Follicular Cell Implantation: An Update on "Hair Follicle Cloning"

Jerry Cooley, MD Charlotte, North Carolina

Disclosure: Dr. Cooley is a consultant for Intercytex, Ltd., a Manchester, U.K. based company which is developing a follicular cell implantation treatment.

Terminology
The concept of culturing hair follicle cells in vitro and re-implanting them falls within the realm of cell therapy or tissue engineering applied to hair loss. The use of the term "cloning" is actually not appropriate when describing the process of expanding hair follicle cells in vitro. "Cloning," from the Greek word "twig," means to create an exact genetic replica by asexual means. The use of fetal or embryonic tissue is often an integral part of this technology. Scientists may refer to cloning a gene, cell, or whole organism, but tissue-engineered organs and tissues, such as hair follicles, are not generally referred to as clones.

I coined the term "follicular cell implantation" to refer to cell therapy for hair loss because I think it best describes what is being done without invoking some magical or mysterious process. Besides "hair follicle cloning," there are a number of other synonyms for this concept including "follicular
neogenesis," "follicular regeneration," and "hair multiplication."

Cell therapy is a burgeoning area of medical research and biotechnology
ventures. Cells and tissues grown and manipulated in the laboratory will be viable treatment options for a whole range of genetic and degenerative diseases in the future. Follicular cell implantation would be one more example to add to this growing list.

Rationale
Of course, the driving force behind follicular cell implantation is the promise of unlimited donor hair. Other potential benefits include the lack of any significant donor scar, the creation of greater density, and a more easily tolerated procedure from the patient's perspective.

Conceptual Framework
The basic idea with follicular cell implantation is: 1) to harvest a small sample of hair follicles from the same donor region used in hair transplantation, 2) to isolate the follicle-inducing cells of the hair follicle and multiply these using cell culture, and 3) reimplant these cells so that new follicles are created in the recipient skin (Figure 1). Because of the cell division that occurs while culturing, countless new cells are created that when implanted result in hundreds of new hair follicles (Figure 2).

Background
The basis for follicular cell implantation began in the fundamental investigation of normal hair growth. It had been known that hair growth results from a dynamic interaction of epidermal and dermal components. This dermal-epidermal interaction is what determines follicle development in the fetus, as well as normal hair shaft production during the anagen phase of the hair growth cycle.

Colin Jahoda and his colleagues showed that the cells of the rat whisker dermal papilla could be grown in vitro and that the cultured cells could induce hair growth when implanted into incisional skin wounds of the rat. Presumably the implanted cells were interacting with native epithelial cells to re-create hair follicles and produce a hair shaft. The process by which implanted cultured dermal papilla cells would induce hair growth is conceptually similar to what happens during fetal development of the hair follicle and the normal anagen phase of the growth cycle (Figure 3).

This work, published in the peer-reviewed medical literature, has been replicated in humans by several investigators including Jahoda, as well as myself, working with Dr. Jim Vogel and reported at the 1996 ISHRS meeting in Nashville, Tennessee. Dr. Walter Unger, the Aderans Research Institute, and several others have also succeeded. However, nothing has appeared in the way of detailed studies to describe the actual techniques used in these human studies. Because of the inherent commercial value of successful research, the importance of protecting intellectual property has overshadowed the impetus to publish.

Obstacles and Challenges
The use of tissue-engineered cells to treat hair loss is conceptually quite simple, but there are numerous obstacles. The fact that research describing follicle induction using implanted follicular cells is almost 20 years old, and yet only inconsistent success in replicating this in humans has been achieved, highlights the complexity and challenges ahead.

Autologous vs Allogeneic
Using allogeneic cells for hair loss, may also be possible because some portions of the hair follicle (e.g., dermal sheath) appear to be "immunologically privileged" and can naturally escape immunologic detection. Jahoda and Reynolds showed that in humans the dermal sheath can be implanted into the skin and induce hair growth in a genetically unrelated person. The sheath apparently lacks cell surface antigens that would identify itself as foreign and is therefore "immunologically privileged." Whether cultured dermal sheath cells could be used as a reliable source of cells and kept in "cell banks" remains a question for future research.

Which Follicular Cells Must Be Implanted?
The potential candidate cells in the hair follicle include (see Figure 4):

Dermal cells

• Dermal papilla
• Dermal sheath

Epidermal cells

• Interfollicular keratinocyte,
• Outer root sheath
• Stem cells from the bulge
• Germinal epithelial cells

The original research by Jahoda indicated the dermal papilla alone may be capable of follicle induction. However, the dermal papilla may not be the ideal candidate and the most recent report by Jahoda showed that cultured dermal papilla failed to induce follicle formation in a wound model. Subsequent research has shown certain beneficial "stem cell"-like properties to the lower dermal sheath such as immunologic privilege, suggesting it may be superior to the papilla for follicle induction.

