Hair Follicle Biology

Understanding the biological system responsible for hair growth.

Hair growth is a continuous biological process controlled by specialised cells, molecular signals, and interactions between different structures within the skin. Each hair grows from a follicle that repeatedly moves through phases of growth, regression, and rest. Understanding this cycle helps explain why hair sheds naturally, why hair density can change, and why different forms of hair loss require different approaches.

Hair is more than the visible strand above the scalp. Its growth begins inside the hair follicle, a specialised structure embedded within the skin.

The follicle contains several types of cells that work together to produce the hair shaft, regulate growth, and support regeneration. Unlike many other structures in the body, hair follicles repeatedly undergo cycles of activity and relative inactivity throughout life.

These cycles are influenced by genetics, hormones, age, local cellular signals, and overall health.

Understanding the biology of the follicle is an important starting point for understanding hair health.

The hair follicle is a complex biological structure with several important components.

Hair bulb

Located at the base of the growing follicle, the hair bulb contains actively dividing cells that contribute to hair shaft formation.

Dermal papilla

A specialised group of cells at the base of the follicle that communicates with surrounding cells and helps regulate follicular growth and activity.

Hair matrix cells

These rapidly dividing cells produce the cells that form the hair shaft. As they mature, they become part of the keratinised hair fibre.

Hair follicle stem cells

These cells help maintain and regenerate the follicle during its growth cycle.

Sebaceous gland

Associated with the follicle, this gland produces sebum, an oily substance that contributes to lubrication of the skin and hair.

Together, these structures support the production and maintenance of hair.

Hair follicles do not remain in a permanent state of growth. They move through a repeating cycle.

Anagen — The Growth Phase

During anagen, cells in the hair matrix actively divide and produce the hair shaft.

Scalp follicles can remain in this phase for several years. The duration varies between individuals and helps determine how long scalp hair can grow.

Catagen — The Transition Phase

Catagen is a relatively short phase in which active growth stops and the lower portion of the follicle undergoes controlled regression.

This is a normal part of the hair cycle.

Telogen — The Resting Phase

During telogen, the follicle is relatively inactive. The existing hair may eventually shed as the follicle transitions into another growth cycle.

Shedding a hair does not necessarily mean that the follicle has stopped functioning.

Why this matters: Hair shedding is a normal biological process. However, excessive shedding, persistent thinning, or changes in hair density may indicate an underlying issue that deserves further assessment.

Hair growth depends on communication between different cell populations within the follicle.

Researchers have identified several signalling pathways involved in follicular development and regeneration, including WNT, SHH, and BMP signalling.

Hormones also influence hair follicles. For example, androgen sensitivity plays an important role in androgenetic alopecia, a common form of progressive hair thinning.

Genetics, ageing, nutritional deficiencies, certain illnesses, medications, and physical or emotional stress can also influence hair growth or shedding.

These factors do not affect every individual in the same way.

Hair thinning can occur through several different mechanisms.

In androgenetic alopecia, susceptible follicles gradually produce finer, shorter hairs through a process known as follicular miniaturisation.

In telogen effluvium, a larger-than-usual number of follicles enter the resting phase, resulting in increased shedding.

Hair breakage is different: the hair shaft becomes damaged and breaks, even though the follicle itself may remain active.

Distinguishing between these processes is important because they have different causes and may require different management strategies.

Healthy hair care involves protecting both the scalp environment and the hair fibre.

Gentle cleansing helps remove excess oil, sweat, and product residue. Appropriate conditioning can reduce friction and help limit hair-shaft breakage.

Adequate nutrition supports normal hair production, particularly when a nutritional deficiency is present.

However, cosmetic products that improve hair feel or reduce breakage should not automatically be considered treatments for follicular hair loss.

The right approach depends on whether the concern involves the scalp, the hair shaft, excessive shedding, or progressive thinning.

• Hair grows from follicles located within the skin.
• Each follicle undergoes repeated growth, regression, and resting phases.
• Genetics, hormones, health, and cellular signalling influence follicular activity.
• Hair shedding, hair breakage, and progressive thinning are different problems.
• Understanding the underlying cause is essential when evaluating hair loss.

Although researchers understand many aspects of hair follicle biology, important questions remain about how to translate laboratory findings into effective treatments for human hair loss.

Discovering a mechanism involved in hair growth does not, by itself, establish that a particular ingredient or cosmetic product can influence that mechanism in a clinically meaningful way.

Krause K, Foitzik K. Biology of the Hair Follicle: The Basics. Seminars in Cutaneous Medicine and Surgery. 2006.

Additional reading: Morphogenesis, Growth Cycle and Molecular Regulation of Hair Follicles. 2022.

Educational note: This article provides general information about hair biology and is not a substitute for individual medical assessment.