Mesenchymal stem cell. Coloured scanning electron micrograph (SEM) of a human mesenchymal stem cell (MSC). MSCs are multipotent stromal (connective tissue) cells that can differentiate into a variety of cell types, including osteoblasts (bone cells), chondrocytes (cartilage cells), and adipocytes (fat cells). The youngest, most primitive MSCs can be obtained from the umbilical cord tissue. Magnification: x4500 when printed 10 centimetres wide. Credit: STEVE GSCHMEISSNER/SCIENCE PHOTO LIBRARY