Hair

A skin appendage with multiple functions, important in thermoregulation, solar radiation protection, and sexual dimorphism.

The hair unit is composed of specialized compartments that are coordinatedly responsible for synthesis of the hair fiber.

Differences in hair morphology in human populations of different ancestors, are believed to be genetically determined.

The hair morphology in human populations of different ancestries, are believed to be genetically determined.

The hair of individuals with African ancestry has a flat and cross-sectional appearance and the hair is usually coiled.

The hair of persons of European ancestry have a oval cross-section, and the hair persons of Asian ancestry has a rounded, more circular perimeter, resulting in straighter hair.

The major signaling center of the hair follicle resides in the dermal papilla.

The dermal papilla consist of a specialize fibroblast population that is responsible for regeneration and cycling of the follicle.

The dermal papilla interacts with overlying epithelial cells, the hair matrix, to generate the hair shaft.

The dermal papilla is dynamic and regulated, and a reduction in the number of cells that make up the dermal papilla contributes to the loss or thinning of hair.

The hair shaft contains complex structural proteins, including many different hair keratin with associated proteins, responsible for formation and growth of the hair shaft.

There are about 50 million hair follicles covering the body with 20% being in the scalp.

The hair follicle is a skin appendage with a primary function to produce the visible hair shaft.

The hair follicle is divided into sections.

The upper sections of the hair follicle are permanent, with the infundibulum running from the opening of the sebaceous gland duct to the site where the hair follicle meets the epidermis, providing a funnel through the epidermis and provides an opening for the hair shaft.

The isthmus at the lower boundary of the sebaceous gland is at the insertion of the arrector pili muscle, and is described as the bulge.

The bulge contains epithelial hair follicle stem cells.

The progeny of the hair follicle stem cells that produce the hair bulb matrix keratinocytes, and contributes to the epidermis.

Hair follicle renewal is maintained by the stem cells associated with each follicle.

Aging of the hair follicle appears to be related to cellular response to the DNA damage that accumulates in renewing stem cells during aging.

The response to DNA damage involves the proteolysis of type XVII collagen by neutrophil elastase.

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

Damage to these cells impairs hair shaft production

Scalp contains an average of 100,000 hairs.

Scalp and facial hair associated with general well-being, social status, sexual attraction.

Hair is often used for social statements and political affiliations.

More than 90% of scalp hairs are growing and are called anagen hairs.

Hair follicle growth occurs in cycles.

Each cycle consists of a long growing phase, anagen, a short transitional phase, catagen, and a short resting phase telogen.

At the end of the resting phase, the hair falls out, xogen, and a new hair starts growing in the follicle beginning the cycle again.

Anagen hair are deeply rooted in subcutaneous fat tissue.

Scalp hair constantly regenerating on the scalp.

Anagen phase of hair growth lasts for most of the 3-7 years and is followed by a 2 week catagen phase.

Catagen phase associated with apoptosis, after which hair goes into the telogen phase, a resting phase, that lasts 3 months.

Telogen hair is more superficially located than anagen hair and is easily pulled out resulting in scalp losses of approximately 100 telogen hairs per day.

Normally, about 40 hairs reach the end of their resting phase each day and fall out.

Scalp hair shafts last 3-7 years and subsequently fall out and are replaced by a new hairs.

Only areas free of hair follicles are the lips, soles and palms.

Before puberty hair is small, straight, and fair termed vellus.

At puberty, with increased levels of androgens, vellus follicles in specific areas develop into terminal hairs which are larger, curlier, darker and more visible becoming sexual hair follicles.

Higher androgen levels are required for the beard development than for the growth of pubic and axillary hair.

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