Hairs

Hairs

Hairs (pili) project above the skin surface over most of the body, except for the sides and soles of the feet, the palms of the hands, the sides of the fingers and toes, the lips, and parts of the external genitalia. They begin to form during embryologic development and are also known as epidermal derivatives because they arise from the epidermis. There are about 2.5 million hairs on the human body, of which over 75% is on the general body surface and not the head. Hairs are structures produced in organs called hair follicles (FIGURE 6-6). They consist of a large amount of dead keratinized cells, dominated by hard keratin. Hair follicles extend from the skin surface into the dermis, containing hair roots that are nourished with dermal blood. Each hair follicle is attached to an arrector pili muscle, which helps the hair shaft (in which keratinization is complete) to stand on end when it contracts. This occurs during emotional upset and cold temperatures, causing gooseflesh or goose bumps. Hairs are pushed upward as epidermal hair cells divide and grow, becoming keratinized and then dying.

Each hair follicle is folded from the epidermal sur-face into the dermis. They may extend into the hypo-dermis of the scalp. Each follicle originates at about 4 mm below the skin surface, expanding to form a hair bulb. A root hair plexus or hair follicle receptor con-sists of a cluster of sensory nerve endings, ­wrapping around each hair bulb. When the hair is bent, these endings are stimulated, meaning that hairs act as touch receptors with extreme sensitivity. Nipple-like dermal tissue makes up a hair papilla, which protrudes into each hair bulb. It contains a knot of capillaries that give nutrients to the growing hair. A fibrous peripheral connective tissue sheath makes up the wall of a hair fol-licle. This derives from the dermis. The other compo-nents of the hair follicle wall are the thickened basal glassy membrane and the inner epithelial root sheath. This sheath becomes thinner as it approaches the hair bulb, with only one layer of epithelial cells covering the papilla.

The hair matrix is the actively dividing part of the hair bulb that produces the hair. It originates in the hair bulge, just a small portion of 1 mm above the hair bulb. Chemical signals that reach the hair bulge cause certain cells to move to the papilla, divide, and produce new hair cells. The older part of each hair is then pushed upward, with the fused cells getting more keratin and dying.

Each hair has three concentric layers known as the medulla, cortex, and cuticle. The central core of a hair is the medulla. It is made up of air spaces and large cells. The medulla is the only hair portion that contains soft keratin. It does not exist in fine hairs.

Surrounding­ the medulla is the bulky cortex, made up of a few layers of flat cells. A single layer of cells forms the outermost cuticle of a hair. In the cuticle, cells are overlapped like shingles, which helps keep each hair shaft from matting with others. Most hard keratin is in the cuticle, which provides strength to the hair shaft and keeps the inner layers compacted tightly together. The ­cuticle experiences the most abrasion and usually­ wears away at the tip. This causes what is commonly known as split ends, in which fibrils of keratin from the cortex and medulla become split. The rough ­surfaces of a hair cuticle are smoothed by hair conditioning products, which give a shiny appearance to a person’s head of hair.

Hair color is reflected by genetics and variations in the pigment produced by melanocytes at the hair papilla. Darker hair has more eumelanin (which is brownish-black), whereas lighter hair has more phe-omelanin (which is reddish-yellow). The different forms of melanin give hair a wide variety of shades, ranging from dark brown to yellow-brown to red. Albinos have white hair because their hair shafts com-pletely lack melanin. Hormonal and environmental factors also influence the hair’s condition. As pigment production decreases with age, hair color lightens. White hair results from a lack of pigment along with the presence of air bubbles in the medulla of the hair shaft. As the proportion of white hairs increases, the overall hair color is described as gray.

Hairs are basically classified as either vellus or terminal. Adult women and newborn children have body hair known as vellus hair, which is fine and pale in color. Terminal hair is coarser and longer, found on the scalp and eyebrows, and often darker than vellus hair. Terminal hairs appear in the axillary and pubic regions of males and females during puberty. On males at puberty, they also appear on the face, chest, and usually the arms and legs. These hairs are stimulated to grow by androgens (primarily tes-tosterone). Large amounts of male hormones cause thick terminal hair growth. Nutrition and hormones influence hair density and growth. Chronic physical inflammation or irritation may cause increased local hair growth. Hair growth that is not cosmetically attractive may be slowed or stopped by electrolysis or laser treatments.

Hair grows at an average rate of 2.5 mm per week, but this varies with sex, age, and body regions. Growth cycles occur in each hair follicle, including an active phase that ranges from weeks to years and a regres-sive phase when hair matrix cells die. The hair folli-cle base and hair bulb then shrink, causing the hair papilla to be moved upward to touch the part of the follicle that does not regress. Then, a 1–3-month rest-ing phase occurs. After this, the cycling area of the follicle is regenerated. Activated bulge cells migrate to the papilla. The matrix is then able to form a new hair. Many proteins control a hair’s life span, and scalp hair follicles are active for up to 10 years before being inac-tivated for several months. Approximately, 90 scalp hairs are lost per day. The eyebrows never reach the length of the scalp hair because each eyebrow follicle is active for only 3–4 months.

Each hair follicle of the body has a limited num-ber of growth cycles, with growing being fastest between the teenage years and the 40s. Hair thinning occurs after this time, because shedding happens more quickly than hair replacement. Both sexes, beginning in middle age, experience a certain degree of hair thinning and/or alopecia (baldness), but it is more commonly seen in men. By age 35, approx-imately 40% of men have visible hair loss, and by age 60 the percentage is about 85%. Scalp hair loss usually begins at the anterior hairline, progressing posteriorly. The hair becomes thinner as vellus hairs being to replace the coarser terminal hairs. However, true baldness (most commonly, male pattern bald-ness ) is not the same situation. It is sex influenced and genetically determined, linked to a gene that activates during adulthood and changes how the hair follicles respond to dihydrotestosterone, which is a metabolite of testosterone.

1. Describe the mechanism that causes hairs to “stand on end” and also causes goose bumps.

2. Explain the tissue that comprises the hair papilla.

3. Describe the hair follicles, the root hair plexus, and the hair matrix.