Growth factor receptors. Illustration of growth factors (yellow and purple) binding to receptors (green and blue) embedded in the membrane of a cell (lower left). The growth factors have been released by other cells (upper right and background). Growth factors are chemicals that stimulate cellular growth, differentiation and maturation, as well as other cellular processes. Growth factors are typically proteins (such as cytokines) or steroid hormones. The phospholipid bilayer structure of the cell membrane is shown, along with the trans-membrane structure of the receptors.