The sea cucumbers skeletogenic mesenchyme arises from the vegetal pole mesoderm during early development. WMISH shows that sea cucumber alx1 transcripts are localized to a population of four cells in the vegetal pole of blastulae (A). During this stage, PpAlx1 is co-expressed with other factors in the mesoderm, as shown here by two-color fluorescent in situ with probes antisense to PpTbr (B) and PpGata4/5/6 (C). Inset is higher magnification view (B and C). The PpAlx1+ cells are among the earliest to ingress at the onset of gastrulation (D). During gastrulation, PpAlx transcripts are localized to the first cells to migrate away from the archenteron (E) and transcripts are later localized in a ring of cells on the dorsal side of the embryo (F, G). In larvae, alx1 transcripts are detected specifically in a localized cluster of cells that corresponds to the position of the skeletal spicule spicule (H; cell cluster is inset). Knockdown of PpAlx1 results specifically in the loss of the skeletal element (arrows) without drastic changes in larval morphology (I vs. J; high magnification shown in K and L). mes. blastula - mesenchyme blastula; EG-early gastrula; d-day; vv-vegetal pole view; control - control MASO injected; KD – MASO knock down.