Embryonic cell lineages in the nervous system of the Glossiphoniid leech Helobdella triserialis

The lines of descent of cells of the nervous system of the leech Helobdella triserialis have been ascertained by injection of horseradish peroxidase (HRP) as a tracer into identified cells of early embryos. Such experiments show that the nervous system of the leech has several discrete embryological origins. Some of the neurons on one side of each of the segmental ganglia derive from a single cell, the ipsilateral N ectoteloblast. Other neurons derive from a different precursor cell, the ipsilateral OPQ cell that gives rise to the O, P, and Q ectoteloblasts. The positions within the ganglion of neuronal populations derived from each of these sources are relatively invariant from segment to segment and from specimen to specimen. Other nerve cord cells derive from the mesoteloblast M; of these four per segment appear to be the precursors of the muscle cells of the connective. The A, B, or C macromeres contribute cells to the supraesophageal ganglion. In preparations in which an N ectoteloblast was injected with HRP after production of its bandlet of n stem cells had begun, the boundary between unstained (rostral) and stained (caudal) tissues can fall within a ganglion or between ganglia. This suggests that each hemiganglion contains the descendants of more than one, and probably two, n stem cells.