The condition in which the entire body of an adult animal or plant consists of a fixed number of cells that is the same in all members of the species. This phenomenon is also called eutely. The largest group of animals exhibiting eutely are the nematode worms, one of the largest of all animal phyla, and of great medical and agricultural importance as parasites of plants, animals, and humans. A plant that exhibits eutely is usually called a coenobium. Many species of semimicroscopic aquatic green algae exist as coenobia, such as the common Volvox and Pandorina.
Numerical limitation occurs in certain organs and organ systems, notably the brain and muscles of annelid worms, mollusks, and vertebrates. A related but different phenomenon, observed for many animal cells when cultured, is that normal cells divide some specific number of times and then stop dividing. Thus the life-span, as measured by number of cell cycles, is limited; for many human cell types this is about 50 cell generations.
In annelids and vertebrates, cell proliferation is more or less continuous throughout life only in those tissues that are subject to wear. Thus, in adults, cell division may be found in the germinative zones of the skin, hair, finger and toe nails, the lining of the alimentary canal, and especially in the blood cell-forming tissues. The muscles and nervous system, however, appear to undergo no cell division after early embryonic or fetal stages. In both earthworms and mammals, including humans, it has been demonstrated that the number of muscle nuclei and muscle fibers, but not fibrils, is fixed early and does not increase with subsequent growth. An earthworm hatches from its egg cocoon with the adult number of muscle fibers and nuclei. A human fetus, about 5 in. (13 cm) from crown to rump, has as many muscle fibers and nuclei as an adult. It has been shown that the number of glomeruli in each kidney of a rat or human, and therefore presumably of any mammal, is fixed before birth, and that the subsequent growth of the glomeruli, either normally or resulting from compensatory hypertrophy after unilateral nephrectomy, is due entirely to the enlargement of cells already present. The same holds true for the cells of the ciliated nephrostomes of earthworms.