1990 Cotsarelis, Sun, Lavker

 
Label-retaining cells reside in the bulge area of pilosebaceous unit: implications for follicular stem cells, hair cycle, and skin carcinogenesis. Cell. 1990;61:1329-37 (Fig. 17).

View Figure  		Fig. 17  [Fig. 6] Hair Cycle: The Bulge Activation Hypothesis (Reproduced with permission.)
 

Message

 
Stem cells, characterized by being slow-cycling, are clustered in the bulge region of mouse follicles, not in the bulb.
 
A bulge-activation hypothesis is proposed, to wit, stem cells situated in "the bulge" are activated by "dermal papilla," following which anagen commences.
 

Critique

 
The mouse follicle displays neither a bulge nor bulges, only a narrow zone, equivalent to the isthmus, to which a muscle of hair erection attaches. Nonetheless, the drawing by Sun depicts "the bulge" as it is portrayed conventionally in a human follicle, namely, as a single bold protuberance.
 
The bulge-activation hypothesis, as conceived by Cotsarelis, Lavker, and Sun, represents a fusion of two concepts developed toward the end of the 19th century, namely, Unna"s notion of "the bulge" as a source of epithelial cells that give rise to a new downgrowth of follicular epithelium at the end of telogen and Garcia"s concept that juxtaposition of a papilla and a productive region at the base of the isthmus is necessary for initiation of anagen.
 
The slow-cycling cells interpreted as stem cells in the region of the bulge by Cotsarelis, Lavker, and Sun cannot be in "the bulge" per se because a mouse follicle has no bulge.