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Hair Follicle
The hair follicle is a tubular epithelial invagination of the epidermis that extends into the dermis and, in larger follicles, may reach the hypodermis. It surrounds the hair root and includes specialized regions involved in hair production, growth cycling, stem cell maintenance, and interaction with sebaceous glands and the arrector pili muscle.
Region-
SystemIntegumentary System
The hair follicle is a tubular epithelial structure that extends from the epidermal surface into the underlying dermis. Large follicles may extend into the hypodermis. It surrounds the root of a hair and contains specialized epithelial and connective tissue components responsible for producing, supporting, and anchoring the growing hair.
Hair follicles are not isolated structures. Most participate in a larger pilosebaceous unit that includes a hair, sebaceous gland, arrector pili muscle, and associated connective tissue, nerves, and vascular structures.
The follicle is highly dynamic. Its lower portion undergoes repeated structural remodeling during the hair growth cycle, while specialized stem cell populations within the permanent portion of the follicle contribute to regeneration.
General Organization
A hair follicle is formed by an invagination of epidermal epithelium into the dermis. The epithelial component is surrounded by a connective tissue sheath derived from the dermis.
The hair shaft occupies the central portion of the follicle and emerges through the skin surface.
Major Regions
The follicle can be divided anatomically into several regions extending from the skin surface toward its deepest portion.
| Region | General Location |
|---|---|
| Infundibulum | From the skin surface to the opening of the sebaceous duct |
| Isthmus | Region below the sebaceous duct extending toward the arrector pili attachment |
| Inferior segment | Lower cycling portion extending toward the hair bulb |
| Hair bulb | Expanded deepest portion surrounding the dermal papilla |
Infundibulum
The infundibulum is the uppermost portion of the hair follicle. It extends from the epidermal surface to approximately the level at which the sebaceous gland empties into the follicular canal.
Its epithelial lining is continuous with the epidermis and undergoes keratinization resembling that of the epidermal surface.
Isthmus
The isthmus lies below the infundibulum and extends through the region associated with the sebaceous gland and arrector pili attachment.
This region forms part of the relatively permanent upper follicle and contains important epithelial stem cell populations.
Inferior Segment
The inferior segment is the deeper portion of the follicle that undergoes extensive changes during the hair cycle. During active growth it extends deeply into the dermis or subcutaneous tissue and terminates in the hair bulb.
Hair Bulb
The hair bulb is the expanded terminal portion of an actively growing hair follicle. It surrounds the dermal papilla and contains rapidly proliferating matrix cells.
The bulb is the principal site at which cells destined to form the hair shaft and inner root sheath are generated.
Dermal Papilla
The dermal papilla is a specialized projection of connective tissue that extends into the base of the hair bulb. It contains capillaries and specialized fibroblast-like cells.
The dermal papilla provides important signaling interactions that regulate matrix cell activity and hair growth.
Hair Matrix
The hair matrix consists of rapidly proliferating epithelial cells surrounding the dermal papilla. These cells divide, differentiate, and move upward to form the hair shaft and inner root sheath.
Matrix cells are among the most actively dividing cells in the body during anagen.
Melanocytes of the Hair Bulb
Melanocytes associated with the hair matrix produce melanin that is transferred to developing hair cells. This pigment contributes to the color of the emerging hair shaft.
Hair Bulb Components
| Component | Function |
|---|---|
| Dermal papilla | Provides vascular support and regulatory signals |
| Matrix cells | Generate cells of the hair shaft and inner root sheath |
| Melanocytes | Supply pigment to developing hair |
| Capillary network | Supports metabolically active follicular tissues |
Hair Root
The hair root is the portion of the hair located within the follicle beneath the skin surface. It is surrounded by the epithelial root sheaths.
Hair Shaft
The hair shaft is the keratinized portion that extends from the follicle and, in most locations, projects above the epidermal surface. It is composed of dead, highly keratinized cells.
Layers of the Hair Shaft
A mature hair shaft may contain three concentric structural components: the medulla, cortex, and cuticle. The medulla may be absent in some fine hairs.
| Layer | Description |
|---|---|
| Medulla | Central region, particularly evident in thicker hairs |
| Cortex | Forms most of the hair shaft and contains densely packed keratin |
| Cuticle | Thin outer layer of overlapping keratinized cells |
Root Sheaths
The hair root is surrounded by concentric epithelial layers collectively known as the root sheaths. These include the inner root sheath and outer root sheath.
Inner Root Sheath
The inner root sheath develops from matrix cells and surrounds the developing hair shaft in the deeper follicle. It helps shape and guide the growing hair.
The inner root sheath does not extend all the way to the skin surface. It disintegrates in the upper portion of the follicle.
Layers of the Inner Root Sheath
| Layer | Position |
|---|---|
| Cuticle of inner root sheath | Innermost, adjacent to hair cuticle |
| Huxley layer | Middle component |
| Henle layer | Outer component |
Outer Root Sheath
The outer root sheath is continuous with the epidermis and surrounds the inner root sheath in the deeper follicle. It represents the downward continuation of epidermal epithelial tissue.
Its structure varies along the length of the follicle and during different stages of the hair cycle.
