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From Martin and Sailer (1962), by permission.

Chapter 11 Skin and Hair Embryology and Hair Types

The hair follicles develop from ectoderm and mesoderm (Fig. 11.3). They first appear in the third month of gestation, and continue to appear through the sixth month. The developing hair follicle grows downward from the epidermis and into the dermis. The deepest part of this hair bud will form the hair bulb. The hair bulb is invaginated by a hair papilla from the mesoderm. The peripheral cells of the hair follicle form the epithelial root sheath, and the surrounding mesenchymal cells differentiate into the dermal root sheath. The epithelial cells of the hair bulb produce the germinal matrix, which proliferates and pushes upward to become keratinized and form the hair shaft. Melanoblasts migrate into the hair bulb, and after differentiating into melanocytes they produce melanin. Hair is first recognizable at the eyebrow, upper lip, and chin areas at about the 20th week of gestation.

The first hair, called lanugo, is fine and colorless, and is usually lost during the perinatal period. It will be replaced by coarser hair, the vellus. This persists over most body areas except the axillary and pubic region, where it is replaced at puberty by coarse terminal hair. In males similar terminal hair appears on the chest and the face. From the mesenchyme around the hair bulb small bundles of smooth muscle form the erector pili muscle, which can make the hair rise off the surface of the skin.

All the hair follicles are present in the fetus. The distribution of hair in later life is mainly due to the difference in growth at the skin surface. Hair color, texture, and distribution need to be examined and recorded, particularly when an ectodermal dyplasia is suspected. The pattern of hair follicle distribution is discussed in Chapter 12.

Hair Color

Hair color depends on the degree of pigmentation. It can vary in different body areas; thus it is useful to record the color of eyebrow and eyelash hair as well as that of scalp and beard.

The hair color can be recorded by describing the shades of color. Examples are black, black-brown, dark-brown, red-brown, light-brown, dark-blond, fair, light, red, and albino. It can also be compared with hair color tables, as used by anthropologists (see Bibliography). Hair color can, of course, be altered by dyeing. The hair color charts for various hair dyes are also useful for recording the color. A sample of hair can also be taken and kept in an envelope in the patient's chart. The accurate color is preserved in such samples if they are not exposed to light.

Hair Texture

Texture of the hair can vary greatly between individuals, even of the same ethnic group (Fig. 11.6). The twisting of a single hair shows great variance along its shaft. Physiologically significant differences in hair structure of various body areas also exist. Body hair is usually curlier than scalp hair. Curly hair is flatter in cross-section compared with straight hair, which is round. The major designations for hair texture also straight, wavy, curly, or narrow curls, but it can be further described as fine, coarse, wiry, stiff, flexible, and so on. Pathological structural anomalies of the hair can best be seen with the microscope (see Bibliography).

Figure 11.6 Normal hair texture forms. From Martin and Saller (1962), by permission.

Straight

Hypopline Slightly wavy Wavy Curly

Small curls

Figure 11.6 Normal hair texture forms. From Martin and Saller (1962), by permission.

Narrow curls

Chapter 11 Skin and Hair Balding

Balding occurs as part of the physiological process of aging. Timing of male baldness depends on the influence of androgens, age, and genetic predisposition. It usually occurs first in the frontoparietal area. This distribution is called "male-pattern" balding.

Female baldness usually occurs later, and the loss of hair is diffuse rather than starting with frontoparietal recession. Female balding is due to random atrophy of follicles and is associated with a decrease in circulating estrogen.

Alopecia

There are various pathological forms of hair loss, the most common is alopecia, which is a premature loss of hair. It can be diffuse or circumscribed, and can involve areas of the scalp or the whole body. It can also be transient or permanent.

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Hair Loss Prevention

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