If dermal cells are implanted alone, they must somehow make contact with epidermal cells present in the recipient site skin. One way of overcoming this problem is to co-implant cultured epidermal cells, such as from the "bulge," outer root sheath, or germinal epithelial cells located in the matrix. But including cultured epidermal cells raises a whole new set of problems that dramatically increases the complexity of the procedure.

Maintaining Inductive Potential
One important finding of Jahoda's research was that after several passages in culture, the papilla cells lost their ability to induce hair growth when re-implanted. Presumably, the cells were changing over time, becoming more like fibroblasts and less like hair-inducing papilla cells.

The limitation of carrying papilla cells through multiple passages could be a major obstacle for the development of a treatment for hair loss. The success of using the cultured hair follicle cells to treat hair loss will in large part depend on the ability to expand their numbers significantly prior to re-implantation so that only a few hairs could give rise to thousands of hairs. If the cells lost their ability to induce follicle formation and hair growth after several passages in culture, achieving this goal would not be possible.

Several techniques have been devised to keep the cultured cells inductive. The
inductive capability of these cells can be verified by using an animal assay (Figure 5). A focus of current research is to identify the changes in gene expression that occur as dermal papilla cells are cultured over time.

Will "Cloned Hair" Look Normal?
Another important question is whether tissue-engineered hair will be cosmetically acceptable. The animal studies to date have not characterized the regenerated hair to determine how closely it matches the donor hair from a cosmetic standpoint (e.g., color, curl, caliber). The regenerated rat whiskers have been described as grossly appearing to be similar to the original whiskers.

Presumably regenerated hairs in humans would also look similar or identical to the "parent" hairs, but this would obviously be an important point to establish. Furthermore, the regenerated follicles must be oriented so that the hair grows at the proper angle and direction.

Economic And Regulatory Hurdles
In addition to the scientific challenges facing the use of implanted hair follicle cells, there are also significant legal and regulatory hurdles. In the United States, the U.S. Food and Drug administration (FDA) would regulate implanted hair cells as a biologic therapy and has proposed a comprehensive regulatory framework for cell-based therapies. Therefore, FDA approval and adherence to these regulatory requirements is a prerequisite before a company can bring follicular cell implantation to market.

The economic costs of developing a government approved cell culture therapy are substantial. The physical facilities must adhere to Good Manufacturing Practice (GMP), which involves costly bureaucratic requirements. Highly trained personnel must be employed to isolate and culture the follicular cells. These economic hurdles may be as significant as the "scientific" hurdles.

Safety
One important safery concern would be whether the implanted cells have any tendency toward tumor formation. The fact that so many other cell types are being used successfully in tissue-engineering applications suggests that tumorigenicity is not a major concern. Tumor formation has not been reported in follicular cell implantation experiments. Also, accepted cell therapy techniques must be adhered to prevent transmission of infectious diseases.

Conclusion
Follicular cell implantation has the potential to overcome many of the limitations of current surgical hair restoration, especially the finite supply of donor hair. Years of animal research have demonstrated the soundness of the basic concept but reports in humans have shown inconsistency and problems with reproducibility. Some of these problems include: 1) identifying the ideal cell type or types of the follicle to isolate and culture, 2) how ro keep the cells inductive throughout the culturing process, and 3) economic and regulatory hurdles to bringing a safe and effective treatment to the marketplace. The prospect of having an unlimited supply of donor hair available to treat hair loss will continue to spur tissue-engineering-based research to overcome these hurdles.

to the top

Figure 1. Conceptual framework for follicular cell implantation. A small piece of skin containing hair follicles is removed from the occipital scalp. The cells of the hair follicle necessary for inducing follicle formation are isolated and grown in culture where they rapidly divide and multiply. The cells are then re-implanted in the skin where they induce new follicle formation resulting in new hair growth. back to article

Figure 2: Photographs of cultured dermal papilla over time. When placed in culture, cells have an inherent capacity to divide and multiply. This is the basis for producing vast numbers of hair follicles from just a few donor follicles. (Courtesy of Intercytex, Ltd.) back to article

Figure 3: Fetal development of the hair follicle, as well as normal anagen hair growth, involves a complex interaction of epithelial (A) and mesenchymal (B) components. In cell therapy, the implantation of dermal papilla cells into the epidermis and subsequent induction of hair follicles mimics this natural process. back to article

Figure 4: Schematic representation of the hair follicle, showing the location of cell types that might be used for cell implantation. GE cells = germinal epithelial cells. back to article

Figure 5: Cultured human dermal papilla and epithelial cells implanted on the backs of immune deficient mice retain their ability to induce follicle formation and produce hair despite being expanded in vitro. (Courtesy of Intercytex, Ltd.) back to article

Transplant Forum International - March/April 2004

to the top