Hair Follicles And Hair

Each hair follicle represents an invagination of the epidermis in which a hair is formed

Hair follicles and hairs are present over almost the entire body; they are absent only from the sides and palmar surfaces of the hands, sides and plantar surfaces of the feet, the lips, and the region around the urogenital orifices. Hair distribution is influenced to a considerable degree by sex hormones; these include, in the male, the thick, pigmented facial hairs that begin to grow at puberty and the pubic and axillary hair that develops at puberty in both genders. In the male, the hairline tends to recede with age, and in both genders, the scalp hair thins with age because of reduced secretion of estrogen and estrogenlike hormones.

The hair follicle is responsible for the production and growth of a hair. Coloration of the hair is due to the content and type of melanin (see page 409) that the hair contains. The follicle varies in histologic appearance, depending on whether it is in a growing or a resting phase. The growing follicle shows the most elaborate structure; thus, it is described here. The hair follicle is divided into three segments:

• Infundibulum, which extends from the surface opening of the follicle to the level of the opening of its sebaceous gland. The infundibulum is a part of the pilosebaceous canal that is used as a route for the discharge of sebum.

• Isthmus, which extends from the infundibulum to the level of insertion of the arrector pili muscle.

• Inferior segment, which in the growing follicle (Fig. 14.14) is of nearly uniform diameter except at its base, where it expands to form the bulb. The base of the bulb is invaginated by a tuft of vascularized loose connective tissue called, not surprisingly, a dermal papilla.

Other cells forming the bulb, including those that surround the connective tissue papilla, are collectively referred to as the matrix, which consists simply of matrix cells. Matrix cells immediately adjacent to the dermal papilla represent the germinative layer of the follicle. Division and proliferation of these cells accounts for the growth of the hair. Scattered melanocytes are also present in this germinative layer. They contribute melanosomes to the developing hair cells in a manner analogous to that in the stratum germinativum of the epidermis. The dividing matrix cells in the germinative layer differentiate into the keratin-producing cells of the hair and the internal root sheath. The internal root sheath is a multilayered cellular covering that surrounds the deep part of the hair. The internal root sheath has three layers:

• The cuticle, which consists of squamous cells whose outer free surface faces the hair shaft.

• Huxley's layer, which consists of a single or double layer of flattened cells that form the middle plate of the internal root sheath.

• Henle's layer, which consists of an outer single layer of cuboidal cells. These cells are in direct contact with the outermost part of the hair follicle, which represents a downgrowth of the epidermis and is designated the external (outer) root sheath.

epidermis dermis eccrine gland arrector pili muscle sebaceous gland internal root sheath f cuticle hair j cortex [medulla

apocrine gland blood vessel matrix connective tissue papilla

Hair Loss Prevention

Hair Loss Prevention

The best start to preventing hair loss is understanding the basics of hair what it is, how it grows, what system malfunctions can cause it to stop growing. And this ebook will cover the bases for you. Note that the contents here are not presented from a medical practitioner, and that any and all dietary and medical planning should be made under the guidance of your own medical and health practitioners. This content only presents overviews of hair loss prevention research for educational purposes and does not replace medical advice from a professional physician.

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