Whisker growth after removal of the dermal papilla and lengths of follicle in the hooded rat.

It is generally agreed that the dermal papilla is a vital component of the hair and feather follicle. Lillie & Wang (1941, 1944) and Wang (1943) have shown for feather follicles that if the dermal papilla, which normally maintains its integrity throughout the growth cycles, and its ectodermal investment are removed from the follicle, feather production ceases. However, if a dermal papilla is transplanted into these 'deprived' follicles, renewed feather growth is induced. An inductive property is also ascribed to the dermal papilla of the hair follicle (Chase, 1955; Cohen, 1961). Chase has suggested that in the hair follicle the dermal papilla retains its integrity throughout the growth cycles and induces renewed follicle activity at the end of telogen. However, relatively little experimental work has been performed on the dermal papilla of the hair follicle to determine the exact role of the papilla in follicle maintenance and hair growth. Crounse & Stengle (1959) consider that the dermal papilla is essential for the maintenance of the hair follicle. They demonstrated that human 'hair roots' degenerate after removal of the dermal papilla when implanted into Millipore chambers in the peritoneum of mice. Similarly Wolbach (1951), Geary (1952) and Billingham (1958) believe that unrenewed hair growth after the application of various physical agents on to large populations of hairs results primarily from the destruction or inactivation of the dermal papilla. It has been shown, from histological studies of anagen hair follicles, that the volume of the matrix in the hair bulb varies directly with the height of the dermal papilla and that the number of mitoses present in the matrix bears a constant ratio to the number of cells in the dermal papilla. Similarly, it has been shown that there is a constant proportional relationship between the volume of the