CH.YBW.CTS.C08.Final.q 6/21/03 1:11 PM Page 112
112 CELLS, TISSUES, AND SKIN
Figure 8.2 Nails are composed of hardened stratified squamous epithelial cells. The different parts of the nail are shown in the diagram here. Nails form in the nail root and grow along the nail bed. The cuticle is comprised of the proximal nail fold, and the white, crescent-shaped area is called the lunula.
homeostasis, and the integumentary system is no exception. Recall that skin is able to synthesize vitamin D when exposed to sunlight. However, this form of vitamin D does not have significant biological activity. Rather, it must be metabolized into an active hormone, first by a chemical modification (hydroxylation reaction) in the liver, followed by a second hydroxylation in the kidneys. Vitamin D promotes absorption of dietary calcium by the small intestine. In turn, calcium is necessary for the proper formation of bones and teeth, as well as for blood clotting and the normal function of nerve and muscle tissues. Thus, by providing the body with vitamin D, the integumentary system is linked to the activity of the digestive, skeletal, muscular, nervous, cardiovascular, and renal systems.