This is an adherent cell that has been made to assume a square shape by constraining it to a square-shaped micropatterned island. The cell has been fixed and stained for filamentous actin (green) and the DNA (blue). This picture has become famous [Textbook cover , Nature article, Whitesides book, Original Article] (It is my picture, although I don't always get credit for it).

This is three Bovine Capillary Endothelial Cells ("3-cell yin yang") that have been constrained to adhere to a circular micropatterned island. They spontaneously break symmetry and self-assemble into a migrating tissue.