Cell clones and segmentation in leech development.

Cell lineage tracer dyes rhodamine-D-peptide and fluorescein-D-peptide were used to study the development of segmentation in embryos of the leech Helobdella triserialis. The earliest overt manifestation of segmentation is in the mesodermal cell layer of the germinal bands, where a repeating pattern of nearly isomorphic mesoblast clusters is seen at an early stage of development. With joint use of both lineage tracers, it is shown that each mesoblast cluster is a clone derived from a single primary mesoblast, as well as being the precursor to an adult mesodermal hemisegment. Not all of the primary mesoblasts give rise to hemisegments, however. Cell clones derived from the first primary mesoblasts produced contribute to the nonmetameric prostomial region, whereas the last primary mesoblasts produced appear to be supernumerary.