Recent discoveries have shown that stem cells exhibit plasticity, enabling the production of cells from other tissues. However, hair follicle neogenesis and follicle cloning remain complex fields with significant potential for improvement and advancement in the future, explains Dr. Konstantinos Anastassakis.
Stem Cells and Hair Follicle Cloning
In recent years, there has been an unprecedented surge of interest in the role of stem cells in regenerative medicine and biomedical engineering. This progress has been accompanied by a deeper understanding of stem cell biology. Primitive hematopoietic stem cells initially sparked this interest due to their unique ability to restore the hematopoietic system in patients undergoing myeloablative therapy.
According to the traditional doctrine of cellular biology, these primitive or tissue-specific stem cells—isolated from a particular tissue—possess the capacity for renewal and differentiation exclusively into cell types of the tissue they reside in. However, in recent years, various studies have challenged this view, demonstrating that tissue-specific stem cells exhibit significant plasticity, meaning they can overcome their tissue origin and generate cells of other tissues. Hematopoietic stem cells (HSCs) have been central to these studies due to their easy isolation from bone marrow. Reports on stem cell plasticity have generated both great enthusiasm and significant skepticism.
The concept of plasticity challenges fundamental principles of developmental biology regarding tissue differentiation restrictions. The ability of adult stem cells to alter their fate holds enormous potential for the treatment of genetic and degenerative diseases. Embryonic stem cells and embryonic germ cells, with appropriate processing, represent an inexhaustible source of stem cells for organ or tissue repair following injury.
Moreover, evidence suggests that mature adult stem cells derived from various sources (e.g., skin) can generate not only their own lineages but also those of other tissues. This often involves overcoming embryonic-origin barriers previously considered insurmountable.
Before exploring the potential applications of stem cells in hair follicle biology, several fundamental concepts must be clarified.
What Are Stem Cells?
Rapidly self-renewing tissues, such as the epidermis and hair follicles, continuously produce new cells to replace dead squamous cells and hairs, respectively. These new cells arise from the ongoing differentiation and maturation of stem cells. However, to define a cell as a stem cell, it must meet three essential criteria:
- Self-renewal
- The ability to differentiate into multiple cell types
- The capacity to restore tissue in vivo
Perhaps the simplest definition of epithelial stem cells is based on their lineage: stem cells are primitive cells that develop into the fully differentiated and specialized cells of post-embryonic tissues.
Cloning Terminology
Before delving into the data analysis, it is essential to clarify the concepts encompassed by the term “cloning” in general, and “hair follicle cloning” in particular.
The term “cloning” has become widespread across many scientific and non-scientific fields, describing a variety of experimental and clinical procedures. This has led to confusion among patients and even healthcare professionals regarding the terminology used in hair follicle cloning research. The following explanation aims to clarify key terms and avoid misunderstandings:
- Clone:
Typically, the term clone describes cells (or organisms) derived from a single parent cell (or individual). The term is mostly used in eukaryotic systems, whereas in prokaryotes (bacteria), the term “colony” is more common. In humans, “clone” is often used in oncology to describe malignant cells originating from a single mutated cancer cell, and in hematology, where it refers to the lineage of cells derived from hematopoietic stem cells.
- Cloning:
Cloning is a term widely used in biotechnology, referring to a range of different procedures across various scientific disciplines, which often causes confusion and makes the term misleading. Molecular biologists use “cloning” to describe the insertion of genes into plasmids, which are then used to produce bacteria expressing a specific gene product. In cell biology, cloning refers to immortalization and expansion (creation of identical daughter cells) from a single cell, a term used for both somatic and embryonic stem cells.
However, due to the high-profile cloning of identical vertebrate animals (e.g., Dolly the sheep), by replacing the egg cell’s nucleus with that of an adult cell (nuclear transfer), the term cloning has become popularly associated with this specific procedure. Given that this technology raises ethical debates and that embryonic stem cell research is legally restricted in many parts of the world, it may be advisable to replace the term “cloning” with more precise terminology.
- Hair Follicle Bioengineering:
The term “cloning” is also used to describe efforts to create hair follicles de novo. Yet, this usage is again misleading. Current technology harnesses the proliferative potential of somatic stem cells found in various compartments of the hair follicle to multiply the follicle itself. Cell proliferation within the hair follicle is limited and does not involve “immortalization” as true cell cloning does. Furthermore, cell proliferation is only one step in generating a new hair follicle; the cells must be properly “assembled” to form a fully functional follicle.
For these reasons, the creation of new hair follicles is more accurately described by terms such as hair follicle bioengineering or folliculo-neogenesis and these terms will be used throughout the rest of the text..
