Morphological and ultrastructural aspects of the dermal papilla during the growth cycle of the vibrissal follicle in the rat.

The dermal papilla is highly implicated as the source of regulatory stimuli upon which the progression of the hair growth cycle depends. Using light and electron microscopes, the large dermal papilla of the vibrissal follicle has been studied during anagen, catagen and telogen phases of the growth cycle. The transition of the papilla from its mature anagen condition to a greatly reduced and morphologically distinct form at telogen is associated with various intra- and extracellular changes. With the reduction in ground substance during catagen papillary cells aggregate as organelles of synthetic function are replaced by lysosomes and autophagic vacuoles. Maximal diminution of the papilla at telogen coincides with the cessation of vibrissa production in the matrix and a prominent thickening of the basal lamina with dense projections into the regressed papilla. These observations are collated with previous studies on pelage follicle morphogenesis and are discussed with reference to the control of the hair growth cycle.