Coloured scanning electron micrograph (SEM) of a network of mesenchymal (red) and vesicular (brown, bottom right) stem cells that have grown three dimensionally in a gel culture. These cells are derived from human umbilical vein endothelial cells (HUVECs) and are still in the early stages of development. The mesenchymal cells have differentiated into fibroblasts, cells that produce connective tissue such as collagen, the main structural protein in the body. The vessel-forming cells are growing as a single layer of cell. HUVECs are able to differentiate into any epithelial cell type. The picture was created as part of a cooperation with Quintessenz Publishing. Magnification: x1,200 when printed at 15cm wide.