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# Hair We produce three kinds of hair during our lives: lanugo, vellus, and terminal hair. ## Hair Types - **Lanugo:** A fine, unpigmented downy hair that first appears on the fetus in the last trimester of development. - **Vellus:** A primary human hair that is similarly fine, unpigmented, or...

# Hair We produce three kinds of hair during our lives: lanugo, vellus, and terminal hair. ## Hair Types - **Lanugo:** A fine, unpigmented downy hair that first appears on the fetus in the last trimester of development. - **Vellus:** A primary human hair that is similarly fine, unpigmented, or lightly pigmented. It is found on the upper and lower limbs. - **Terminal Hair:** Usually coarser, pigmented, and longer than vellus. It grows on the scalp, eyebrows, and eyelashes. At puberty, it replaces vellus hair in the axillary and pubic regions. Under the influence of testosterone, it will form a beard on the face. ## Hair Structure and Follicles Three zones can be recognized along the length of a hair: 1. **Hair Bulb:** A swelling at the base where the hair originates in the dermis. It consists of epithelial cells and surrounds a small hair papilla. 2. **Hair Root:** The zone of the hair extending from the bulb to the skin surface. 3. **Hair Shaft:** The third and final portion of the hair that extends beyond the skin surface. Hair production involves a specialized type of keratinization that occurs within the **hair matrix**, a structure immediately adjacent to the hair papilla in the hair bulb. Epithelial cells near the center of the hair matrix divide, producing new cells that are gradually pushed toward the surface. - **Medulla:** The soft core of the matrix. It is composed of loosely arranged cells and air spaces, and it contains flexible, soft keratin. - **Cortex:** Several layers of flattened cells closer to the outer surface of the developing hair form the relatively hard cortex. - **Cuticle:** A single cell layer around the cortex forms the cuticle, which coats the hair. The **hair follicle** is an oblique tube that surrounds the hair root. It always extends into the dermis and sometimes into the subcutaneous layer. The cells are organized into two concentric layers. - **Connective Tissue Root Sheath:** Originates from the dermis. - **Epithelial Tissue Root Sheath:** Originates from the epidermis. ## Sensory Reception Hair follicles have associated tactile receptors (root hair plexuses) that detect light touch (see section 16.2a). ## Visual Identification Hair characteristics are important in determining age and in identifying individuals. ## Hair Color Hair color is determined by the melanin that is synthesized in the matrix adjacent to the hair papilla. Variations in hair color reflect genetically determined differences in the structure of the melanin. - **Environmental factors:** May influence the color of the hair. - **Production of Pigment:** Decreases as a person ages, making hair lighter in color. - **Gray Hair:** Results from a gradual reduction of melanin production within the hair follicle. - **White Hair:** Occurs due to a complete stoppage of melanin production. ## Hair Growth and Replacement There are three stages of the hair growth cycle: anagen, catagen, and telogen: 1. **Anagen Phase:** Active phase of growth where living cells of the hair bulb are rapidly growing, dividing, and transforming into hair. It is the longest part of the growth cycle. It lasts for about 18 months to as much as 7 years, depending on the body location of the hair (e.g., scalp, eyebrows) and the genetics of the person. - Each hair strand grows about one-third of a millimeter per day, which equals 0.5 to 1.0 cm per month. - On a normal scalp, 80-95% of follicles are in the anagen phase. 2. **Catagen Phase:** A brief regression period where cell division ceases and the follicle undergoes involution. This very short phase lasts for about 3 to 4 weeks. 3. **Telogen Phase:** The resting phase. It is the phase when the hair is shed. After 3 to 4 months in the telogen phase, the cells of the hair bulb start regrowing, and the follicle re-enters the anagen phase. ## Functions of Hair The millions of hairs distributed on the surface of the human body have important functions: - **Protection:** Hair on the head protects the scalp from sunburn and injury. Hair within the nostrils entraps particles and prevents their entry deeper into the respiratory system. Hair within the external ear canal protects the ear from insects and foreign particles. Eyelashes protect the eyes. - **Heat Retention:** Hair on the head prevents the loss of conducted heat from the scalp to the surrounding air. Individuals who have lost their scalp hair release much more heat through the scalp than those who have a full head of hair.