Cells lose their ability to divide over time unless they become cancerous. This limit to cellular replicative capacity (Hayflick's limit or phenomenon) can be demonstrated in fibroblasts removed from the umbilical cord of newborns and cultured in vitro. The fibroblasts divide only until they are dense enough to contact each other--a phenomenon called contact inhibition. If diluted, the fibroblasts divide again until maximum density is reached. Studies have shown that the loss of replicative capacity does not depend on the total amount of time cells are cultured (chronologic age) but on the number of divisions (biologic age).