Hair follicles and their function

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Hair follicle function

Hair follicles are part of the "pilosebaceous unit" in the skin. The pilosebaceous unit is the hair follicle structure that makes the hair plus the attached oil secreting sebaceous gland. These two properties - the manufacturing of hair fiber and the secretion of oil - are perhaps the most visible of the pilosebaceous unit's functions.

However, the hair follicle structure does much more than that. More recently it has been realized that hair follicles can contribute to our health and also to the development of diseases and cancer in the skin. There are over 2 million hair follicles in our body skin and they have a significant potential to affect us both in beneficial and in adverse ways.

Recent studies have shown that hair follicle can act as conduits into our skin. They are in effect little tubes into our skin through which chemicals and pathogens like viruses and bacteria can enter. It has been shown that chemicals are absorbed into the skin much more quickly through hair follicles than through adjacent sections of skin that don't have hair follicles. Some viruses like to take up residence in hair follicles and to some extent they are protected from the body's immune system inside hair follicles. In effect they can hide away and avoid being destroyed by our body's defense system.

Studies have also shown that, at least experimentally, if skin cancer develops in an area where hair follicles are present, the skin cancer cells will signal to the adjacent hair follicles and encourage them to make blood vessels. The hair follicle produced blood vessels can then help feed the skin cancer making it worse.

As such, hair follicles can be both a benefit to us but also they can be harmful to us in some circumstances. Their production of hair fiber and oils are not the only things that hair follicles do for us.

Beneficial functions of the pilosebaceous unit

  • Hair follicles protect the developing human fetus in uterus during the last trimester of gestation by producing vernix caseosa.
  • In the postnatal periods, hair follicles produce oils for epidermal skin surface protection.
  • Hair follicles also provide a sensory apparatus through extensive nerve innervation around the pilosebaceous unit.
  • Hair follicles produce pheromones by associated apocrine glands in the arm pits.
  • Hair follicles facilitate hair follicle renewal and skin regeneration by providing a stem cell reservoir.
  • Hair follicles provide a cell reservoir for, and mediation of, skin angiogenesis.
  • Hair follicles help in fiber pigmentation and epidermal repigmentation through a melanocyte pigment cell reservoir.
  • Hair follicles provide a reservoir of Langerhans cells for possible immunosurveilance.

Detrimental functions of the pilosebaceous unit

  • Hair follicles harbor pathogens and may provide a conduit for pathogen entry into the body.
  • Loss of hair follicle immune privilege provides an antigenic target for the immune system and development of folliculotropic diseases with an autoimmune component.
  • Hair follicles often play the role of key mediators of stress signaling through sensitivity to and synthesis of neuroendocrine activities.
  • Hair follicle stem cell mutations may lead to skin cancer tumor development.
  • Adjacent skin neoplasias can tend to “hijack” hair follicle angiogenesis and skin remodeling properties for their own benefit.
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