Figure 1

Progression of MHC -positive cell lineages during sea urchin development. MHC and Trop1 expression in the gastrula (40 to 48 h), prism, (48 to 60 h), early (60 to 72 h) and late pluteus stage (72 to 96 h) . Expression of MHC (A, F, K, P) and Trop1 (B, G, L, Q) was localized by colorimetric in situ hybridization. The embryos in panels A and B are shown in a vegetal view; the ones in panels C-T are viewed along the animal top/vegetal down (A/V) axis. The inset in panel I is double fluorescent in situ hybridization (FISH) showing the co-expression of CapZ (green) and MHC (red) in myoblasts. Double FISH indicates that Trop1 (green) and MHC (red) are co-expressed in the muscles of a late pluteus (inset in Q). FISH was used to localize the expression of MHC (red; C, D, H, I, M, N, R, S). Bright field images (C, H, M, R) and confocal stacks (D, I, N, S) that include 4',6-diamidino-2-phenylindole (DAPI) (blue) staining are shown. Embryos in C, D, E and T are shown as lateral views with the oral side to the right. The ciliary band and gut internal cilia were stained anti-acetylated tubulin (shown as green in S). The inset is a magnified view of the muscle fibers from a single confocal plane. A schematic representation of the progression of MHC-positive cells in the formation of the muscle fibers is shown (E, J, O, T). Ventrolateral processes (v), the cardiac sphincter (cs; red arrow), the pyloric sphincter (ps; black arrow) and the anal sphincter (as; yellow arrow) are indicated. Muscle fibers are indicated as white (N, S) or black (O, T) arrows.