Why Only the Back of the Head Keeps Its Hair — Reading Donor Dominance Through Stem Cell Conditioned Media2026.07.07
“When I look at myself in the mirror, my M-shape hairline and receding edges are what stand out. When I peek at the top, the crown looks thin” — and yet, the back of the head still holds a full head of hair. This puzzling asymmetry is something many people struggling with hair loss have experienced. In fact, there is a medical reason behind this phenomenon, and understanding it is closely linked to how we design treatment for hair loss and hair regenerative medicine using stem cell conditioned media.
Why does only the hair on the back of the head remain until the very end? Behind this is a phenomenon known as “Donor Dominance.” Proposed by Dr. Orentreich in New York in 1959, this concept is both the scientific basis of why hair transplantation works and a reflection of the true nature of AGA (androgenetic alopecia) — a condition that is anything but simple.
In this article, we explain the molecular mechanism that protects hair on the back of the head, and then from the perspective of AVAN TOKYO Ginza, Dr. Moriwaki outlines what conditioned media therapy can and cannot aim for against this “difference in sensitivity.”
Key Points of This Article
・The reason the back of the head keeps its hair the longest is a phenomenon called “donor dominance,” caused by differences in DHT sensitivity between scalp regions
・Follicles at the front and crown of the scalp have more type II 5-alpha reductase and are strongly affected by DHT, while occipital follicles are inherently less sensitive
・Hair transplantation moves follicles “while they retain their occipital character,” a clinical application of donor dominance
・Stem cell conditioned media is not a therapy that changes the DHT sensitivity of a follicle itself; rather, it aims to send signals that return miniaturized follicles to the growth (anagen) phase by adjusting the surrounding environment
・Preserving the follicles at the back while lifting the follicular environment at the front and crown — this asymmetric design is essential to any treatment plan
Why Does the Back of the Head Hold On? — The “Donor Dominance” Phenomenon
In hair transplantation, follicles are harvested from the back of the head and moved to the front or crown. The reason this approach has worked clinically for decades lies in the observation that “even after being transplanted, the follicles retain their occipital character.” This is the concept of donor dominance. Even placed in the vulnerable environment of the frontal scalp, occipital follicles continue to resist thinning — revealing that each follicle carries a “regional personality” of its own.
The Core: Regional Differences in DHT Sensitivity
The main driver of thinning and shedding in AGA is testosterone being converted to dihydrotestosterone (DHT) by 5-alpha reductase, and DHT binding to androgen receptors in the follicle. When this binding continues, the growth phase shortens and the follicle miniaturizes.
What matters here is that 5-alpha reductase is not uniformly distributed across the scalp. In particular, type II 5-alpha reductase is more abundant in follicles at the front and crown, and less so in occipital follicles. Even within the same head, occipital follicles are less exposed to DHT effects and are also thought to express relatively less androgen receptor. As a result, the hair on the back is less affected even at the same circulating testosterone level.
Gene Expression Profiles of Occipital Follicles
Recent research shows that follicles from the back and front of the head differ in their gene expression profiles themselves. It is not just about hormone sensitivity — the character of the dermal papilla cells varies by region. This “innate character” is carried over even after transplantation, which is why hair grafts survive.
Part of why AGA is said to be “untreatable” is that this sensitivity gap cannot be fully flipped by drugs. Finasteride and dutasteride inhibit 5-alpha reductase, but they do not change the androgen receptor expression level in the follicle itself. This leaves a “gap” that medication alone cannot fully bridge.
Stem Cell Conditioned Media Targets a Layer Beyond “Sensitivity Differences”
So what can conditioned media do against this “asymmetry between the back and front of the head”? To be blunt: this therapy is not a drug that changes the DHT sensitivity of the follicle itself. That boundary needs to be drawn honestly. What it targets is a different layer — the “microenvironment (niche)” surrounding the follicle.
The Idea of Returning Miniaturized Follicles to Anagen
Miniaturized follicles have not been completely lost — many are simply having difficulty entering the growth phase. The diverse growth factors contained in stem cell conditioned media, such as VEGF, IGF-1, HGF, and KGF, have been shown at the basic research level to potentially act on pathways for dermal papilla activation, angiogenesis, and maintaining the anagen phase.
