Phase contrast microscopy of classical myosphere cultures. A When cultured in suspension, progenitor/stem cells initially correspond to very few small and refringent rounded cells. Within the first days (days 1–3) some of them could appear in clusters of two to four cells. B Then (days 3–6), they began to grow according to atypical modalities as they aggregated into microspheroid colonies composed of a few dozen of joined cells that occasionally could also appear superposed. C In the following days (days 6–10), these floating myospheres pursued to grow in size and number, reaching several hundreds of cells. D After 10 days, the myospheres appeared as spheroid complexes that are very dense in cells and voluminous. They can be maintained by serial passages as suspended myospheres for at least several weeks without losing their proliferation ability. Scale bars: 25 µm. Related to the work done on in vitro characterization of canine MuStem cells .