Restoration of hair-inductive activity of cultured human follicular keratinocytes by co-culturing with dermal papilla cells.

Hair follicle outer root sheath (ORS) cells can be expanded in vitro, but often lose receptivity to hair-inducing dermal signals. Recent studies have shown hair-inductive activity (trichogenicity) can be restored in rat ORS cells expanded with a fibroblast feeder by co-culturing with rat vibrissae dermal papilla (DP) cells. In this study, we investigated whether the trichogenicity of human ORS cells can be restored by co-culturing with human DP cells. ORS cells from human scalp hair follicles were cultured independently or with DP cells for 5 days and implanted into nude mice alongside freshly isolated neonatal mouse dermal cells. Although there was no hair induction when monocultured ORS cells were implanted, it was observed in co-cultured ORS cells. We also observed differential regulation of a number of genes in ORS cells co-cultured with DP cells compared to monocultured ORS cells as examined by microarray. Taken together, our data strongly suggest that human DP cells restore the trichogenicity of co-cultured ORS cells by influencing ORS gene expression through paracrine factors.

Knockdown of FOXA2 Impairs Hair-Inductive Activity of Cultured Human Follicular Keratinocytes.

Reciprocal interactions between hair-inductive dermal cells and epidermal cells are essential for de novo genesis of hair follicles. Recent studies have shown that outer root sheath (ORS) follicular keratinocytes can be expanded in vitro, but the cultured cells often lose receptivity to hair-inducing dermal signals. In this study, we first investigated whether the hair-inductive activity (trichogenicity) of cultured human ORS follicular keratinocytes was correlated with the cultivation period. ORS follicular keratinocytes from the scalp were cultured for 3, 4, 5, or 6 weeks and were then implanted into nude mice along with freshly isolated neonatal mouse dermal cells. We observed that the trichogenicity of the implanted ORS cells was inversely correlated with their cultivation period. These initial findings prompted us to investigate the differentially expressed genes between the short-term (20 days) and long-term (42 days) cultured ORS cells, trichogenic and non-trichogenic, respectively, by microarray analysis. We found that forkhead box protein A2 (FOXA2) was the most up-regulated transcription factor in the trichogenic ORS cells. Thus, we investigated whether the trichogenicity of the cells was affected by FOXA2 expression. We found a significant decrease in the number of induced hair follicles when the ORS cells were transfected with a FOXA2 small interfering RNA versus control small interfering RNA. Taken together, our data strongly suggest that FOXA2 significantly influences the trichogenicity of human ORS cells.

Future horizons in hair restoration.

This article reviews the history of hair follicle regeneration from follicular fragments and dissociated cells. The challenges of trichogenic in vitro culture and subsequent delivery into the patient are discussed, as well as cosmetic acceptance, recent achievements on regeneration of human hair follicles, and new potential cell sources for hair regeneration.