Glassy Membrane
Outside the outer root sheath is a thickened basement membrane commonly called the glassy membrane. It separates the epithelial follicle from the surrounding connective tissue sheath.
Connective Tissue Sheath
The epithelial components of the follicle are surrounded by a connective tissue sheath derived from the dermis. This sheath contains collagen fibers, fibroblasts, vessels, and associated neural structures.
Concentric Organization of the Follicle
| From Center Outward | Structure |
|---|---|
| 1 | Hair shaft |
| 2 | Inner root sheath |
| 3 | Outer root sheath |
| 4 | Glassy membrane |
| 5 | Connective tissue sheath |
Bulge Region
The bulge is a specialized region of the outer root sheath near the attachment of the arrector pili muscle. It contains important epithelial stem cells.
These stem cells contribute to regeneration of the cycling portion of the hair follicle and can participate in epidermal repair following injury.
Hair Follicle Stem Cells
Hair follicle stem cells are relatively quiescent populations capable of generating progenitor cells when activated. Their behavior is controlled by signals from surrounding epithelial, mesenchymal, neural, and immune environments.
Sebaceous Gland
Most hair follicles are associated with one or more sebaceous glands. Their ducts empty sebum into the follicular canal, usually within the upper follicle.
Sebum then travels toward the skin surface along the hair shaft and follicular canal.
Arrector Pili Muscle
The arrector pili is a small bundle of smooth muscle associated with the follicle. It extends from the connective tissue surrounding the follicle toward the superficial dermis.
Contraction pulls on the follicle and produces piloerection, commonly known as goosebumps.
Pilosebaceous Unit
| Structure | Relationship |
|---|---|
| Hair follicle | Central epithelial structure |
| Hair | Produced within the follicle |
| Sebaceous gland | Usually empties into the follicular canal |
| Arrector pili muscle | Attaches to follicular connective tissue |
| Associated nerves | Participate in sensory and autonomic functions |
Hair Follicle Innervation
Hair follicles are surrounded by sensory nerve endings that can detect displacement of the hair shaft. Movement of the hair therefore provides a mechanism for detecting light mechanical stimuli at the skin surface.
Hair Root Plexus
A network of sensory nerve endings around the follicle is commonly called the hair root plexus. Bending or movement of the hair stimulates these endings.
Blood Supply
The follicle receives vascular support from vessels within the surrounding dermis. Capillary loops associated with the dermal papilla support the highly metabolically active matrix during hair growth.
The hair shaft itself is avascular.
Hair Growth Cycle
Hair follicles repeatedly cycle through periods of active growth, regression, rest, and shedding. This cycle allows the follicle to regenerate a new hair after the previous growth phase ends.
Anagen
Anagen is the active growth phase. The follicle is long and deeply positioned, matrix cells proliferate rapidly, and a hair shaft is continuously produced.
The duration of anagen varies greatly among body regions and is a major determinant of potential hair length.
Catagen
Catagen is a relatively brief regression phase. Matrix proliferation ceases, the lower follicle involutes, and the dermal papilla becomes separated from the former active bulb as the follicle shortens.
Telogen
Telogen is the resting phase. The follicle is relatively short and inactive compared with an anagen follicle.
Exogen
Exogen refers to release or shedding of the old hair shaft. Shedding can overlap with other phases of the follicular cycle.
Hair Cycle Phases
| Phase | Main Event |
|---|---|
| Anagen | Active follicular growth and hair production |
| Catagen | Regression of the lower follicle |
| Telogen | Resting stage |
| Exogen | Release of the old hair |
Follicular Remodeling
The upper portion of the follicle remains relatively stable, while much of the lower follicle is repeatedly reconstructed and regressed during successive hair cycles.
This remarkable regenerative behavior depends on interactions among follicular stem cells, progenitor cells, and the dermal papilla.
Hair Types
Hair follicles differ in size and produce different types of hair depending on anatomical location, age, and hormonal influences.
Vellus Hair
Vellus hairs are fine, short, lightly pigmented hairs covering much of the body surface. Their follicles are relatively small.
Terminal Hair
Terminal hairs are larger, longer, thicker, and usually more heavily pigmented. Examples include scalp hair and, after puberty, hair in several androgen-responsive regions.
Vellus Versus Terminal Hair
| Feature | Vellus Hair | Terminal Hair |
|---|---|---|
| Diameter | Fine | Thicker |
| Length | Short | Potentially long |
| Pigmentation | Usually light | Usually more prominent |
| Follicle size | Smaller | Larger |
Regional Variation
Hair follicles vary considerably across the body. Their density, depth, size, orientation, hair type, and growth-cycle duration differ according to anatomical location.
Hair follicles are absent from certain specialized regions, including the palms and soles.
Scalp Follicles
Scalp follicles are typically large terminal hair follicles with relatively long anagen phases. Their deep bulbs may extend through the dermis into the subcutaneous tissue.
Hormonal Influence
Hair follicles respond to numerous hormonal signals. Androgens are particularly important in determining the transformation and behavior of follicles in specific body regions.
The response to androgens differs by anatomical site, which helps explain why the same hormonal signals can promote terminal hair growth in some areas while contributing to follicular miniaturization in susceptible scalp regions.
