" Discussion: Over the last decade, major advances have been made in both understanding and treatment of hair loss. At least three distinct breakthroughs increased hope and interest for future research: development of selective 5α-reductase inhibitors, development of specific cell culture systems, identification of the physiology of the hair follicle bulge, and their reservoir of stem cells.
These last two developments have made the hair transplantation process possible with the development of specific cell culture systems as the subject of recent studies of combined medical and surgical treatments (25, 26). The hair follicle bulge serves as an excellent reservoir of epithelial stem cells (27). This fascinating structure has been already identified in limited areas such as the bottom of the intestinal crypts, the limbal zone of the cornea, and the subventricular zone of the brain (7).
In theory, several steps are needed for succes sful hair follicle generation under in vitro conditions: isolation of epithelial and dermal papilla cell populations, expansion of their numbers by culture, maintenance of the proliferative and inductive properties of each cell linage, and the provision of exogenous signals to enhance their interaction. However, the final trichogenic ability should be assessed in an in vivo model. In this study, we attempted to isolate, culture, and test the ability of epithelial stem and dermal papilla cells to induce hair growth in an in vivo model. Immunofluorescence staining showed that the human hair scalp expressed both CD200 and K15; hence, epithelial stem cells could be isolated and cultured in vitro. Regarding CD200 expression in bulge area (between the SG and insertion of the APM), we selected CD200 for more analysis. Isolation of epithelial cells and evaluation of CD200 expression after culture showed the presence of hair stem cells. Expression of alkaline phosphatase by cultured dermal papilla cells showed inductive properties of these cells in vitro. Toluidine blue showed that after the third A D B E C CELL JOURNAL(Yakhteh), Vol 19, No 2, Jul-Sep (Summer) 2017 267 Nilforoushzadeh et al. passage, dermal papilla cells had the capability to produce extracellular matrix.
Although injection of mixed dermal papilla and epithelial cells led to hair growth in mice in our experiment, none of the mice that only received dermal papilla cells had visible hair growth. It seemed that while dermal papilla cells could not induce detectable hair follicles when injected alone, they assisted with hair growth when mixed with epithelial cells. Inamatsu et al. (14) evaluated the ability of rat embryonic and adult dermal papilla cells in inducing hair growth on a piece of rat’s sole skin.
Both cultured dermal papilla cells and an intact dermal papilla between the epidermis and dermis resulted in the generation of hair follicles. They concluded that epidermal cells reprogram embryonic processes when exposed to embryonic dermal papilla cells, whereas they directly initiate anagen of the hair cycle when they receive stimulatory signals from adult rat dermal papilla cells. Aoi et al. (15) evaluated the ability of rat dermal papilla cells to induce hair follicle growth in five transplantation methods in vivo.
They observed new hair follicle regeneration in the rat’s sole skin regardless of the transplantation method. However, the hemi-vascularized sandwich (HVS) method was superior in terms of number and maturity of follicles, which was attributed to direct epithelialmesenchymal signaling and better vascularization/ oxygenation. Several possible explanations exist for the observed discrepancy between reported results. First, the inductive ability of dermal papilla cells might decrease after multiple passages.
However, two recent developments were associated with increased hair-inductive activity of cultured dermal papilla cells: first, the co-culture of epidermal keratinocytes with dermal papilla cells prolonged the hair-inductive property of dermal papilla cells. This feature was mostly attributed to unknown soluble factors secreted by epidermal keratinocytes, which maintained the hair-inductive capability of dermal papilla cells during their population doublings in culture. Second, Wnt signaling from external source resulted in a preserved hairinductive capability of dermal papilla cells (28). Although alkaline phosphatase and toluidine blue staining showed trichogenic properties of our cultured dermal papilla cells, additional factors might be needed to confirm trichogenicity of these cells in the culture. Second, the culture condition was another possible explanation for reduced trichogenicity. However, Kang et al. (29) evaluated the hair-inducing capacity of dermal papilla cells in three-dimensional (3D) spheroid cultures. Mixtures of mouse epidermal cell with two-dimensional (2D) - or 3D-cultured dermal papilla cells were implanted into nude mice and compared with implantation of epidermal cells alone.
Only combined mouse epidermal and mesenchymal implantation induced new hair follicle formation. Interestingly, various 3D dermal papilla passages resulted in hair follicle formation, while no new hair follicles were obtained with the same 2D cultures. Conclusion We reported that injection of adult human cultured hair epithelial cells in combination with cultured dermal papilla cells without any matrix induced the growth of detectable hair in a mouse model. Although preliminary, these findings demonstrated that cultured epithelial and dermal papilla cells could open a new window to treat hair loss, especially in non-scarring alopecia. "
