During embryonic development, cells differentiate very rapidly, becoming increasingly specialized, with the rate of proliferation decreasing over time. Some, such as human cardiac muscle cells, are then not able to divide again, while others are required to replace lost cells (e.g., epithelial cells, skin fibroblasts) via, for example, apoptosis. All told, cell proliferation amounts to a careful balance with cell death so that in adulthood there is a constant number of cells in tissues and organs..
See Apoptosis (or cell death), Cell, Cell theory, Differentiation (embryology), Epithelium, Fibroblasts, Mitosis, Olfaction. Progenitor cells, Stem cells