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The nude mouse graft model for testing the hair-forming ability of selected cell populations has considerable potential for providing insights into factors that are important for hair follicle development and proper hair formation. We have developed a minimal component system consisting of immature hair follicle buds from newborn pigmented C57BL/6 mice and adenovirus E1A-immortalized rat vibrissa dermal papilla cells. Hair follicle buds contribute to formation of hairless skin when grafted alone or with Swiss 3T3 cells, but produce densely haired skin when grafted with a fresh dermal cell preparation. The fresh dermal cell preparation represents the single cell fraction after hair follicles have been removed from a collagenase digest of newborn mouse dermis. It provides dermal papilla cells, fibroblasts, and possibly other important growth factor-producing cell types. Rat vibrissa dermal papilla cells supported dense hair growth at early passage in culture but progressively lost this potential during repeated passage in culture. Of 19 E1A-immortalized, clonally derived rat vibrissa dermal papilla cell lines, the four most positive clones supported hair growth to the extent of approximately 200 to 300 hairs per 1–2 cm 2 graft area. The remaining clones were moderately positive (five clones), weakly positive (three clones), or negative (seven clones). Swiss 3T3 cells prevented contraction of the graft area but did not appear to affect the number of hairs in the graft site produced by dermal papilla cells plus hair follicle buds alone. The relatively low hair density (estimated 1–5% of normal) resulting from grafts of hair follicle buds with the most positive of the immortalized dermal papilla cell clones compared to fresh dermal cells suggests that optimal reconstitution of hair growth requires some function of dermal papilla cells partially lost during the immortalization process and possibly the contribution of other cell types present in the fresh dermal cell preparation, which is not supplied by the Swiss 3T3 cells. The current graft system, comprising hair follicle buds and immortalized dermal papilla cell clones, provides an assay for positive or negative influences on hair growth exerted by added selected cell types, growth factors, or other substances. Characterization of the phenotype of the dermal papilla cell lines, which differ in their ability to support hair growth when grafted with hair follicle buds, may provide insight into specific dermal papilla cell properties important for their function in this system.

Combinations of cultured and uncultured epidermal and dermal cell preparations from newborn and perinatal mice were grafted onto the backs of athymic nude mouse hosts to elucidate the cellular requirements for skin appendage formation. All epidermal populations studied, including a total epidermal keratinocyte preparation from trypsin-split skin, developing hair follicle buds isolated from epidermis, and preformed hair follicles isolated from dermis, make haired skin when grafted with fresh dermal cells. Only pre-formed hair follicles produce haired skin on grafts without an additional dermal component. Hair follicle buds grafted alone or with cultured dermal cells will reconstitute skin but without appendage formation. Thus, cells or factors present in fresh, but not cultured, dermal cells are essential for supporting hair growth from budding follicles, whereas more developed (pre-formed) follicles appear to contain all the necessary components for hair formation. Dissociation of isolated hair follicles by trypsin/ethylenediaminetetraacetic acid prior to grafting is permissive for hair growth, suggesting that follicle cells can be re-induced or reassociate in vivo. Dermal papilla cells, microdissected from rat vibrissal follicles and cultured for up to 14 passages, stimulate hair growth from follicle buds and influence the quality of hair growth from pre-formed hair follicles. Thus, dermal papilla cells maintain inductive capacity in culture and contribute to the reconstituted skin. This reconstitution model should be useful for identifying cell populations within the hair follicle compartment necessary for hair growth and for examining the effects of specific gene products on hair follicle growth and development in vivo.