Intrinsic segmental identity of segmental founder cells of the leech embryo.

Segmentation occurs in several animal phyla, and the cellular mechanisms generating this structural periodicity vary considerably. In the leech, an annelid worm, segmental founder cells arise through a fixed cell lineage (Fig. 1), and come together in a longitudinally repeating array through a stereotyped pattern of morphogenesis. In this paper we demonstrate that founder cells forced to differentiate in a foreign segmental environment give rise to their normal, segment-specific clones of neuronal descendants, even in segments in which those neuronal phenotypes would not normally be observed. These findings indicate that the individual founder cells possess segmental identity at or shortly after the time of their birth, and further suggest that such identities are established by a mechanism in which the parent stem cell 'counts' mitotic cycles.