Cells that express MyoD mRNA in the epiblast are stably committed to the skeletal muscle lineage

Abstract

The epiblast of the chick embryo contains cells that express MyoD mRNA but not MyoD protein. We investigated whether MyoD-positive (MyoDpos) epiblast cells are stably committed to the skeletal muscle lineage or whether their fate can be altered in different environments. A small number of MyoDpos epiblast cells were tracked into the heart and nervous system. In these locations, they expressed MyoD mRNA and some synthesized MyoD protein. No MyoDpos epiblast cells differentiated into cardiac muscle or neurons. Similar results were obtained when MyoDpos cells were isolated from the epiblast and microinjected into the precardiac mesoderm or neural plate. In contrast, epiblast cells lacking MyoD differentiated according to their environment. These results demonstrate that the epiblast contains both multipotent cells and a subpopulation of cells that are stably committed to the skeletal muscle lineage before the onset of gastrulation. Stable programming in the epiblast may ensure that MyoDpos cells express similar signaling molecules in a variety of environments.

Ancestral Myf5 gene activity in periocular connective tissue identifies a subset of fibro/adipogenic progenitors but does not connote a myogenic origin

“…This possibility contrasts with the clear demonstration that cells that have expressed MyoD during embryogenesis are essentially all committed to the myogenic lineage (Gerhart et al, 2007; Kanisicak et al, 2009). Whereas satellite cells uniformly demonstrate ancestral expression of MyoD (Kanisicak et al, 2009), adipogenic cells that have been occasionally observed in satellite cell cultures and initially considered to represent non-myogenic derivative of satellite cells (Asakura et al, 2001; Shefer et al, 2004), were proven negative for MyoD-driven reporter, hence, not of satellite cell origin (Starkey et al, 2011).…”

Noggin producing, MyoD-positive cells are crucial for eye development

“…Although cells from this primitive epithelium are generally considered to be pluripotent, a small subpopulation within this tissue of the chick embryo expresses mRNA for the skeletal muscle specific transcription factor MyoD and a cell surface antigen recognized by the G8 monoclonal antibody (MAb) (George-Weinstein et al, 1996; Gerhart et al, 2000; Gerhart et al, 2001; Gerhart et al, 2004a; Strony et al, 2005). MyoD mRNA positive (MyoD+) epiblast cells do not synthesize detectable levels of MyoD protein or other skeletal muscle genes, including Myf5, Myogenin and sarcomeric myosin (George-Weinstein et al, 1996; Gerhart et al, 2000; Gerhart et al, 2007). …”

“…MyoD+ cells labeled with the G8 MAb also are integrated into embryonic organs lacking skeletal muscle, including the heart and brain (Gerhart et al, 2006; Gerhart et al, 2007). In these locations, they continue to express MyoD mRNA and the G8 antigen and are not induced to form cardiac muscle or neurons.…”

The Avian Embryo as a Model System for Skeletal Myogenesis

“…Furthermore, the uncovering of TCF/LEF and Gli binding sites upstream of the MYF5 locus in mouse indicates that these pathways are direct effectors of MYF5 activation (Borello et al 2006). An unexpected twist to these findings is a series of studies from George-Weinstein's group, which indicate that mesodermal cells have a tendency to undergo myogenesis in the absence of inducing factors [Gerhart et al (2007) and references therein] and it is therefore possible that the secreted factors WNT and SHH in fact reinforce a pre-existing bias towards a myogenic path.…”