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## 6.2b Hair ### Learning Objectives - Describe the structure, distribution, and function of hair. - Describe how hair changes throughout life (i.e., the hair growth cycle). ### Hair Hair is found almost everywhere on the body except the palms of the hands and palmar surface of the fingers, the...

## 6.2b Hair ### Learning Objectives - Describe the structure, distribution, and function of hair. - Describe how hair changes throughout life (i.e., the hair growth cycle). ### Hair Hair is found almost everywhere on the body except the palms of the hands and palmar surface of the fingers, the sides and soles of the feet and toes, the lips, and portions of the external genitalia. The general structure of hair and its relationship to the integument are shown below. ### Hair type and distribution A single hair, or pilus, has the shape of a slender filament; it is composed of keratinized cells growing from hair follicles that extend into the dermis, and often deeper into the underlying subcutaneous layer. Differences in hair density are due primarily to differences in the texture and pigmentation of the hair. ### Figure 6.10 Hair - Hair is a derivative of the epithelium, a hair grows from a follicle extending from the epidermis into the dermis. - Photomicrographs of a hair follicle - SEM of a hair emerging from its follicle. - **(a)** - Cuticle - Medulla - Cortex - Hair follicle - Connective tissue root sheath - Epithelial tissue root sheath - Matrix - Hair papilla - Root - Hair bulb - Arrector pili muscle - **(b)** - Shaft - Hair papilla - **(c)** - Connective tissue root sheath - Epithelial tissue root sheath - Hair follicle - Medulla - Cortex - Matrix - Hair bulb ### What did you learn? Why is the nail body darker in color, while the free edge of the nail is lighter colored? Most of the nail body appears darker or pinkish because of the blood flowing in the underlying capillaries; the free edge of the nail appears lighter in color because there are no underlying capillaries. At the nail root and the proximal end of the nail body, the nail bed thickens to form the ***nail matrix***, which is the actively growing part of the nail. The ***lunula*** (lūnū-lă: luna = moon) is the whitish, semilunar (semicircular) area of the proximal end of the nail body. It has a lighter-colored appearance because a thickened stratum basale obscures the underlying blood vessels. Along the lateral and proximal borders of the nail, folds of skin called ***nail folds*** overlap the nail. The ***eponychium*** (ep-ō-nik'ē-ŭm; onyx = nail), also known as the ***cuticle*** (kū'ti-kl), is a narrow band of epidermis extending from the margin of the nail wall onto the nail body. The ***hyponychium*** (hi-pō-nik'e-um) is the area of thickened epithelium underlying the free edge of the nail.