Morphological features of undifferentiated/differentiated human umbilical cord mesenchymal stromal cells cultured on different surfaces. (A) Undifferentiated cells were characterized by a fibroblast-like cell morphology during proliferation on a tissue culture polystyrene surface. (B) Differentiated cells formed a cell-clustering structure (arrow) in a Matrigel™-coated Transwell insert following culture in a pseudo-three-dimensional system for 21 days. (C) Differentiated cells showed morphology with stress fibers on a 0.1% gelatin solution-coated hard surface. (D) In comparison, differentiated cells formed a cluster structure on a 100% Matrigel™ soft surface (see high-magnification insert in (D)). (C), (D) Cells were seeded on the wells of glass chamber slides precoated with 0.1% gelatin solution to improve cell attachment on the glass surface or 100% Matrigel™ to create a soft gel surface for cell clustering behavior respectively. They were cultured for up to 7 days, then fixed and stained for actin distribution by fluorescein isothiocyanate–phalloidin (green). Cell nuclei were counterstained with propidium iodide (red). Scale bars = 50 μM.