All cells of a multicellular organism except the gametes come from mitosis. Tissues are collections of cells that are performing the same function. Muscles and skin would be different tissues. Put together several kinds of tissues and you get an organ like the heart or the kidney. Several organs would be an organ system, like digestive system or circulatory system. Even though every cell (but gametes) has the same DNA, specialization occurs because parts of the DNA in tissues are turned on or off by protein messengers.