Most pigment cells during embryogenesis arise from the cranial or truncal portion of the neural crest and migrate to the skin, hair bulbs, choroid of the eye, the inner ear, leptomeninges, and other tissues. Cells of the retinal pigment epithelium come from a different source, namely, the primitive forebrain, and are involved in the formation of the retina and the optic nerves and tracts. Most pigment cells in all parts of the body seem to be constant in number and function until approximately middle age (the fourth or fifth decade of life). Thereafter, the number of melanocytes in the skin, hair, and eyes and the number of nevi begin to decrease. One function of pigment cells may be to eradicate oxygen radicals that are responsible in part for inducing malignancies and are also involved in the aging process. Possibly one result of the loss of melanocytes from the various organs is acceleration of the aging process in a permissive environment for the development of malignancies.