Morphological change of PBMCs-derived EPCs in vitro. Adherence, sequential changes and differentiation of PBMCs are observed under inverted optical microscopy. A) At day 0, freshly isolated PBMCs were plated on fibronectin and the majority of cells are non-adherent with a rounded morphology. B) At day 3, the adherent cells appear either as single cells or as irregular colony-like structures. C) At day 5, colonies are better defined and form a central cluster of round cells with elongated spindle-like cells at the periphery. D) At day 10, cells show a flat monolayer of spindle-shaped cells.