Fig. 1. ED10 quail pectoral muscle contains fast and slow muscle fibers in nearly equal amounts, while the chicken pectoral muscle contains only fast muscle fibers. (A) Quail muscle fibers express fast MyHC (blue), and quail-specific antigen (QCPN) (red) are frequently visible both within muscle fibers and within the stroma. (B) Approximately half of the quail muscle fibers express a slow MyHC isoform (green). (C) Chicken muscle fibers at this stage also express fast MyHC, but (D) no fibers from the center of the muscle expressed slow MyHC. At this stage of development cross sections through the pectoral muscles of both the chicken and the quail show the typical embryonic muscle cell phenotype. Arrows indicate a fiber expressing fast and slow MyHCs, while the asterisks identify the position of a pure fast fiber. Panels A and B, and C and D, are photos of the same field of the quail and chicken respectively, photographed to determine fiber-type. Scale bar is 100 µm.