a hormone that stimulates the growth and division of cells in tissue culture. The term was invented in 1922 by the French pathophysiologist A. Carrel, who noticed that fibroblastic growth was stimulated by leukocytes, as well as by embryo and tumor cells. The term later became less common, since substances differing in chemical composition were discovered to be capable of stimulating cell growth.
At the present time, many factors contained in extracts made from leukocytes are known to be capable of stimulating the growth of tissue cells in artificial media. One such factor causes lymphocytes to become lymphoblasts that subsequently proliferate. A second such factor, isolated from monocytes, stimulates the formation of colonies of granular leukocytes in a lethally irradiated organism. This factor, which has a molecular weight of about 30,000, was discovered to be a lipoprotein containing neuraminic acid. Thus, what was thought to be a trephone is actually a large group of substances, each with its own name.