098 The Shedding Phase of the Mouse Hair Growth Cycle, Exogen, is Coupled to Anagen and Generates a Unique Hair Shaft Base

Y. Milner1, J. Sudnik, M. Filippi, M. Kashgarian2, and K. Stenn, Skin Biology TRC, Johnson & Johnson, Skillman, NJ, Myers Skin Biochemistry Laboratory, Department of Biological Chemistry, Life Sciences Institute, The Hebrew University, Jerusalem, Israel1, Department of Pathology, Yale University School of Medicine, New Haven, CT2

The normal hair follicle undergoes phases of growth (anagen), regression (catagen), and rest (telogen). It has long been recognized that at some time at the end of the cycle the hair shaft sheds. This shedding phase, recently termed "exogen", has been given little attention and thus is poorly understood. It was the purpose of this study to characterize the exogen phase in the mouse. Using the C57Bl/6 strain we develop a method for collecting ("vacomb") and quantifying shed hair (spectrophotometry) from newborn and adult mice. We found that the base of shed hairs differ from the base of telogen hairs (by their elongated shape, nibbled contour, and poorly preserved cytologic detail) and that this difference is demonstrable by light and electron microscopies. During spontaneous and induced hair cycles, although there is a basal amount of hair shedding in all phases of the cycle, the greatest amount of shedding occurs during mid to late anagen. The hair shafts that are shed in any phase show chiefly exogen morphology. As seen by metabolic labeling the hair shafts that shed are formed, for the most part, during the previous cycle. These studies present a method for collecting and quantifying shed hair, and documents, for the first time, that exogen occurs predominately during anagen and that its shaft base is unique. Collectively, these findings underscore the existence and distinctiveness of the exogen phase of the hair cycle.