Hair Follicle

1. Overview

The hair follicle is a complex, tubular structure embedded in the dermis and extending into the hypodermis, responsible for the growth and anchoring of hair. As a dynamic mini-organ of the integumentary system, it plays a critical role in hair production, skin regeneration, thermoregulation, and sensory perception. Hair follicles are involved in cyclical activity including growth (anagen), regression (catagen), and rest (telogen), and they house several specialized cells including keratinocytes, melanocytes, and stem cells.

2. Location

Hair follicles are found in nearly all skin regions of the body except:

• Palms of the hands

• Soles of the feet

• Lips

• Nipples

• Some parts of the external genitalia

They are especially dense on the scalp, face, and pubic areas. The depth and size of the follicle vary depending on the type of hair it produces (terminal vs. vellus hair).

3. Structure

Each hair follicle is composed of several layers and associated structures, organized into three main segments:

1. Upper segment

• Infundibulum: From the skin surface to the opening of the sebaceous gland duct.

• Isthmus: Extends from the sebaceous duct to the insertion point of the arrector pili muscle.

2. Lower segment

• Hair bulb: Enlarged base of the follicle housing the dermal papilla and matrix cells.

• Matrix: Actively dividing keratinocytes and melanocytes responsible for producing the hair shaft and pigment.

3. Associated components

• Inner root sheath (IRS): Surrounds and guides the growing hair shaft.

• Outer root sheath (ORS): Continuous with the basal layer of the epidermis.

• Dermal papilla: Highly vascularized connective tissue providing nutrients and signaling cues.

• Sebaceous glands: Lubricate the hair and skin with sebum.

• Arrector pili muscle: Smooth muscle that elevates the hair during piloerection (goosebumps).

4. Function

Hair follicles perform several crucial functions:

• Hair production: Generate hair shafts through proliferation and differentiation of matrix cells.

• Pigmentation: Melanocytes transfer melanin to keratinocytes in the matrix, coloring the hair.

• Secretion: Associated sebaceous glands secrete sebum to maintain skin and hair hydration.

• Stem cell reservoir: Houses epithelial stem cells in the bulge region that contribute to follicle regeneration and wound healing.

5. Physiological role(s)

The hair follicle contributes to various systemic and skin-specific processes:

• Thermoregulation: Hair helps insulate the body; piloerection traps warm air in cold environments.

• Sensory function: Follicles are richly innervated and sensitive to tactile stimuli, aiding mechanosensation.

• Barrier maintenance: Sebum secretion supports the skin’s lipid barrier, deterring microbes and preventing dehydration.

• Tissue regeneration: Hair follicle stem cells assist in epidermal repair following injury.

• Hormonal responsiveness: Follicular activity is influenced by androgens, cortisol, and thyroid hormones, affecting hair growth and distribution.

6. Clinical Significance

Hair follicles are involved in various dermatological, endocrine, and autoimmune conditions:

• Alopecia:

  • Androgenetic alopecia: Hormone-sensitive miniaturization of scalp follicles.

  • Alopecia areata: Autoimmune attack on the hair bulb leading to sudden, patchy hair loss.

  • Scarring alopecia: Destruction of follicles due to chronic inflammation or trauma.

• Folliculitis:

  • Bacterial or fungal infection of the follicle, often resulting in pustules or painful nodules.

• Hirsutism:

  • Excess androgen-induced terminal hair growth in females in a male pattern distribution.

• Trichotillomania:

  • A psychological disorder involving compulsive hair pulling, which may damage follicles and cause scarring.

• Laser hair removal:

  • Targets melanin in the follicle to disrupt hair growth, used for cosmetic or medical reasons.

• Chemotherapy-induced hair loss:

  • Matrix cells in the bulb are highly mitotic and are affected by chemotherapeutic agents, resulting in temporary hair loss.