Applications of Stem Cells
Recent breakthroughs in understanding stem cell biology have led to significant advancements in techniques that utilize individual cells to create mature organs, including the small intestine, mammary gland, and teeth. These groundbreaking achievements suggest that clinical applications of such technologies may become feasible in the near future.
Substantial clinical benefits are already being realized, such as bone marrow transplantation for the treatment of leukemias and corneal transplantation to restore vision lost due to chemical burns. These successes have been made possible because stem cells have been identified and isolated from the affected tissues themselves.
Levels of Complexity
In the effort to transform stem cells into bioproducts useful for regenerative medicine, cellular organization and the molecular interactions required for their proper function depend on a hierarchy of morphological complexity, presenting significant challenges for both research and clinical application:
- Level 1 Complexity: Bone marrow stem cells have already been applied clinically because blood cells do not require spatial organization, and differentiated blood cells can function immediately upon entering circulation. These stem cells represent the first level of complexity.
- Level 2 Complexity: Secretory tissues that release molecules such as insulin and dopamine represent the second level. While spatial organization of the secreting cells is less critical, strict regulation of synthesis and secretion of these potent biological molecules is essential.
- Level 3 Complexity: Tissues whose cellular morphology, structure, and organization are critical for function fall under the third level. These include the skin, cartilage, and bone.
- Level 4 Complexity: The most complex categories require not only correct architecture but also integration at the functional level, as seen in the cardiovascular and nervous systems.
Hair follicles used in hair transplantation belong to the third level of complexity, as they require intricate, three-dimensional organization to form fully functional follicular structures.
In normal skin regeneration, different cell types originating from two distinct germ layers give rise to multiple cell types, which further require interaction between these layers and proper anatomical orientation to function effectively. These demands make the “cloning” of such tissues even more complex.
Fundamental Concepts of Hair Follicle Anatomy and Embryology
It is universally accepted that a finely tuned sequence of reciprocal interactions between epithelial and mesenchymal cells is essential for hair follicle morphogenesis. In the embryonic skin, hair follicles develop through the coordinated interaction of epithelial and mesenchymal components. Generally, dermal cells are considered the inducers, while epithelial cells act as responders in the hair follicle formation process, although the signaling exchange between these cells is mutual and highly complex.
Unlike other organs, each hair follicle undergoes self-regeneration during every life cycle, mimicking its embryonic development. In fact, it is the only tissue in the mammalian body with this ability. Postnatally, the lower segment of the hair follicle is remodeled with each new cycle through the interaction between epithelial stem cells in the bulge region and adjacent mesenchymal-origin dermal papilla cells (DPCs).
Hair follicle morphogenesis proceeds via a cascade of molecular signals exchanged between cells of the embryonic ectoderm and the underlying mesenchyme. Zheng et al. (2010) demonstrated that the mechanism by which cells form hair follicles through organogenesis is strikingly conserved across mammalian species, earning the description “universal.” The mammals studied by Zheng et al. are evolutionarily distant— their last common ancestor lived approximately 120 million years ago—yet they share nearly identical hair follicle formation pathways both in vivo and in vitro from isolated cells.
Hair follicle formation begins with the emergence of the placode, followed by the formation of dermal condensates—loose clusters of mesenchymal cells in the skin that eventually give rise to the dermal papilla. Cells in the condensate induce a reduction in the growth rate of the placode. These two cell types further interact via reciprocal signaling, leading to follicle maturation. Finally, six major morphogenetic molecular signaling pathways regulate hair follicle development and its lifecycle: fibroblast growth factor (FGF), transforming growth factor-beta (TGF-β), sonic hedgehog (Shh), Wingless (Wnt) pathway, and the neurotrophin and homeobox gene families.
It is evident that orchestrating the actions of these molecules and processes is exceedingly complex and sensitive. As we will see, it remains currently impossible to replicate this process artificially.
What Kind of Hair Follicle Do We Want to “Construct”?
Hair follicles produced through folliculoneogenesis must meet certain fundamental anatomical and physiological criteria. Although it may seem obvious, it is important to begin by defining the exact characteristics of the hair follicle we aim to create via biomedical engineering:
- The proximal end of the appendage must exhibit the characteristic hair follicle structure, with the epithelial portion emerging from the distal end of the follicle, and the dermal papilla located at the base of the follicle.
- Proliferating cells should be positioned proximally, while differentiated cells reside deeper, distally, demonstrating a clear proximal-distal pattern of development.
- The follicle should consist of concentric layers including an outer and inner epithelial sheath, cortex, and medulla.
- The product of the hair follicle—the hair shaft—must form a unique molecular structure.
