Following mitosis, the cytoplasm of a cell divides in two, as seen in these scanning electron micrographs (a. 3,750x; b. 3,750x; c. 3,190x).

From Scanning Electron Microscopy in Biology, by R. G. Kessel and C. Y. Shih. © 1976 Springer-Verlag.

produce this growth factor. This is why an animal's licking a wound may speed healing.

Growth factors are used as drugs. Epidermal growth factor (EGF), for example, can hasten healing of a wounded or transplanted cornea, a one-cell-thick layer covering the eye. Normally these cells do not divide. However, cells of a damaged cornea treated with EGF undergo mitosis, restoring a complete cell layer. EGF is also used to help the body accept skin grafts and to stimulate healing of skin ulcers that occur as a complication of diabetes.

Space availability is another external factor that influences the timing and rate of cell division. Healthy cells do not divide if they are surrounded by other cells, a phenomenon called contact (density dependent) inhibition.

H How do cells vary in their rates of division?

Which factors control the number of times and the rate at which cells divide?

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