Significance and Use 5.1 This guide describes markers involved in myoblast differentiation that can be used to screen stem cells to help define myogenic capacity. Stem cells include pluripotent and multipotent stem cells capable of differentiating into several different mesenchymal cells, including skeletal muscle myoblasts. 5.2 To assess myogenesis in cells derived and not derived from muscle, markers are measured to accurately define the changes in transcription and structural proteins that regulate differentiation, fusion, and myotube formation. Discussion of these markers is important to understand why they are recommended. 5.3 Myogenic Differentiation: 5.3.1 Myogenic differentiation is a highly regulated process controlled by paired box (Pax) transcription factors and the myogenic regulatory factor (MRF) family. During early differentiation in adults, myogenic progenitors such as activated satellite cells or myoblasts express Pax3 and Pax7. Pax3 and Pax7 transcription factors switch the cells toward a myogenic fate, and repress myocyte differentiation (2), priming the cell for later MRFs. To form muscle, the family of MRFs is required to terminally differentiate myoblasts and form myofibers. These regulatory proteins belong to a superfamily of basic helix-loop-helix transcription factors that consists of myogenic differentiation factor 1 (Myod1), myogenic factor 5 (Myf5), myogenin (Myog), and myogenic factor 6 (Myf6). In the initial stages of myogenic differentiation, Myod1 and Myf5 are the first MRFs to be expressed, and trigger increased production of Myog and Myf6 (3). Increased intracellular Myog and Myf6 induces terminal differentiation of myoblasts into myocytes, leading to fused myotubes. 5.4 Forming Myotubes: 5.4.1 While myogenic markers describe differentiation, fusion into multinucleated myotubes is an important factor in muscle biology. Myoblasts differentiate into a fusogenic phenotype characterized by multiple fusion markers. One marker of note is m-cadherin. M-cadherin is reported to be involved in myoblast fusion and to regulate myotube development (4). Therefore, assessment of fusion markers in addition to myogenic differentiation markers would favor a cell phenotype capable of forming muscle. In support of this, studies have shown that despite expression of myogenic differentiation genes, cells not expressing m-cadherin were unable to fuse and form muscle. These results suggest that in addition to myogenic differentiation markers, fusion markers should be considered given their importance as indicators of whether a cell is able to fuse (5). This guide will enumerate published methods to measure and quantify myoblast fusion markers.Scope 1.1 Myogenic differentiation is a process regulated by specific transcription factors and signaling molecules that have been shown to induce a myogenic phenotype. Transcription factors mark the stages of myogenesis and act as benchmarks for use in myogenic assays. 1.2 This guide applies to mammalian cells but does not apply to non-mammalian cells as the myogenic markers for non-mammalian cells can be different than those described here. 1.3 This guide proposes appropriate markers to measure when conducting myogenic differentiation assays. This guide describes the stages for multipotent stem cell differentiation toward myoblasts and myotubes. This guide provides information about the appropriate methods to determine myogenic differentiation. This guide does not provide information about media, supplements, or substrates that drive differentiation toward a myogenic phenotype. 1.4 The purpose of this guide is to act as an aid for work performed in the area of skeletal myogenesis. Using this guide, researchers should be able to understand which skeletal muscle markers are best suited for experiments. This guide will improve consistency for studies of myogenic differentiation of multipotent stem cells by identifying appropriate markers for each stage leading to myocyte differentiation. It should be noted that myoblast differentiation in vitro may not be predictive of results that may be obtained in vivo. 1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of regulatory limitations prior to use. 1.6 This international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
