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.