Hair Growth Cycle

The hair growth cycle is the repeating sequence of growth, regression, rest, and shedding undergone by individual hair follicles. Rather than producing hair continuously throughout life, each follicle repeatedly remodels itself through a series of coordinated phases.

The major phases are anagen, the active growth phase; catagen, the regression phase; telogen, the resting phase; and exogen, the phase in which the old hair is released or shed.

Hair follicles generally cycle independently rather than all entering the same phase simultaneously. This asynchronous behavior helps prevent normal physiological cycling from producing complete loss of hair from a region at one time.

Overview of the Hair Cycle

Phase	Main Event
Anagen	Active follicular growth and production of the hair shaft
Catagen	Rapid regression of the lower follicle
Telogen	Relative follicular rest
Exogen	Release and shedding of the old hair

Dynamic Nature of the Hair Follicle

The hair follicle is unusual among adult organs because much of its lower portion repeatedly undergoes controlled destruction and reconstruction.

The upper permanent portion of the follicle remains relatively stable, while the lower cycling portion changes dramatically between anagen, catagen, and telogen.

Permanent and Cycling Portions

Follicular Component	Behavior During the Cycle
Infundibulum	Relatively permanent
Isthmus	Relatively permanent
Bulge region	Stem cell-containing permanent region
Inferior follicle	Undergoes major cyclic remodeling
Hair bulb	Prominent during anagen and regresses during catagen

Anagen

Anagen is the active growth phase of the hair cycle. During this phase, the follicle is elongated and its deepest portion may extend into the deep dermis or subcutaneous tissue.

Cells of the hair matrix proliferate rapidly around the dermal papilla. Their descendants differentiate to form the hair shaft and inner root sheath.

Hair Production During Anagen

The growing hair shaft is produced continuously as matrix-derived cells divide, move upward, differentiate, accumulate keratin, and become incorporated into the hair.

As long as the follicle remains in anagen, the hair can continue to increase in length.

Anagen Hair Bulb

During anagen, the hair bulb is large and surrounds the dermal papilla. Matrix cells in this region have high mitotic activity and require substantial metabolic support.

Dermal Papilla During Anagen

The dermal papilla lies within the base of the bulb and provides important signaling interactions that regulate the activity of surrounding epithelial cells.

Its close relationship with the matrix is essential for normal production of the hair shaft.

Hair Pigmentation During Anagen

Follicular melanocytes are active during anagen and transfer melanin to developing hair cells. Hair pigmentation is therefore closely coordinated with active hair production.

Duration of Anagen

The duration of anagen differs dramatically among body regions. Scalp follicles can remain in anagen for years, allowing scalp hair to grow to considerable length.

Follicles producing shorter body hairs generally have much shorter growth phases.

Anagen and Hair Length

The maximum potential length of a hair depends strongly on how long its follicle remains in anagen, together with the rate at which the hair shaft is produced.

A long anagen phase permits long hair, while a short anagen phase limits hair length even when the follicle is otherwise healthy.

Characteristics of Anagen

Feature	Anagen State
Matrix proliferation	High
Hair production	Active
Follicle length	Long
Hair bulb	Well developed
Dermal papilla relationship	Closely associated with matrix
Pigmentation	Active in pigmented hairs

Catagen

Catagen is the short transitional phase between active growth and rest. During catagen, proliferation of matrix cells stops and the lower portion of the follicle undergoes controlled regression.

The follicle becomes substantially shorter as the epithelial structures of the lower segment involute.

Regression of the Lower Follicle

Catagen involves programmed cellular changes, including apoptosis within selected epithelial compartments. The large anagen bulb disappears as the lower follicle regresses.

Dermal Papilla During Catagen

As the lower follicle involutes, the dermal papilla moves upward relative to its deep anagen position and remains associated with the regressing follicular epithelium.

Preservation of the dermal papilla is important because it participates in signaling required for a future growth cycle.

Club Hair Formation

As active production ceases, the proximal end of the existing hair becomes keratinized and forms a club hair. This hair can remain retained within the follicle during the subsequent resting phase.

Characteristics of Catagen

Feature	Catagen State
Matrix proliferation	Stops
Hair shaft production	Ceases
Lower follicle	Regresses
Hair bulb	Involutes
Club hair	Forms
Duration	Relatively brief

Telogen

Telogen is the resting phase of the hair cycle. The follicle is much shorter than during anagen and is not actively producing a new elongated hair shaft.

The club hair may remain anchored within the follicle during this period.

