Development of the esophageal muscles in embryos of the sea urchin Strongylocentrotus purpuratus.

Development of the esophageal muscles in embryonic sea urchins is described using light- and electron microscopy. The muscles develop from processes of about 14 cells of the coelomic epithelium that become immunoreactive to anti-actin at about 60 h (12-14 degrees C). Initially, each myoblast extends a single process with numerous fine filopodia around the esophagus. By 72 h the processes have reached the midline and fused with those from cells of the contralateral coelomic sac. Myoblasts begin to migrate out of the coelomic epithelium between 72 and 84 h. By 72 h the processes stain with the F-actin specific probe NBD-phallacidin. The contractile apparatus is not evident in transmission electron-microscopic preparations of embryos at 70 h, but by 84 h the contractile apparatus is present and the muscle cells are capable of contraction. Because the myoblasts migrate free of the coelomic epithelium and are situated on the blastocoelar side of the basal lamina, it is suggested that that they should be considered as a class of mesenchymal cells.