MyoD-positive epiblast cells regulate skeletal muscle differentiation in the embryo

Abstract

MyoD mRNA is expressed in a subpopulation of cells within the embryonic epiblast. Most of these cells are incorporated into somites and synthesize Noggin. Ablation of MyoD-positive cells in the epiblast subsequently results in the herniation of organs through the ventral body wall, a decrease in the expression of Noggin, MyoD, Myf5, and myosin in the somites and limbs, and an increase in Pax-3–positive myogenic precursors. The addition of Noggin lateral to the somites compensates for the loss of MyoD-positive epiblast cells. Skeletal muscle stem cells that arise in the epiblast are utilized in the somites to promote muscle differentiation by serving as a source of Noggin.

Unique precursors for the mesenchymal cells involved in injury response and fibrosis

“…To determine whether G8 pos cells that responded to epithelial wound healing 24 or 72 h postinjury were indeed progeny of G8 pos cells present at T0, the fixed explants were labeled again with the G8 mAb (1:40), this time followed by an IgM secondary antibody conjugated to Alexa Fluor 488 (Invitrogen-Molecular Probes). Previous fate mapping studies in which G8 pos cells are tracked from the epiblast to embryonic tissues demonstrate that G8 mAb that tags G8 pos cells in the epiblast remains associated with these cells throughout the study and does not transfer to surrounding cells (16). A similar labeling protocol was used for the time-lapse studies, and images were acquired with a Coolsnap HQ camera (Photometrics) on a Nikon Eclipse microscope using Image-Pro Plus software (Phase 3 Imaging Systems).…”

“…G8 pos cell ablation in epithelial explants followed a procedure previously described for ablation of G8 pos cells within the epiblast of the chicken embryo (16). For these studies, ex vivo lens epithelial explants were prepared in SFM.…”

“…G8 cells were labeled for tracking according to procedure described in ref. 16. Briefly, the lens ex vivo epithelial explants were incubated at T0 in Media 199 containing the G8 mAb (1:40) for 45 min at room temperature, rinsed in Media 199, and incubated in rhodamine-conjugated IgM antibody (Millipore) for 30 min at room temperature.…”

“…G8 and MyoD mRNA expressing epiblast cells are capable of undergoing myogenesis when removed from the embryo and placed in culture (12)(13)(14). In vivo, these cells are incorporated into somites where they function instead as cellsignaling centers that promote the myogenic differentiation of surrounding skeletal muscle progenitor cells through their release of Noggin, a bone morphogenetic protein inhibitor (16). G8 pos / MyoD pos cells also are incorporated into tissues that lack skeletal muscle (12,(17)(18)(19), including the embryonic lens (20), where our studies now suggest they play a principal role in wound repair and are a source of disease-causing myofibroblasts.…”

Algorithm of myogenic differentiation in higher-order organisms

“…The current model suggests that a combinatorial effect between the Wnt proteins and Shh is required for myogenesis (Borello et al, 2006;Cairns et al, 2008;Stern et al, 1995), whereas other studies present data showing that the Wnt proteins and Shh antagonize each other (Lee et al, 2000;Lee et al, 2001). Conversely, it was demonstrated that a combinatorial effect between the Wnt proteins and noggin is also sufficient for myogenesis (Reshef et al, 1998), and that noggin addition lateral to the somites can promote myogenesis even when MyoD-expressing cells are removed from the somite (Gerhart et al, 2006). Studies in the field agree that Bmp4 inhibits myogenesis (Hirsinger et al, 1997;Marcelle et al, 1997;Pourquie et al, 1996;Reshef et al, 1998;Sela-Donenfeld and Kalcheim, 2002;Watanabe and Le Douarin, 1996).…”