Telogen Follicle

A telogen follicle lacks the large proliferative bulb characteristic of anagen. The dermal papilla is positioned near the lower end of the shortened epithelial follicle.

Biological Activity During Telogen

Although telogen is commonly called a resting phase, the follicle is not biologically inert. Signaling interactions continue and help determine when stem and progenitor cell populations will initiate the next anagen phase.

Characteristics of Telogen

Feature	Telogen State
Active hair production	Absent
Follicle length	Short
Club hair	Usually retained initially
Matrix activity	No active anagen matrix
Regenerative signaling	Continues in preparation for future growth

Exogen

Exogen is the process by which the club hair is released from the follicle and shed from the body surface.

Exogen is often discussed separately from anagen, catagen, and telogen because shedding does not always correspond exactly to the transition between the other phases.

Hair Shedding

Normal shedding is a physiological consequence of follicular cycling. Individual hairs are released while other follicles in the same region may remain in active growth.

This asynchronous pattern allows hair density to be maintained despite continuous turnover.

Exogen and New Hair Growth

A new anagen hair may begin developing while an older club hair remains within the follicular canal. The old hair can subsequently be displaced and shed as the new growth progresses.

Club Hair

A club hair is the fully keratinized hair formed after active matrix growth stops. Its proximal end has a characteristic club-like shape and lacks an actively growing bulb.

Club hairs are normally associated with late stages of the follicular cycle and eventual shedding.

Initiation of a New Anagen Phase

A new growth cycle begins when epithelial stem and progenitor populations become activated and reconstruct the lower follicle.

Signals exchanged between the epithelial follicle and the dermal papilla are central to this transition.

Bulge Stem Cells

The bulge region of the follicle contains important epithelial stem cells. These cells are relatively quiescent for much of the cycle but can become activated during follicular regeneration.

Hair Germ

A population of epithelial progenitor cells located near the dermal papilla during the resting phase is often referred to as the hair germ. These cells participate in early events leading to the next anagen phase.

Dermal Papilla Signaling

The dermal papilla functions as a specialized mesenchymal signaling center. Communication between dermal papilla cells and follicular epithelial cells helps determine whether the follicle remains quiescent or enters a new growth phase.

Follicular Regeneration

Structure	Role in Regeneration
Bulge	Contains long-lived epithelial stem cells
Hair germ	Contains progenitor cells involved in early regeneration
Dermal papilla	Provides regulatory mesenchymal signals
Matrix	Rapidly proliferating population formed during active anagen

Asynchronous Cycling

In humans, neighboring hair follicles commonly occupy different phases of the cycle. One follicle may be actively producing hair while another is resting or shedding.

This differs from species in which large groups of follicles can cycle synchronously.

Scalp Hair Cycle

Most healthy scalp follicles are normally in anagen at any given time, while smaller proportions are in catagen and telogen.

The long anagen phase of scalp follicles explains why scalp hair can achieve much greater lengths than most body hair.

Body Hair Cycle

Many body hair follicles have shorter anagen phases and relatively different proportions of time spent in other phases. This limits the length of hairs on regions such as the arms, legs, and trunk.

Regional Variation

Region or Hair Type	General Cycling Characteristic
Scalp hair	Long anagen permits substantial length
Eyebrow hair	Shorter growth phase limits length
Eyelashes	Limited growth duration maintains short hair length
Body hair	Generally shorter anagen than scalp hair

Factors Affecting the Hair Cycle

Hair cycling is influenced by multiple local and systemic factors. These include genetics, age, anatomical location, hormonal signals, nutrition, inflammation, medications, systemic illness, and the local follicular microenvironment.

Hormonal Regulation

Hormones can alter the duration and behavior of different phases of the hair cycle. The effects vary substantially among body regions.

Androgens

Androgens have region-specific effects on hair follicles. They promote conversion of vellus hairs to terminal hairs in several body regions during puberty.

In genetically susceptible scalp follicles, androgen signaling can instead contribute to progressive follicular miniaturization and shortening of the effective growth phase.

Thyroid Hormones

Normal thyroid function contributes to normal hair follicle physiology. Disturbances in thyroid hormone status can be associated with changes in hair growth and shedding patterns.

Age

Aging can alter follicular cycling, hair shaft diameter, pigmentation, and the proportion of follicles producing terminal hairs. These changes vary by anatomical region and individual biology.

Hair Cycle and Chemotherapy

Because anagen matrix cells divide rapidly, they can be particularly vulnerable to drugs that target rapidly proliferating cells.

Damage to actively growing follicles can result in rapid hair loss known as anagen effluvium.

