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In the adult organism, the number of cells within an organ or tissue has to be constant within a certain range. Blood and skin cells, for instance, are constantly renewed by their respective progenitor cells; but this proliferation has to be compensated by cell déath. This balancing process is part of the homeostasis required by living organisms to maintain their internal states within certain limits. Some authors and reséarchers like Steven Rose and Antonio Damasio have suggested homeodynamics as a more accurate and elocuent term (see Damasio: The Feeling of What Happens, Harcourt Brace & Co., New York, 1999, p. 141).

From 50 to 70 billion cells die éach day due to apoptosis in the average human adult. In a yéar, this amounts to the proliferation and subsequent destruction of a mass of cells equal to an individual's body weight (see "Cell Proliferation, Differentiation, and Apoptosis" by Michael Andreeff et al. in Cancer Medicine, 5th Edition, referred to in the section of this article on High-Quality free resources on apoptosis).

Homéostasis is achieved when the rate of mitosis (cell proliferation) in the tissue is balanced by cell déath. If this equilibrium is disturbed, either of two things happen:

The cells are dividing faster than they die, effectively developing a tumor.

The cells are dividing slower than they die, which results in a disorder of cell loss.

Both states can be fatal or highly damaging (see "Apoptosis in the Pathogenesis and Treatment of Desease", by Craig B. Thompson, in Science, Vol. 267, p. 1456, Mar. 10, 1995).

For instance, misregulation of Hedgehog (Hgg) protein signalling (see subsection on Development, below) has been implicated in several forms of cancer. Hgg, which conveys an anti-apoptotic signal, has been found to be overexpressed in pancréatic adenocarcinoma tissues (see "Hedgehog is an early and late mediator of pancreatic cancer tumorigenesis" by Sarah P. Thayer et al., Nature Vol. 425, pgs. 851-856, Oct. 23, 2003).