- Each follicle must be associated with a sebaceous gland.
- Each follicle should have the capacity to shed the old hair shaft (apoptosis) while maintaining stem cells and dermal papilla cells (DPCs) for the next growth cycle.
- The intrinsic ability of the follicle to regenerate a new hair shaft through repeated life cycles must be preserved.
Thus, for a bioengineered product to be rightfully called a “hair follicle,” all the above criteria must be fulfilled. Failure to meet any of these stages will result in defective follicular structures and consequently inadequate hair follicle formation.
Stem Cells, Hair Loss, Hair Transplantation and Expected Benefits
The concept of cultivating hair follicles through stem cell cloning to generate an unlimited number of new follicles is highly appealing from a theoretical standpoint. Marritt’s insightful study demonstrated that the average person affected by Androgenetic Alopecia (AGA) can lose up to 50% of their hair without noticeable thinning.
On average, a person has approximately 50,000 follicular units (FUs) across the entire scalp, with a total of around 120,000 to 150,000 individual hair follicles. The safe donor area accounts for roughly 25% of this total (about 12,500 FUs), while the remaining 37,500 FUs are at risk from AGA, as they reside in hormone-sensitive regions of the scalp. From the 12,500 FUs in the safe donor zone, approximately 50%, or roughly 6,250 FUs, can be safely harvested and transplanted without visibly thinning the donor site—according to a reverse interpretation of Marritt’s findings. Thus, a total of 6,250 FUs are available to cover an area originally containing 37,500 FUs. If evenly distributed, this results in about one-sixth of the original hair density.
Even when grafts are strategically placed to create the appearance of full coverage in certain areas, de novo folliculogenesis in the laboratory remains the only true solution to the limited FU supply. Additionally, this approach could prevent extensive trauma and scarring of the donor site that is typically required to harvest thousands of grafts.
The ideal “hair follicle cloning” scenario would begin with harvesting follicles from a small segment of the permanent hair zone, isolating stem cells, and then expanding these cells in vitro to generate a large number of follicles or manipulating them to produce hair follicles directly in culture.
Finally, these stem cells or lab-grown follicles would be implanted into bald areas of the scalp, where they would no longer be subject to the hormonal influences that drive AGA.
However, as promising and straightforward as this theory sounds, there are formidable obstacles to achieving successful follicle cloning.
A significant challenge is that cultured Dermal Papilla Cells (DPCs) appear to lose their inductive capacity in vitro after a limited number of passages. Another concern is whether these “cloned hairs” will look natural—will the follicles grow in the correct orientation, ensuring hairs emerge at proper angles relative to the scalp, or will they grow chaotically? Additional critical issues include long-term safety, given the oncogenic potential of stem cells, and the high cost associated with any such method, as will be discussed further.
Hair follicle cloning is, therefore, far from a simple matter
Stem Cell Research on Hair Follicles to Date
Many skin conditions—such as skin cancer, chronic wounds, skin atrophy, hypertrichosis, and androgenetic alopecia (AGA)—can be understood as disorders of adult stem cells. Since epidermal and hair follicle stem cells serve as the sole source of stem cells for both tissue types, understanding the regulation of their proliferation and differentiation is key to comprehending disorders related to these processes. Moreover, isolating, culturing, and expanding epithelial cells are crucial steps for treating skin disorders through biomedical tissue engineering.
Given the high demand for an effective and definitive treatment for AGA, regenerative cell therapy using Dermal Papilla Cells (DPCs) has been extensively studied as a potential option. Insights into folliculogenesis can be drawn from our current understanding of the morphogenesis and development of normal hair follicles.
In brief, hair follicle neogenesis can theoretically be achieved through one or more of the following approaches:
- Isolation, culture, and implantation of stem cells into the scalp to generate new hair follicles in situ at previously bald sites (full neogenesis).
- Isolation, culture, and implantation of stem cells that integrate into existing follicles and “redirect” them toward a different morphology. Specifically, in areas where follicles have miniaturized (vellus follicles), stem cell implantation and integration can induce production of mature hair follicles (morphogenic switch).
- Laboratory creation of de novo hair follicles that can be implanted into the scalp as fully functional units (proto-hairs), either unsupported or with external scaffolding.
Practically speaking, to date, experiments have not conclusively determined which approach is the most effective and feasible. In most studies, when DPCs are injected into the dermis, it remains unclear whether the cells induce new follicle formation from adjacent epithelium—that is, true follicle neogenesis—or simply integrate into neighboring follicles to enlarge them, converting miniaturized follicles into mature ones.
Stay tuned for the continuation of our series, where we will explore stem cells and hair follicle cloning in more detail.
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