Hair Pigmentation
Hair pigmentation depends on melanin produced by follicular melanocytes and transferred to developing keratinocytes in the hair matrix. The amount and type of melanin contribute to differences in hair color.
Graying of Hair
Progressive loss or reduced activity of pigment-producing follicular melanocytes contributes to the development of gray and white hair with aging.
Follicular Keratinization
Cells produced in the matrix undergo specialized keratinization as they move upward. This process forms the highly resistant hair shaft and differs from ordinary epidermal keratinization.
Acne and the Hair Follicle
Acne vulgaris is fundamentally a disorder of the pilosebaceous unit. Abnormal keratinization within the follicular canal contributes to obstruction and comedone formation.
Sebum production, Cutibacterium acnes, and inflammation also contribute to the disease process.
Folliculitis
Folliculitis refers to inflammation centered on hair follicles. It can result from infectious, inflammatory, mechanical, or other causes.
Ingrown Hair
An ingrown hair occurs when a hair grows into or re-enters the surrounding skin rather than emerging normally from the follicular opening. This can produce local inflammation.
Alopecia
Alopecia refers broadly to hair loss. Different forms of alopecia affect different components or stages of the follicular system.
Androgenetic Alopecia
In androgenetic alopecia, genetically susceptible scalp follicles undergo progressive miniaturization under androgen-related influences. Terminal hairs become progressively finer and shorter.
Alopecia Areata
Alopecia areata is an immune-mediated disorder in which inflammatory processes target actively growing hair follicles, producing non-scarring hair loss.
Scarring Alopecia
In scarring alopecia, destructive inflammation can permanently damage the follicular epithelial stem cell region. Loss of the regenerative capacity of the follicle can result in permanent hair loss.
Clinical Importance of the Bulge
The stem cell-rich bulge is particularly important in disorders that damage hair follicles. Destruction of this region can prevent normal follicular regeneration.
Hair Transplantation
Hair transplantation relocates intact follicular units from donor regions to recipient sites. Successful transplantation requires preservation of the essential regenerative structures of the follicle.
Follicular Unit
A follicular unit is a naturally occurring grouping of one or more hair follicles together with associated sebaceous glands, arrector pili structures, connective tissue, vessels, and nerves.
Histological Identification
Hair follicles can appear very different depending on the plane of section and stage of the hair cycle. Longitudinal sections reveal the length and layered organization of the follicle, while transverse sections demonstrate concentric epithelial and hair structures.
Longitudinal Section
A longitudinal section through an active follicle may demonstrate the hair bulb, dermal papilla, matrix, developing hair shaft, inner root sheath, outer root sheath, sebaceous gland, and arrector pili muscle.
Transverse Section
A transverse section can clearly demonstrate the concentric relationship between the hair shaft, root sheaths, glassy membrane, and connective tissue sheath.
Key Structures of the Hair Follicle
| Structure | Key Role |
|---|---|
| Hair bulb | Expanded growth region of active follicle |
| Dermal papilla | Regulatory mesenchymal structure and vascular interface |
| Hair matrix | Produces hair and inner root sheath cells |
| Inner root sheath | Shapes and guides developing hair |
| Outer root sheath | Epidermally derived epithelial sheath |
| Bulge | Contains important epithelial stem cells |
| Sebaceous gland | Releases sebum into follicular canal |
| Arrector pili | Produces piloerection |
| Hair root plexus | Detects movement of the hair |
Key Features of the Hair Follicle
| Feature | Key Point |
|---|---|
| Origin | Epidermal epithelial invagination |
| Location | Extends through dermis, sometimes into hypodermis |
| Central structure | Hair root and shaft |
| Growth center | Hair matrix in bulb |
| Mesenchymal regulator | Dermal papilla |
| Stem cell region | Bulge |
| Associated gland | Sebaceous gland |
| Associated muscle | Arrector pili |
| Sensory structure | Hair root plexus |
| Growth phases | Anagen, catagen, telogen, and exogen |
Anatomical and Functional Importance
The hair follicle is one of the most structurally complex appendages of the skin. It combines epithelial, mesenchymal, vascular, neural, glandular, muscular, and pigment-producing components within a highly organized regenerative unit.
At its base, matrix cells surrounding the dermal papilla generate the hair and inner root sheath. More superficially, the outer root sheath contains the bulge region, which houses stem cells capable of regenerating the cycling follicle.
The follicle also serves as the central structure of the pilosebaceous unit. Sebaceous glands release sebum into its canal, the arrector pili muscle attaches to its outer tissues, and sensory nerve endings detect movement of the associated hair.
Its repeated progression through growth, regression, rest, and shedding makes the hair follicle a particularly important model of adult tissue regeneration. Disorders affecting different follicular compartments can produce acne, folliculitis, altered hair growth, or temporary and permanent forms of alopecia.
The hair follicle therefore links skin anatomy, epithelial regeneration, connective tissue signaling, sensory function, sebaceous secretion, pigmentation, and hair growth.
Published on October 15, 2026
Last updated on October 15, 2026
Last updated on October 15, 2026