The experimental induction of whisker growth in the hooded rat by implantation of dermal papillae.

Dermal papillae are regenerated and whiskers are produced after the experimental removal of up to the lower third of the vibrissa follicle, but if more of the follicle is removed neither event occurs (Oliver, 1966 a, b). Similarly, when lengths of the lower third of the vibrissa follicle wall are transplanted into ear skin, whiskers are again produced, but not if lengths of wall are taken from within the upper two-thirds of the follicle (Oliver, 1967). From these experiments it was clear that the outer root sheath and the adherent mesenchymal layer, from which the new papillae are apparently derived, are the essential tissues in the regeneration process. The failure of papilla regeneration in the upper two-thirds of the follicle could be explained in several ways. It is possible that the outer root sheath at this level is incapable of supporting whisker growth or of stimulating papilla formation. Alternatively, the cells of the mesenchymal layer at this level, unlike those in the lower third of the follicle, may be incompetent to form a dermal papilla. In an attempt to analyse these factors, vibrissa dermal papillae were implanted into the bases of whisker follicles from which more than the lower third of the root had been removed.