In other words, this therapy does not create “follicles with occipital-level resistance.” Instead, it tries to send signals that pull miniaturized, dormant follicles back into the growth phase, even while their sensitivity remains high. The tendency to lose hair at the front and crown cannot itself be changed, but performance within the potential the follicle still has can be lifted — that is the practical goal.
Lifting the Whole Scalp Environment
The frontal and crown areas are places where multiple disadvantages overlap: high DHT sensitivity plus microinflammation, decreased blood flow, sebum imbalance, and more. What conditioned media can aim for is to soften this stack of negatives and bring the scalp environment closer to neutral. Even if the sensitivity barrier remains, bringing environmental negatives closer to zero allows the follicle to draw out the ability it originally has. This idea is discussed repeatedly in our other columns; see also our Hair Regenerative Medicine Column Index for more.
Treatment Design — “Protect the Back, Focus on the Front and Crown”
In clinical practice, understanding donor dominance directly shapes how treatment is distributed. Because the back of the head resists thinning, it is also preserved as a potential future donor resource for hair transplantation. There is no need to blindly stack RF treatments or high-frequency injections on the occipital area. It is more reasonable to concentrate treatment on lifting the environment at the front and crown.
That said, the back is not “absolutely immune to thinning.” Occipital density can decrease gradually with age, and local factors such as pressure, scarring, or seborrheic dermatitis can still affect it. For AGA treatment guidelines and differential diagnosis of scalp diseases, the Japanese Dermatological Association guidelines are also useful. Whether you can carefully design this asymmetry — protecting the occipital area while focusing on the front and crown — is the most important perspective for making stem cell conditioned media work.
“Starting while the back is still full” is a big advantage, both in terms of preserving donor resources and in terms of psychological breathing room. Once occipital density starts to decline, the available options narrow significantly.
Frequently Asked Questions
Q. If I use stem cell conditioned media, will the hair at the front become as strong as the hair at the back?
Unfortunately, conditioned media is not a drug that changes the DHT sensitivity of a follicle itself. It cannot transform frontal follicles into “having the same character as occipital follicles.” However, it can work in the direction of adjusting the environment around miniaturizing follicles at the front and crown to bring them back into the growth phase, so it can be expected to compensate for part of the disadvantage of high sensitivity. There is individual variation, and how change appears differs by person.
Q. Should I also inject conditioned media into the back of the head?
Because the back of the head resists thinning and holds value as a future donor resource for hair transplantation, treatment is generally designed to focus first on improving the environment at the front and crown. If there is a clear reason for the occipital area — such as local inflammation or pressure-induced alopecia — it is considered individually. That said, occipital follicles are still affected by aging and external factors, so it is not a “never inject” zone.
Q. Which should come first — hair transplantation or stem cell conditioned media?
Generally, the phase of “protecting and nurturing the hair you have” using oral/topical medications and conditioned media comes first. If density gaps remain after that, hair transplantation is considered next. It is not uncommon to continue conditioned media after transplantation to protect the original hair as well. The order should be decided individually based on progression and personal goals.
Q. Is donor dominance equally strong in everyone?
There is individual variation. Family history, progression speed, and age affect how strong occipital resistance is. During consultation, we use a microscope to compare hair caliber and hairs per follicular unit between the front and back, objectively evaluating how much has been preserved. If “the back is also thinning,” the treatment design needs to be revisited early.
Q. How long does it take to judge the effect of conditioned media?
Because of the hair cycle, evaluating change objectively takes at least 3–4 months, and often around 6 months. It is important to combine multiple metrics — photos, trichoscopy, hair caliber measurement — rather than relying on a single subjective impression. Effect varies individually and depends on progression and combined therapies.
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Medical Supervision: Shin Moriwaki, MD (Supervising Physician)
Member, Japan Society of Aesthetic Surgery (JSAS) / Member, American Academy of Aesthetic Medicine
ECFMG Certificate holder
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