Making skeletal muscle from progenitor and stem cells: development versus regeneration

Developmental progression of myogenesis and myogenic gene expression. Top panel: presomitic mesoderm cells express Pax3 (pale blue) and low levels of Myf5 (pale yellow) transcripts. Epithelial somite stage is omitted. Second panel: appearance of sclerotome, dermomyotome (Pax3⁺), and primary (1°) myogenic cells (Myf5⁺) at the dorsal medial edge. Pax3 expression is at a higher level at the lateral edge. Third panel: Pax7 expression emerges and overlaps with Pax3⁺ cells in the central dermomyotome in more mature somites. Bottom panel: vertical division of the central dermomyotomal cells, which give rise to the secondary (2°) myogenic progenitors. These cells give rise to more progenitors and Myf5⁺ or MyoD⁺ myogenic cells. The lateral Pax3⁺ cells give rise to migrating myoblasts entering the ventral body wall and limbs. Keys to cells with specific gene expression are at the bottom.

Gene expression and signaling regulation during adult muscle regeneration. CD34⁺Sca1⁺ side‐population cells can be converted into Pax7⁺ cells via Wnt/β‐catenin signaling. Pax7⁺ satellite cells (red) are associated with the myofiber (pale green) with peripheral myonuclei (white). Upon injury, Pax7⁺ cells either go through symmetric division for self‐renewal via the Wnt/PCP pathway or asymmetric division to give rise to committed progenitors (Pax7⁺Myf5⁺, orange) involving Dll/Notch signaling. These cells expand (as transit amplifying cells) and express MyoD (yellow). Myf5 is presumed (Myf5 (?)) to be expressed in transit amplifying cells and Pax7⁻MyoD⁺ myoblasts (yellowish green). Myogenin (green) is turned on in differentiated myocytes, which eventually fuse to form the new myofiber (dark green), which has the characteristic of centrally located myonuclei. Wnt/β‐catenin can also convert Pax7⁺ cells into fibroblasts. There are at least two steps of muscle regeneration involving FGF signaling: FGF6 affects MyoD⁺ cells, while FGF4R4 affects myofiber number and size. Gene expression for each stage is confined to those covered in the text and does not represent a full list. Wnt, FGF, and Notch pathways that regulate specific steps of myogenic progression are represented by black lines next to the defined steps.

Pax7 expression is detected in satellite cells of the mouse tibialis anterior muscle. (a) Immuno‐electron microscopy (EM) detection of endogenous Pax7 by a monoclonal antibody (Developmental Studies Hybridoma Bank, Iowa, USA), followed by a horse radish peroxidase (HRP)‐conjugated goat anti‐mouse IgG1 antibody (Molecular probes, Grand Island, NY, USA) and enzymatic reaction using the DAB substrate (Vector Labs, Burlingame, CA, USA). The sample was fixed in Zamboni's fixative following the protocol in Chen et al.50 (b) Transmission electron microscopy (TEM) of samples from tamoxifen‐treated Pax7‐Cre‐ER^T2; LacZ reporter mice, fixed in 4% paraformaldehyde, reacted with X‐gal substrate, following the procedure in Kanisick et al.34 Arrows in (a) indicate reacted DAB deposits in the nucleus, in (b), reacted X‐gal precipitates in the cytoplasm. Some X‐gal precipitates are often seen next to satellite cells, likely due to substrate diffusion during enzymatic reaction. Scale bar in (b). b, basement membrane; m, muscle fiber; ps, plasmalemmal surface; sc, satellite cell.