Fig. 4

Anterior part of the shark hypobranchial muscles is marked by the expression of Lbx1. a–d Whole-mount in situ hybridization of S. torazame embryos at stage 27.5. a, b Expression of Lbx2 in ventral (a) and left lateral (b) views. c, d Expression of Lbx1 in ventral (c) and left lateral (d) views. Arrows show the expression in the anterior extremity of HBM primordium. e–g” Transverse sections of a stage 29 embryo at the levels indicated in h (ventral view stained with MyHC antibody). e’–g’ The enlarged views of the areas marked in e–g. f” and g” are the adjacent sections to f’ and g’, respectively, hybridized with different probes. e’ Lbx1 is expressed in the medial HBM precursor cells (arrowhead) located dorsal to the hyoid arch-derived muscles (double arrowheads). f’, f” Lbx1 is expressed in the medial HBM precursor cells, whereas Lbx2 expression is restricted in the bilateral HBM primordium (f”). g’, g” Earliest differentiation of HBM occurs in the cells in which Lbx2 is downregulated. i At stage 29.5, the anterior medial part of HBM differentiates as coracomandibularis (cmd) muscle. j At stage 30, the anterior tip of early m. coracoarcualis (cac, originally Lbx2-positive) extended anteriorly, forming coracohyoideus (chy) muscle connecting cac muscle and basihyal cartilage, overlapping laterally with m. coracomandibularis. Coracomandibularis now forms a single medial bundle of muscle fibers that connects the coracoarcualis muscle and the lower jaw. k Meckel’s cartilage expressing Sox9 (asterisk). l, m The cac muscle has four segments corresponding to the adjacent pharyngeal arches (pa3-6). l Ventral view. m Left ventrolateral view. ht, heart; m, mouth. Scale bars, a–l 0.5 mm; m 0.2 mm