Besides the term hair follicle is a common name. This term refers to all anatomical structures that are involved in anchoring the hair in the skin surrounding the place of origin of hair formation.
Anatomy and Function
Simplified, one can imagine the hair follicle as a thread-like invagination in the skin, which is surrounded by special concentric cell layers. These layers comprise an outer epithelial hair root sheath, also called stratum basale, a unicellular cell layer, the so-called companion layer, and an inner epithelial hair root sheath, which in turn can be divided into three further layers. These layers are called the Henle layer, the Huxley layer and the cuticle.
The bottom of the hair follicle is formed by the hair bulb, also called hair bulb, while the roof of the hair follicle is called hair funnel or infundibulum. The entire hair follicle is surrounded by a layer of connective tissue, which embeds it in the surrounding tissue and makes it movable in relation to it. The outer epithelial hair root sheath consists of several cell layers, which are very rich in glycogen and provide important nutrients for hair growth.
These cells are unkeratinized in the area of the entire hair follicle. At the seamless transition to the epidermis, the epidermis, the cells of this layer are now keratinized. The unkeratinized “companion layer” is connected to the already keratinized Henle layer via numerous desmosomes, adhesive contacts between the cells.
The “Companion layer” glides together with the entire inner epithelial hair root sheath towards the surface of the outer epithelial hair root sheath, where it is ground off and the hair is released. This complex of layers serves the hair as an anchorage in the skin, since the side of the cuticle facing the hair has barb-like structures on its surface. The hair bulb is slightly widened like a flask and contains the so-called matrix cells and melanocytes.
Matrix cells are cells that divide very readily, which are initially unkeratinized and in a later keratinized state make up the majority of the hair. Melanocytes are cells that produce pigment and define the hair color. This pigment is released to the matrix cells, which means that it reaches the tip of the hair during hair growth.
The hair funnel represents the opening of the hair bellows for the hair. Through this it reaches the surface and can grow unhindered. A sebaceous gland and a sweat gland open into the hair funnel. Just below the sebaceous gland, a small muscle on the connective tissue root sheath inserts, which can raise the hairs of the body to goose bumps.
