The first description of the hair follicle bulge by Franz von Leydig.

account of the initial descriptions of the bulge is given by Stöhr [8] . Stöhr provides a long list of publications mentioning the bulge and briefly discusses an alleged first description by the famous Austrian dermatologist Ferdinand von Hebra [11] . However, in the publication in question, Hebra merely reports a thickening of the hair follicle root sheath in lichenoid lesions; he does not employ the term ‘Wulst’ and since there is no accompanying picture, it is rather questionable whether the alterations he described correspond to the bulge of a normal hair follicle. According to Stöhr, the first description of the bulge of a normal hair follicle was made by Unna [7] . Here, I present evidence that the first description and detailed depiction of the hair follicle bulge was made almost two decades earlier by Franz von Leydig (1821– 1908), a German histologist and zoologist most commonly known for the discovery of the testicular interstitial cells bearing his name, in an article entitled ‘About the outer coats of mammals’ [12] . Leydig ( fig. 1 ) obtained a medical doctorate degree from the University of Würzburg in 1847. He also spent his early career in Würzburg before becoming a professor of zoology in Tübingen in 1857. In 1875, Leydig moved to the University of Bonn, where he stayed until his retirement in 1887. He died in 1908 in the town of his birth, Rothenburg ob der Tauber. More than 20 years ago, Cotsarelis et al. [1] published an article in which they showed that a region of the mouse hair follicle known as the ‘bulge’ contains a population of slow-cycling label-retaining cells. Additional studies revealed that these cells are multipotent stem cells, whose activation during early anagen allows the initiation of a new hair cycle (the so-called ‘bulge activation hypothesis’ [2] ). Since then, the biological properties of the bulge stem cells have been intensively studied [3, 4] . Although additional sites bearing stem cells have been identified along the hair follicle [5, 6] , the importance of the bulge as the essential reservoir of hair follicle stem cells remains undisputable. Before the 1990s, the bulge attracted little attention from researchers, mainly because it was rarely identified in routine paraffin sections of adult human hair follicles. With the sudden interest in the bulge, a variety of statements regarding its first description were made. Authors often credit Paul Gerson Unna [7] and Philipp Stöhr [8] for the ‘discovery’ of the bulge, a (erroneous) belief that can be traced back to The Biology of Hair Growth , a classic dermatology textbook [9] . The more recent history of bulge research has been reviewed by Arve Madsen [10] , who acknowledges that the structure was initially named ‘Wulst’ by different German authors. The most detailed Published online: August 31, 2011