Quick Facts
The hair follicle is one of the tubular invaginations of the epidermis that enclose the hairs, and from which they grow. It is divided into upper and lower segments: the upper comprises the follicular infundibulum (extending from the free surface to the sebaceous gland) and the isthmus (extending from the sebaceous gland to the arrector pili muscle); the lower comprises the hair bulb and the part sometimes referred to as the stem. In the lower part, the hair cuticle is surrounded by the inner and outer root sheaths, enclosed within a fibrous dermal sheath (Dorland, 2011).
Structure/Morphology
The hair follicle consists of a hair shaft and bulb. It is a down growth of the epidermis, with its long axis usually traveling obliquely through the skin layers. The hair follicle can extend as far as the hypodermis; however, it can also be superficial in the reticular layer of the dermis. A membrane, known as the glassy membrane, separates the dermis from the epithelium of the hair follicle.
In histological sections, hair follicles may have different appearances. This is because the hair undergoes cycles of growth and loss. In the anagen phase, the hair is actively growing, and the follicle is at is maximal extent of development. During the catagen phase, hair growth ceases and the hair follicle shrinks. During a resting period, known as the telogen phase, the inferior portion of the follicle is missing. This phase leads back to the anagen phase and the follicle will regenerate to its full extent (Standring, 2016).
The follicles here depict the anagen follicle and it is composed of several parts. The most inferior segment is the hair bulb, which extends from the base of the hair follicle to the follicular bulge, to which the arrector pili muscle attaches. The bulb contains a dermal papilla that inserts centrally to the bulb. Cells similar to that of the stratum basale of the epidermis line the dermal papilla forming the hair matrix. It contains large melanocytes that supply melanin, or pigment, to the keratinizing cells of the hair matrix, thus determining hair color. Cells in the dermal papilla divide and differentiate, giving rise to the cortex, medulla, and cuticle of the hair. Peripheral cells in the bulb contribute to the internal and external root sheaths of the hair follicle.
The bulb of the hair extends to the arrector pili muscle at the follicular bulge. Between the arrector pili muscle and the entry site of the duct from the sebaceous gland is the isthmus, that leads to the pilary canal, or infundibulum. The pilary canal holds the hair and also forms the route for the discharge of sebum secreted by the sebaceous glands. Below the sebaceous duct, the hair and the follicular wall are intimately connected, however, above the duct, the hair becomes free in the intraepidermal pilary canal (Standring, 2016).
Function
The hair follicle is responsible for the production, growth and maintenance of hair.
References
Dorland, W. (2011) Dorland's Illustrated Medical Dictionary. 32nd edn. Philadelphia, USA: Elsevier Saunders.
Standring, S. (2016) Gray's Anatomy: The Anatomical Basis of Clinical Practice. Gray's Anatomy Series: Elsevier Limited.
