Segmentation in leech development

Segments in glossiphoniid leeches, such as Helobdella triserialis, are the products of sterotyped cell lineages that yield identifiable cells from first cleavage. Cell lines generating segmental tissues are separated from those generating prostomial tissues early in development. Segments arise from five bilateral pairs of longitudinal columns of primary blast cells that are generated by five bilateral pairs of embryonic stem cells called teloblasts. There are four ectodermal cell lines (N, O, P and Q) and one mesodermal cell line (M) on each side of the embryo. In normal development, each cell line generates a segmentally iterated set of identified definitive progeny comprising a mixture of cell types. In the M, O and P cell lines, each blast cell generates one segment's worth of definitive progeny (segmental complement). But the clones of blast cells in each of these three cell lines interdigitate longitudinally with cells of the adjacent clones from the same line, so that the clone of an individual m, o and p blast cell is distributed across more than one segment. Thus, there is no simple clonal basis for morphologically defined segments. In the N and Q cell lines, two blast cells are required to produce one segmental complement of definitive progeny; in each of these two cell lines, two classes of blast cells (nf and ns, qf and qs) are produced in exact alternation. Primary n and q blast cells are about the same size and are produced at the same rate as blast cells for the o and p bandlets, but the longitudinal extent of their clones is roughly half that of the o and p blast cells' clones.(ABSTRACT TRUNCATED AT 250 WORDS)