Understanding how hair grows helps you make better decisions about treating hair loss.
Most people think of hair as simple strands. It’s not.
Each hair follicle is a complex, living mini-organ that cycles through growth, rest, and regeneration throughout your entire life. This article explains how that cycle works, why hair loss happens, and how modern treatments like CRP and CIT use this science to restore hair.
What Will I Learn?
Why Hair Matters More Than You Think
Hair serves no vital biological function. You don’t need it to survive.
But its psychological impact is enormous.
As dermatologist Richard L. De Villez, MD noted in his research on hair biology:
“Hair provides no vital function for humans, but its psychological effect is nearly immeasurable. Luxurious scalp hair expresses femininity for women and masculinity for men.”
This explains why hair loss can be so distressing, and why people go to significant lengths to address it.
Hair Growth By the Numbers
Your scalp contains between 100,000 and 150,000 individual hair follicles.
That number is set at birth. You don’t grow new follicles as an adult.
Here’s what the research shows about hair growth rates:
That works out to roughly half an inch per month for scalp hair.
Interestingly, scalp hair grows slightly faster in women than in men.
The Hair Growth Cycle Explained
Every hair follicle on your body cycles through three distinct phases.
This cycle repeats throughout your entire life. Understanding it explains why hair loss treatments work the way they do.
Anagen: The Growth Phase
This is when your hair actively grows.
During anagen, cells in the hair matrix divide rapidly. The follicle reaches its maximum length. Pigment is produced.
On your scalp, anagen lasts 4 to 8 years. That’s why scalp hair can grow so long.
Eyelashes and body hair have much shorter anagen phases, just 1 to 6 months. That’s why they stay short.
At any given time, about 90% of your scalp hairs are in anagen.
Catagen: The Transition Phase
This is a short transitional period lasting about 2 to 3 weeks.
Growth stops. The follicle shrinks. The hair detaches from its blood supply.
De Villez describes the onset of catagen as “an interruption of medullary mitotic activity and the simultaneous cessation of melanogenesis.” In plain terms, the hair stops growing and stops producing pigment.
Only about 1% of your hairs are in catagen at any time.
Telogen: The Resting Phase
The follicle rests. The hair sits dormant.
Telogen lasts about 2 to 4 months for scalp hair.
At the end of telogen, the old hair sheds, and a new anagen phase begins.
About 10% of your scalp hairs are in telogen at any time.
This is why losing 50 to 100 hairs per day is completely normal. Those are telogen hairs completing their cycle.
The Dermal Papilla: The Brain of the Follicle
At the base of every hair follicle sits a small cluster of cells called the dermal papilla.
Think of it as the follicle’s control center.
The dermal papilla contains fibroblasts, blood vessels, and signaling cells that regulate hair growth.
Research has shown that hair follicles without a dermal papilla cannot produce hair. If you transplant dermal papilla cells to a follicle with a damaged matrix, a new matrix will form, and hair production resumes.
This is why CIT (Cole Isolation Technique) focuses on preserving stem cells in the dermal papilla region. Damage the papilla, and you damage the follicle’s ability to regenerate.
What Actually Causes Balding
Here’s what most people get wrong: balding isn’t about follicles dying.
It’s about follicles shrinking.
De Villez explains it clearly:
“The balding process is a conversion of the follicles so that they produce vellus hairs rather than terminal hairs.”
Your thick, visible terminal hairs gradually become thin, colorless vellus hairs.
The follicle is still there. It’s still producing hair. But the hair is so fine you can’t see it.
The Role of DHT
Testosterone converts to dihydrotestosterone (DHT) in certain cells via an enzyme called 5-alpha-reductase.
In genetically susceptible follicles, DHT binds to receptors and triggers miniaturization.
Each hair cycle produces a slightly thinner hair. Over years and decades, terminal hairs become vellus hairs.
This is why DHT-blocking medications like finasteride can slow hair loss. They reduce the hormone causing miniaturization.
It’s also why hair transplants work. Follicles from the back and sides of the scalp are genetically resistant to DHT. When transplanted, they keep that resistance.
Why This Science Matters for Hair Restoration
Understanding hair biology isn’t just academic. It directly informs how modern treatments work.
CRP and Growth Factors
CRP (Cytokine-Rich Plasma) delivers concentrated growth factors directly to follicles.
These growth factors stimulate the dermal papilla, prolonging anagen and encouraging thicker hair production.
CRP also promotes blood vessel formation around follicles. Better blood supply means better nutrient delivery to the hair matrix.
This is why CRP can accelerate graft growth after transplants. The growth factors push follicles into active anagen faster.
CIT and Stem Cell Preservation
Cole Isolation Technique (CIT) is designed around follicle biology.
The minimally invasive extraction leaves stem cell remnants in the donor area. This precision requires understanding each patient’s unique follicular anatomy and angles.
