[1] It resides in the dermal layer of the skin and is made up of 20 different cell types, each with distinct functions.

[1] The function of hair in humans has long been a subject of interest and continues to be an important topic in society, developmental biology and medicine.

[1] For centuries, humans have ascribed esthetics to scalp hair styling and dressing and it is often used to communicate social or cultural norms in societies.

In addition to its role in defining human appearance, scalp hair also provides protection from UV sun rays and is an insulator against extremes of hot and cold temperatures.

Anatomically, the triad of hair follicle, sebaceous gland and arrector pili muscle make up the pilosebaceous unit.

Attached to the follicle is a tiny bundle of muscle fiber called the arrector pili.

Also attached to the follicle is a sebaceous gland, which produces the oily or waxy substance sebum.

Proteolysis of collagen leads to elimination of the damaged cells and then to terminal hair follicle miniaturization.

There is also a shedding phase, or exogen, that is independent of anagen and telogen in which one or several hairs that might arise from a single follicle exits.

A hair pulled out in this phase will typically have the root sheath attached to it which appears as a clear gel coating the first few mm of the hair from its base; this may be misidentified as the follicle, the root or the sebaceous gland by non-health care professionals.

Scalp hair stays in this active phase of growth for 2–7 years; this period is genetically determined.

[3] Studies of Witka et al. 2020 has shown the role of microbiome in the biology, immunology and diseases of scalp hair follicle.

In each of these methods, naturally occurring groupings of one to four hairs, called follicular units, are extracted from the hair restoration patient and then surgically implanted in the balding area of the patient's scalp, known as the recipient area.

These follicles are extracted from donor areas of the scalp, or other parts of the body, which are typically resistant to the miniaturization effects of the hormone DHT.