Development of the arm’s musculature. Developmental stages of the arms shown in each column are indicated at the top margin, staining/gene name of gene expression (purple) shown in each row is indicated on the left margin. Arms are displayed in an oral view with proximal to the left. (A to E) First muscle fibers detected by phallacidin staining appear at stages 24 to 25 (B) in the area of the longitudinal (arrows) and transverse muscle layer (arrowheads). At stage 26 (E), both muscle layers increase in complexity. (F to J)
Ov-Mhc is not detected in early limb bulges (F) and first appears at stages 20 to 21 in large cells (arrowheads) within the area of the future transverse muscle layer (G). From stages 22 to 23 onwards, Ov-Mhc is also expressed in the area of the future longitudinal muscle layer (arrows) (H to J). (K to O)
Ov-Actin is expressed throughout the arm’s development. From stages 24 to 25, it strongly localizes in the areas of the future transverse (arrowheads) and longitudinal muscle layers (arrows) (N to O). (P to T)
Ov-Tm is expressed in the inner cell mass during early arm outgrowth (P, Q). Starting stages 22 to 23, it becomes localized to cells in the future longitudinal (arrows) and transverse muscle layers (arrowheads) (R to T). Asterisk in (E) and dotted line indicate the area of the axial nerve cord; dashed line marks the area of the growth zone of the arm. Arrowheads denote muscle cells/muscle fibers in the area of the transverse muscle layer; arrows point at muscle cells/muscle fibers in the area of the longitudinal muscle layer. Scale bar (refers to A to T): 100 μm.