Anagen Effluvium

Anagen effluvium is hair loss resulting from disruption of actively growing anagen follicles. It can occur after exposure to certain cytotoxic agents and other severe insults to matrix cell proliferation.

Telogen Effluvium

Telogen effluvium occurs when an increased number of follicles enter telogen following a physiological or pathological trigger. Increased shedding becomes apparent after the affected hairs progress through the resting and shedding stages.

Potential Triggers of Telogen Effluvium

Trigger Category	Examples
Physiological stress	Major systemic stress or significant physiological change
Illness	Severe or febrile illness
Nutritional disturbance	Significant deficiency or restrictive intake
Hormonal change	Major endocrine or reproductive changes
Medications	Certain drugs can alter follicular cycling

Postpartum Hair Shedding

Changes in follicular cycling around pregnancy and the postpartum period can produce noticeable shedding. Hormonal conditions during pregnancy can prolong growth in some follicles, followed by a shift toward telogen after delivery.

Androgenetic Alopecia

In androgenetic alopecia, susceptible follicles undergo progressive miniaturization. The growth phase becomes shorter and the hairs produced become progressively finer and shorter.

Alopecia Areata

Alopecia areata is an immune-mediated disorder affecting hair follicles, particularly those in active growth. Follicular cycling becomes disrupted, producing areas of non-scarring hair loss.

Scarring Alopecia

In scarring forms of alopecia, inflammation can destroy the permanent follicular structures and stem cell compartments required for regeneration. Hair loss may therefore become permanent.

Hair Cycle and Follicular Miniaturization

Repeated shortening of anagen and reduction in follicular size can gradually transform a terminal hair-producing follicle into one producing a finer, shorter hair.

This process is particularly important in androgenetic alopecia.

Clinical Assessment of the Hair Cycle

Hair disorders can be evaluated using clinical examination, hair-pull testing, microscopy, dermoscopy, biopsy, and other methods depending on the clinical situation.

Understanding which phase is affected can help explain the timing and pattern of hair loss.

Histology of Anagen Follicles

Anagen follicles are long and deeply situated. A prominent bulb surrounds the dermal papilla, and the matrix shows active proliferation and differentiation.

Histology of Catagen Follicles

Catagen follicles show regression of the lower segment, loss of the active matrix, and progressive shortening of the follicle.

Histology of Telogen Follicles

Telogen follicles are short and lack an active hair bulb. A club hair may be present within the follicular canal.

Histological Comparison of Hair Cycle Phases

Feature	Anagen	Catagen	Telogen
Follicle depth	Deep	Decreasing	Relatively superficial
Matrix	Highly active	Regressing	Inactive
Hair production	Active	Stopping	Absent
Bulb	Large	Regresses	Absent as an active bulb
Club hair	Not characteristic of active growing hair	Developing	Present

Hair Cycle and Hair Density

Visible hair density depends not only on the number of follicles but also on the proportion of follicles actively producing substantial hair shafts, hair diameter, shaft length, pigmentation, and shedding rate.

Hair Cycle and Hair Length

Hair length is not determined simply by how fast hair grows. The duration of anagen is equally important because growth stops when the follicle leaves the active phase.

Key Features of the Hair Growth Cycle

Feature	Key Point
Anagen	Active hair growth
Catagen	Follicular regression
Telogen	Resting phase
Exogen	Hair shedding
Growth center	Hair matrix during anagen
Mesenchymal regulator	Dermal papilla
Stem cell compartment	Bulge region
Scalp hair length	Strongly influenced by long anagen duration
Normal cycling pattern	Generally asynchronous among human follicles
Clinical relevance	Altered cycling contributes to several forms of hair loss

Anatomical and Functional Importance

The hair growth cycle demonstrates the remarkable regenerative capacity of the hair follicle. During anagen, the follicle constructs a deep, highly active growth apparatus capable of continuously producing a keratinized hair shaft. During catagen, much of this apparatus undergoes controlled regression. The follicle then enters telogen before initiating another regenerative cycle.

This repeated remodeling depends on communication between the permanent epithelial compartments of the follicle and the dermal papilla. Stem cells in the bulge and nearby progenitor populations provide the cellular basis for reconstruction of the lower follicle.

The duration and regulation of these phases determine many visible characteristics of human hair, including its length, density, rate of shedding, and response to hormonal and pathological influences.

The hair growth cycle therefore links hair follicle anatomy, epithelial stem cells, dermal papilla signaling, keratinization, pigmentation, endocrine regulation, regeneration, and clinical hair loss.