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Patterning of fast and slow fibers within embryonic muscles is established independently of signals from the surrounding mesenchyme

¹ Department of Medicine, Stanford University School of Medicine, Stanford, California, 94305-5151, USA
² Institute of Anatomy II, University of Freiburg, Albertstrasse 17, POB 111, D-79104 Freiburg, Germany

View larger version (38K): [in a new window]   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.

View larger version (39K): [in a new window]   Fig. 2. Quail muscle fibers formed within chicken pectoral stroma show the quail pattern of fiber-type. Quail somites were transplanted into chicken embryos at ED2 and the pectoral muscles were isolated and sectioned at ED10. (A) The transplanted half of the pectoral muscle contains a substantial number of quail-derived muscle fibers, as evidenced by quail-specific antigen staining (red) in fast MyHC-expressing fibers (blue). (B) Approximately half of the quail-derived muscle fibers express slow MyHC (green). (C and D) At the same position within the contralateral control side of the chicken pectoral muscle, there is no quail contribution to either muscle or stromal cells, and as expected, no slow fibers are found within this region. Scale bar is 100 µm.

View larger version (48K): [in a new window]   Fig. 3. Chicken muscle fibers formed within quail pectoral stroma show the chicken pattern of fiber-type. Chicken somites were transplanted into quail embryos at ED2 and the pectoral muscles were isolated and sectioned at ED10. (A) As evidenced by the lack of staining for the quail-specific antigen (red) within muscle fibers (blue), the half of the pectoral muscles receiving a contribution from transplanted somites contain muscle fibers derived from the chicken. (B) Chicken fibers within the chimeric pectoral muscle rarely express slow MyHCs. (C) Muscle fibers on the contralateral control side of this pectoral muscle are exclusively derived from quail somites as evidenced by the ubiquitous quail-specific marker, and (D) many of the quail muscle fibers within this location are of the slow phenotype (green). Scale bar is 100 µm.

View larger version (30K): [in a new window]   Fig. 4. Chicken muscle fibers formed in transplanted quail somatopleure show the chicken pattern of fiber-type. Donor quail somatopleure replaced that of the host chicken embryo adjacent to somites 16-23 in ED2 embryos, and the pectoral muscles were isolated and sectioned at ED10. (A) The absence of quail-specific antigen staining (red) in muscle fibers (blue) demonstrates the chicken origin of these fibers in the chimeric pectoral muscle. (B) Chicken fibers within the chimeric pectoral muscle rarely express slow MyHCs. (C,D) At the same position within the contralateral control side of the chicken pectoral muscle, there is no quail contribution to either muscle or stromal cells, and as expected, no slow fibers are found within this region. Scale bar is 100 µm.

View larger version (19K): [in a new window]   Fig. 5. Model of primary fiber-type patterning in avian hypaxial muscles. An intrinsic commitment to either a fast or a slow fiber-type lineage occurs in myogenic precursors while still within the somite. Myogenic precursors migrate into the limb stroma where extrinsic signals operate on fiber-type-committed myoblasts. One mechanism to establish the size and fiber-type pattern of individual anatomic muscles is through expansion of the appropriate myoblast population. In the case of the pectoralis muscle shown here, myoblasts committed to form fast fibers are preferentially amplified in the chicken, while in the quail, both fast- and slow-fiber-committed myoblast populations are expanded.