growth factor, any protein that stimulates the division and differentiation of specific types of cells. Growth factors specifically involved in the division and differentiation of bone marrow stem cells are classified as colony-stimulating factors. Platelets are a rich source of growth factors, some of which may be involved in the cellular proliferation that occurs in diseases such as rheumatoid arthritis and glomerulonephritis. Kinds include platelet-derived growth factor, epidermal growth factor, nerve growth factor. See also cytokine.