Hair

1. Overview

Hair is a keratinized filamentous structure that emerges from the epidermis and is rooted in the dermis. As a key appendage of the integumentary system, hair serves a range of functions, including protection, thermoregulation, sensory perception, and social signaling. Each hair is produced by a hair follicle, which is a dynamic mini-organ that undergoes cycles of growth, regression, and rest. While humans have hair over most of their body, its density, length, and pigmentation vary by location and genetic factors.

2. Location

Hair is distributed across nearly all parts of the skin except for specific glabrous areas. Common hair-bearing sites include:

• Scalp: Dense, pigmented terminal hair.

• Face: Including eyebrows, eyelashes, and beard regions.

• Arms, legs, chest, and back: Vellus or terminal hair depending on sex and age.

• Axillae and pubic area: Terminal hair that develops at puberty.

Hair is absent on the palms, soles, lips, eyelids (except eyelashes), and parts of the genitalia.

3. Structure

Hair has both visible and internal components, divided into:

• Hair shaft: The part of the hair that extends above the skin surface. It consists of three concentric layers:

  • Medulla: The innermost core (may be absent in fine hair).

  • Cortex: Contains keratin and melanin, determining strength and color.

  • Cuticle: Outermost layer of overlapping keratinized cells that protect the shaft.

• Hair root: Located below the skin surface within the follicle.

• Hair follicle: A tubular invagination of the epidermis extending into the dermis and often the hypodermis. It includes:

  • Bulb: The base of the follicle housing the dermal papilla and matrix cells.

  • Matrix: Actively dividing cells that produce the hair shaft.

  • Dermal papilla: A mesenchymal structure supplying blood and growth factors to the matrix.

• Associated structures:

  • Sebaceous glands: Secrete sebum into the follicle to lubricate hair and skin.

  • Arrector pili muscle: Smooth muscle that contracts to make hair stand up (piloerection).

4. Function

Hair performs several biological and protective functions:

• Thermal insulation: Hair on the scalp and body helps conserve heat, particularly in colder environments.

• UV protection: Scalp hair protects against ultraviolet radiation exposure.

• Sensory input: Hair follicles are associated with nerve endings that respond to displacement, aiding in tactile sensation.

• Barrier function: Eyelashes and nasal hairs trap debris and prevent foreign particles from entering the eyes and airways.

• Sexual dimorphism and identity: Hair patterns influenced by androgens contribute to visual gender differences and social identity.

5. Physiological role(s)

Hair contributes to a range of physiological functions:

• Growth cycle regulation: Hair follicles undergo cyclic phases—

  • Anagen: Active growth phase.

  • Catagen: Regression or transitional phase.

  • Telogen: Resting phase, followed by hair shedding (exogen).

• Pigment synthesis: Melanocytes in the hair matrix synthesize eumelanin or pheomelanin, giving hair its color.

• Hormonal responsiveness: Hair follicles respond to androgens and other hormones, influencing distribution and growth rate (e.g., beard growth, hirsutism).

• Immune modulation: Follicles secrete cytokines and antimicrobial peptides, playing a role in skin immunity.

6. Clinical Significance

Hair is involved in several dermatologic and systemic disorders:

• Alopecia:

  • Androgenetic alopecia: Genetic, hormone-dependent hair loss (common male/female pattern baldness).

  • Alopecia areata: Autoimmune condition causing patchy hair loss.

  • Telogen effluvium: Stress-induced shedding due to an abnormal shift into the telogen phase.

• Hirsutism:

  • Excess terminal hair growth in women in a male pattern distribution, often due to androgen excess (e.g., PCOS).

• Trichotillomania:

  • A psychological disorder involving compulsive hair pulling.

• Folliculitis:

  • Inflammation of hair follicles, often due to bacterial infection or irritation.

• Gray hair:

  • Age-related or premature reduction in melanin production in hair follicles, leading to depigmented hair shafts.

• Laser hair removal and treatments:

  • Medical and cosmetic techniques target hair follicles for long-term hair reduction or treatment of follicular disorders.