Multiple Inductive Signals Are Involved in the Development of the Ctenophore Mnemiopsis leidyi

Ctenophores possess eight longitudinally arrayed rows of comb plate cilia. Previous intracellular cell lineage analysis has shown that these comb rows are derived from two embryonic lineages, both daughters of the four e1 micromeres (e11 and e12) and a single daughter of the four m1 micromeres (the m12 micromeres). Although isolated e1 micromeres will spontaneously generate comb plates, cell deletion experiments have shown that no comb plates appear during embryogenesis following the removal of e1 descendents. Thus, the m1 lineage requires the inductive interaction of the e1 lineage to contribute to comb plate formation. Here we show that, although m12 cells are normally the only m1 derivatives to contribute to comb plate formation, m11 cells are capable of generating comb plates in the absence m12 cells. The reason that m11 cells do not normally make comb rows may be attributable either to their more remote location relative to critical signaling centers (e.g., e1 descendants) or to inhibitory signals that may be provided by other nearby cells such as sister cells m12. In addition, we show that the signals provided by the e1 lineage are not sufficient for m1-derived comb plate formation. Signals provided by endomesodermal progeny of either the E or the M lineages (the 3E or 2M macromeres) are also required.