Fig. 5.

PMCs re-arrange themselves as the endoskeleton grows and branches. (A and B) Late gastrula-stage embryo showing the spacing of 64 PMCs in the skeletal rudiment (Hodor and Ettensohn 1998). (C and D) Pluteus stage of embryo showing the 64 PMCs spaced farther apart as the syncytium accommodates the larger, more complex skeleton (Hodor and Ettensohn 1998). (E) Diagram of the growth and branching of the original tri-radiate skeletal rudiment. Modified from Armstrong and McClay (1994). The tri-radiate skeleton begins to form between a tetrad of cells in the middle of the ventrolateral cluster immediately beneath the BE–DVM intersection. Shortly thereafter, one of the branches (#3) of the tri-radiate branches again to produce the post-oral skeletal arm (#5) that grows toward the ventral side, and another rod (#4) that grows toward the dorsal side. Elements #4, #1, (that crosses toward the posterior midline), #2 (that runs anterior-from the initial tri-radiate parallel to the DVM), and #7 (that extends dorsally on the anterior side) provide the framework inside which the body of the pluteus differentiates. The anterior arms (#6) and the postoral arms (#5) along with the anterior–posterior body rod (#2) provide the “catch basin” for larval food that is then streamed into the mouth. Once skeletogenesis is underway, as rod #2, elongates anteriorly, parallel to the DVM, it reaches a site two-third of the way to the animal pole where it branches to extend the ventral arm (#6) that establishes the oral hood of the larva, and the dorsal rod (#7) that provides the anterior skeletal element of the body (Note: anterior is the site closest to the original animal pole whereas posterior is closest to the site of the blastopore). The branchpoint along the DVM is a second site of VEGF production (Duloquin et al. 2007), suggesting that VEGF is secreted and controls skeletal branching (Knapp et al. 2012). In each case, skeletal arms originate and extend from either the BE or from a location along the DVM, and rods encasing the body grow parallel to either the BE or the DVM. The one exception to this is the dorsal extension of rod #7 of the anterior body, suggesting that there is a region of ectoderm patterning that remains to be discovered. A recent publication on ectoderm specification suggests that another band of ectoderm may exist along this location, and if so, it is an excellent candidate for the patterning of band #7 (Li et al. 2014).