These stem cells, located in the bulge region of the follicle, can regenerate new follicles.
With ACell treatment, 30% to 40% of the extraction sites regenerate. That’s 300 to 400 extra hairs per 1,000 grafts.
Traditional extraction methods damage these stem cells. CIT preserves them.
Stem Cell Treatments
Stem cell therapies work by targeting the dermal papilla and hair matrix.
Injected stem cells release growth factors that can reactivate dormant follicles.
Early research shows they may even reverse some miniaturization, converting vellus hairs back toward terminal hairs.
This is cutting-edge science, but it’s grounded in the same biology De Villez documented decades ago.
What Determines Hair Color and Texture
Hair color comes from melanin produced by melanocytes in the hair bulb.
Black and brown hair contains eumelanin. Red hair contains pheomelanin. Blonde hair has incompletely melanized eumelanosomes.
Gray hair? The melanocytes are still there, but they’ve stopped producing pigment effectively.
Hair texture depends on follicle shape:
Straight follicles produce straight hair. Curved follicles produce curly hair.
De Villez notes:
“Mongoloids have straight hair because their hair follicles are straight and perpendicular to the surface of their skin. Blacks have spiral-shaped hair because their hair follicles are curved.”
This matters for hair transplants. Curly hair requires different extraction techniques than straight hair to avoid follicle damage.
How Nutrition Affects Hair Growth
Your follicles need nutrients to produce hair.
Protein is essential. Hair is primarily keratin, a protein built from amino acids, particularly cysteine.
Severe malnutrition visibly affects hair. De Villez observed:
“Individuals who are on diets that are unsuitable for weight loss, children who are starving and who have kwashiorkor disease, and adolescents who are suffering from anorexia nervosa all grow hair that is fine, short, and either unpigmented or copper-colored.”
Crash diets can trigger telogen effluvium, where large numbers of follicles simultaneously enter the resting phase.
For healthy hair, maintain adequate protein, iron, zinc, biotin, and vitamins A, D, and E.
The Bottom Line
Hair growth is a complex biological process.
Your follicles cycle through growth, transition, and rest phases continuously. Balding occurs when follicles miniaturize, not when they die.
This science explains why modern treatments work:
CRP delivers growth factors that stimulate the dermal papilla and prolong anagen.
CIT preserves stem cells that enable donor regeneration.
Stem cell therapies may reactivate dormant follicles.
DHT blockers slow miniaturization at its hormonal source.
Understanding the biology helps you make informed decisions about your hair restoration options.
Ready to discuss what might work for you? Schedule a free consultation with our team.
Frequently Asked Questions about Hair Growth
How fast does hair grow?
Scalp hair grows about 0.44 mm per day at the crown and 0.39 mm per day at the temples.
That’s roughly half an inch per month or 6 inches per year.
Growth rates vary slightly by individual. Women’s scalp hair tends to grow slightly faster than men’s.
Why do some hairs grow longer than others?
Hair length is determined by the duration of the anagen (growth) phase, not growth speed.
Scalp hair has an anagen phase of 4 to 8 years, allowing it to grow very long.
Eyelashes and body hair have anagen phases of just 1 to 6 months, which is why they stay short.
Is it normal to lose hair every day?
Yes. Losing 50 to 100 hairs per day is completely normal.
About 10% of your scalp hairs are in the telogen (resting) phase at any time. When telogen ends, those hairs shed.
This is part of the normal cycle. New hairs replace the shed ones.
Do bald people still have hair follicles?
Yes. Balding isn’t about follicles dying.
The follicles miniaturize, producing thin, colorless vellus hairs instead of thick terminal hairs.
The follicles are still there. This is why some treatments can potentially reverse miniaturization
What is the dermal papilla and why does it matter?
The dermal papilla is a cluster of cells at the base of each follicle that controls hair growth.
It contains blood vessels, signaling cells, and stem cells.
Research shows follicles without a functioning dermal papilla cannot produce hair. This is why CIT is designed to preserve stem cells in this region.
Can nutrition affect hair growth?
Absolutely. Hair is made of protein (keratin) and requires nutrients to grow.
Severe deficiencies in protein, iron, zinc, or vitamins can cause hair thinning or shedding.
Crash diets are a common trigger for telogen effluvium, where many hairs simultaneously enter the resting phase and shed.
References
1. De Villez RL. Hair Growth Study. Division of Dermatology, University of Texas Health Science Center, San Antonio.
2. Paus R, Cotsarelis G. The biology of hair follicles. N Engl J Med. 1999;341(7):491-497. PubMed
3. Stenn KS, Paus R. Controls of hair follicle cycling. Physiol Rev. 2001;81(1):449-494. PubMed
4. Messenger AG, Rundegren J. Minoxidil: mechanisms of action on hair growth. Br J Dermatol. 2004;150(2):186-194. PubMed
