Which cells in the sea urchin embryo form the larval skeleton?
Large micromeres derived from vegetal pole during unequal fourth cleavage autonomously specify as primary mesenchyme cells after ingression. Upon entering blastocoel they express conserved transcription factors Alx1, Ets1, Tbr and downstream skeletogenic gene battery including MSP130, Sm50 and collagen. Guided by ectodermal patterning cues such as VEGF and FGF, these cells migrate and fuse into syncytial cables depositing calcium carbonate to form triradiate spicules that elongate into pluteus larval skeleton. Small micromeres contribute to germline, while Veg2 becomes secondary mesenchyme and Veg1 forms endoderm, not skeleton.
Ref: Davidson et al., Gene regulatory network for sea urchin skeletogenesis, Science; Gilbert Ch 8: Micromere specification.