Morphology, proliferation, cell cycle, and apoptosis of WJ MSCs (Wharton’s jelly-derived mesenchymal stem cells) in the early, middle, and late phases. (a) Representative phase-contrast micrographs showed that there were many small raised cells with a fibroblast-like appearance in the early phase, (b) then some cells gained an irregular and flat morphology in the middle phase, and (c) more cells appeared to be irregular and flat in the late phase. Arrows indicated the irregular and flat cells. Bar: 100 μm. (d) CCK8 (cell counting kit-8) assay showed that the proliferation rate of WJ MSCs decreased gradually in long-term culture in vitro. (e) S and G2 phase block phenomenon might exist at higher passages (P14 and P21) compared to P7. (f) There were no significant differences in apoptosis among three phases.