Hair Follicles: Structure, Function & Diseases

The hair follicle refers to the structures that surround the hair root. The hair follicle serves to anchor the hair in the skin.

What are hair follicles?

The hair follicle is the body structure that surrounds the human hair root. It also bears the name hair follicle. Human hair is composed of keratinized skin cell layers. Like nails, hair also belongs to the skin appendages. Important components of the hair are the hair shaft, the hair bulb and the hair root. The hair shaft is the visible section of the hair that protrudes from the skin. Anchored in the skin is the hair root, which extends into the dermis and sometimes into the subcutis. In the hair bulb, a bulb-like thickening, the hair root finds its end. The root is located in the hair follicle, which forms the root sheath. Following the hair follicle is a sebaceous gland, one of whose functions is to lubricate the hair. The number of hair follicles varies depending on the region of the body. Most follicles are present on the forehead. Thus, there are about 1200 hair follicles per square centimeter there.

Anatomy and structure

The human hair follicle is composed of two layers. These are the outer epithelial hair root sheath and the inner epithelial hair root sheath, which surround the hair follicle along its length. The outer hair root sheath can be classified as a continuation of the stratum germinativum epithelii (stratum basale epithelii) into the hair funnel. The hair funnel is a depression in the skin, which has the shape of a funnel. It is found in the area where the hair emerges from the skin. The outer hair root sheath surrounds the hair region that runs inside the skin. The same applies to the hair root. In addition, it serves as an envelope for the inner epithelial hair root sheath. This has small horny scales that align against the hair root, which fixes the growing hair in the follicle. The inner hair root sheath surrounds the hair shaft and the hair root in the hair follicle. The matrix cells, which are located at the outer hair root edge, are responsible for the descent of the layer. The inner epithelial hair root sheath is composed of the cuticle, the Huxley layer and the Henle layer. While the cuticle forms the inner layer, the Henle layer marks the outer layer. The Huxley layer is the middle section of the inner hair root layer. Cornification also occurs in the cells of the inner epithelial hair root layer, firmly attaching the hair in the hair follicle. While the horny scales are directed towards the hair root, the hair scales, on the other hand, point towards the hair tip. In this way, they interlock. Within the hair follicle, several glands open. These produce and excrete sebum as well as other substances such as fragrances.

Function and tasks

Among the tasks of the hair follicle is the production of hair. Already in the 6th week of pregnancy, the creation of about five million hair follicles occurs. However, after the birth of the human being, no new follicles are created. Moreover, not every hair follicle actually grows a hair. However, the follicle has the ability to produce several types of hair in the course of life. The development of hair follicles on a man’s face does not begin until puberty, which eventually results in the growth of beard hair. In addition, a man’s pubic hair develops during this period. The hair follicle also provides anchorage for the hair, giving it a firm hold. Furthermore, the maturing of some glands takes place in the hair follicle. These produce sebum or substances such as fragrances. Under the sebaceous glands of the hair follicle are located the musculi arrectores pilorum, which are the hair bellows muscles. They have the ability to erect the hair of the person, which is noticeable by the appearance of the so-called goose skin. In the hair follicle also end some fine nerve fibers. They are important for the sense of touch and control the hair bellows muscles, which are sympathetically innervated.

Diseases

One of the most common impairments of the hair follicle is hair folliculitis. It is also called hair folliculitis. If the inflammation appears on the beard hair, it is called folliculitis barbae.Typical symptoms of folliculitis are reddened skin, swelling, severe itching and the formation of pustules. In addition, an abscess may form. The disease begins with a mild inflammation of the hair follicle. In the further course, painful nodules often form, which are filled with pus. Furthermore, the hair root inflammation can spread from one hair follicle to the next. The center of the pustules is typically penetrated by a hair. However, folliculitis is a harmless disease. Treatment is usually only necessary if the inflammation spreads to the entire hair follicle or if a furuncle or carbuncle or abscess develops. Inflammation of the hair follicle is caused by bacteria such as staphylococci. These occur naturally on human skin. Due to intense sweating or acne, there is a risk that the hair follicle closes due to sebum or dead skin cells, providing favorable conditions for the germs to multiply. In some cases, however, folliculitis is also triggered by weaknesses in the immune system or certain pre-existing conditions such as diabetes mellitus.