Appearance of hair follicle-inducible mesenchymal cells in the rat embryo

Rat vibrissa follicle morphogenesis starts around 13 days of gestation. By day 14 mesenchymal cells have already aggregated as 'condensations' beneath the initial hair bud. Some of the mesenchymal cells will form a dermal pap illa, having profound effects on hair follicle formation. The appearance of follicle-inducing mesenchymal cells in the process of vibrissa follicle de velopment was examined. Mesenchymal cells were isolated from the developing site of vibrissa follicles at 13 days or at later stages and amplified in mass culture, harvested and transplanted in association with the epithelium . It was demonstrated that 13-day mesenchymal cells did not induce any hair bulbs but those from 14 days or later stages could induce hair-producing n ew bulbs or new follicles depending on the association with the follicle ep ithelium or with the glabrous sole epidermis of the adult rats, respectivel y. Further, clones having hair bulb-inducing ability were obtained from 14- and 15-day mass-cultured mesenchymal cells. Based on these and other resul ts, it was concluded that mesenchymal cells having follicle-inducing abilit y are present at least by 14 days in the future whisker pad region. This su ggests that the differentiation of the dermal papilla cells must start befo re the initial hair bud stage.