast cell sublineage. The first two commitment events occur just prior to cell divisions which segregate the committed sublineage from the remainder of the blast cell clone, suggesting that the committed state is manifested by only one of the two daughter cells produced at those divisions.