CH.YBW.CTS.C08.Final.q 6/21/03 1:11 PM Page 107
Skin Derivatives 107
responsible for producing hair. The outer layer is composed of dermal connective tissue. It provides blood vessels and physical reinforcement. The inferior end (bottom) of a hair follicle is enlarged, forming a structure called the hair bulb. Each hair bulb is wrapped by a knot of sensory nerve endings, the hair root plexus. This anatomical arrangement allows hair to act as a sensitive touch receptor permitting us, for instance, to feel insects crawling on our skin (hopefully before they have a chance to sting or bite us). Hair is formed by division of cells in the matrix , a growth zone located in the hair bulb. The matrix displays active mitosis because it is continuous with the stratum basale, the epidermal layer capable of cell division. The newly formed cells are nourished by blood vessels located in the papilla , which is an indentation of the dermal connective tissue at the base of the follicle. As daughter cells continue to divide, they are pushed farther away from the growing region and also become keratinized. Shortly thereafter they die. Thus, the bulk of hair is composed of non-living cells. Hair growth depends on several factors, including nutritional status, gender, and age, as well as circulating levels of some hormones. For instance, poor nutrition will result in poor hair growth. In addition, the hormone testosterone encourages growth of hair. Although scalp hair typically grows an average of 2 millimeters per week, each follicle goes through a series of growth cycles. Initially, there is an active phase, which generally lasts 2 to 6 years. This is followed by a resting phase, where the follicle is inactive for several months. Following the resting phase, the matrix proliferates again, forming a new hair that will replace the old one, which has already fallen out or will be pushed out. Because follicles generally spend more time in an active phase, we only shed about 90 hairs from our scalp each day. Interestingly, the active phase for eyebrow hair is only three