Segmentation in Leeches

Of the three classes of annelids defined by traditional morphological approaches, only the leeches (Hirudinea) have retained their status as a monophyletic group, which is now firmly nested within the paraphyletic oligochaetes and polychaetes, as judged by molecular phylogenetics. Leeches have a paradigmatically segmented body plan, including spatially coherent metameres in derivatives of all three germ layers, and the segmental ganglia of their ventral nerve cord provide convenient sets of individually identifiable, phenotypically distinct cells. Glossiphoniid leeches, including species in the genus Helobdella, provide useful models for studying segment formation at the cellular level. Their relatively large and hardy embryos cleave via a pattern of fixed lineages that is ancestral for clitellate annelids (oligochaetes plus leeches), giving rise to experimentally accessible and individually identifiable segmentation stem cells (teloblasts) from which segmental mesoderm and ectoderm arise. Microinjecting embryonic blastomeres with lineage tracers permits the precise analysis of cellular processes underlying segment formation. Among other things, such cell lineage analyses have revealed (1) that segmentation in this taxon is driven by lineage-dependent processes in which cell clones intermingle across segment boundaries, in contrast to the boundary-driven processes of insect and vertebrate models, in which cell clones are restricted from mingling across morphological boundaries; and (2) the existence of “grandparental” stem cell lineages, in which sequentially produced stem cell progeny adopt two distinct fates in exact alternation.