The molecular identity
of serotoninergic apical cells is beginning to be elucidated:
the 5HT cell included in our PrImR analysis appears to
specifically express lmx1ab, a LIM homeodomain factor
implicated in serotoninergic specification in nematodes
[57], and nk2.1, demarcating serotoninergic cells in sea urchin [36] and, possibly, in the Ptychodera tornaria [75].
Likewise, fezf has been proposed to correlate with serotoninergic fate in the sea urchin [32].