DEVELOPMENTAL POTENTIAL OF MUSCLE-CELL PROGENITORS AND THE MYOGENIC FACTOR SUM-1 IN THE SEA-URCHIN EMBRYO

During sea urchin development, esophageal muscle arises from secondary mesenchyme cells, descendants of the vegetal plate that delaminate fr om the coelomic epithelium at the end of gastrulation. In lithium-indu ced exogastrulae, where vegetal plate descendants evert rather than in vaginate, myogenesis occurs normally, indicating that myocyte progenit ors do not have to be near the future stomodeum for differentiation to occur. Vegetal plate descendants isolated along with the extracellula r matrix at different times during gastrulation produce differentiated myocytes in culture as monitored by staining with a myosin heavy chai n antibody. Vegetal isolates prepared at mid-gastrulation or later con sistently produce differentiated myocytes whose form and position rese mbled their counterparts in the intact embryo, whereas vegetal isolate s prepared a few hours earlier while capable of gut differentiation, a s evidenced by the de novo synthesis of the endodermal surface marker Endo 1, did not produce differentiated myocytes. These results suggest that sometime after early gastrulation, a subset of secondary mesench yme cells are competent to differentiate into muscle cells. RNase prot ection assays showed that the accumulation of sea urchin myogenic fact or (SUM-1) mRNA is likely to be coincident with the earliest demonstra ble commitment of myogenic precursors. Premature expression of SUM-1 c oding sequences in mesenchyme blastulae resulted in the activation of muscle-specific enhancer elements, demonstrating that SUM-1 can functi on precociously in the early embryo. However, SUM-1 expressed in this manner did not activate the endogenous MHC gene, nor induce premature or ectopic production of muscle cells.