The Biology, Structure, and Function of Eyebrow Hair

Other hormones that modulate hair growth include estrogens, thyroid hormones, glucocorticoids, retinoids, prolactin, and growth hormone.¹

Minoxidil has been used for patients with severe hypertension and was incidentally observed to result in hair growth. The mechanism by which minoxidil induces hair growth is not fully elucidated, but may be related to its action on opening of the adenosine triphosphate (ATP) sensitive potassium channel (K[ATP] channel). K(ATP) channel opening in dermal papilla cells at the base of the follicle is thought to play a role in stimulating hair growth.¹⁵ As a vasodilator, it may promote more oxygen, blood, and nutrients to the proliferating dermal papilla cells. Other studies have shown that minoxidil stimulates the production of growth factors such as vascular endothelial growth factor in cultured dermal papilla cells, and that these growth factors might stimulate hair growth.¹⁶ It has also been shown that minoxidil promotes the survival of human dermal papilla cells, thereby prolonging anagen, through proliferative and anti-apoptotic effects.¹⁷

Prostaglandin analogues, such as bimatoprost, have been shown to induce eyelash growth, although the mechanism is unclear. It is suggested that hypertrichosis is probably a result of the induction of the anagen phase in the telogen phase follicles of the eyelashes and prolonging the anagen phase.¹⁸ Anecdotal cases describe significant eyebrow hair growth after use of topical prostaglandin analogues for eyebrow hypotrichosis.¹⁹

In contrast to AGA, hirsutism and hypertrichosis result from a prolonged anagen stage with an abnormal enlargement of hair follicles; small, fine vellus hairs transform into large, terminal hairs. This can be caused by medications or hormonal factors (eg, dysfunction in the adrenal glands or ovaries resulting in hyperandrogenism).

Anagen effluvium is caused by the cessation of anagen, often due to antineoplastic/chemotherapeutic drugs, which disrupt the rapidly proliferating bulb matrix cells. As a result, the hair shaft becomes narrower, with subsequent breakage and loss of the hair. Because the stem cells of the hair follicles are typically spared, a new hair bulb may be regenerated once the medication is stopped. Hair loss usually begins 1 to 2 weeks after chemotherapy is started and is most noticeable by 1 to 2 months.²⁰ The scalp hair is usually most affected, but all body hair, including eyelashes and eyebrows, can be affected.²¹

Inflammatory alopecias (such as lichen planopilaris and discoid lupus erythematosus) can lead to permanent scarring hair loss, whereas others (such as alopecia areata and telogen effluvium) are nonscarring and potentially reversible. In scarring alopecias, the inflammation usually involves the superficial portion of the follicle, including the bulge area, resulting in permanent destruction of the stem cells necessary for the regeneration of the follicle. In contrast, the acute follicular inflammation in alopecia areata targets the hair bulb in the subcutaneous fat, resulting in disruption of the anagen stage. Because the bulge area is spared, the hair follicle has the potential to generate a new hair bulb and hair shaft once the inflammation has resolved.

Hair disorders associated with rare congenital hair defects, such as Netherton’s syndrome and ectodermal dysplasias, are caused by mutations in keratins or other structural proteins. Netherton’s syndrome is an autosomal recessive disease caused by mutations in serine protease inhibitor Kazal-type 5, encoding the serine protease inhibitor lympho-epithelial Kazal-type-related inhibitor. It presents with an atopic diathesis, allergic reactions, and icthyosiform dermatitis. A clue to diagnosis is the examination of eyebrow hairs, which will characteristically show trichorrhexis invaginata, in which the distal portion of the shaft is invaginated into the proximal portion. ²² Eyebrows may be the sole site of involvement in many patients with Netherton’s syndrome, and the findings more prominent than in the scalp.²²

In summary, eyebrow hair serves many important biologic functions, including sensory transmission and protection from the elements, as well as playing an important role in cosmesis and expression. The hair follicle originates from complex mesenchymal- epithelial interactions during embryogenesis. Similar molecular mechanisms underlie hair follicle cycling during one’s lifetime. Knowledge of the hair follicle structure and cycle is key to understanding the pathogenesis of the different types of alopecia, as well as developing targeted therapies for hair loss.

Jennifer V. Nguyen MD has no conflicts of interest to disclose.