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Dermatopathology: Practical & Conceptual July - September 1995
Mammals, Other Than Man, Do Not Have Follicular Bulges: Implications for the Bulge-Activation Hypothesis
Robert W. Dunstan, D.V.M., MS.
Keith A. Linder, D.V.M.
Examination of sites of attachment of arrector pill muscles in eight nonhuman mammalian species failed to reveal a true bulge from follicles in any of them. Although in most species very slight thickening of the outer sheath could be appreciated at the site of attachment, there were no signs of a knobby protuberance like that which characterizes "the bulge" in humans. Not only were we unable to identify a bulge on follicles in any of the nonhuman mammalian species, but we could not find, prior to 1990 when the bulge-activation hypothesis first was advanced, a single reference to a bulge analogous to that in humans in any of the mammalian species studied by us.
This contrasts strikingly with statements by Lavker et al. to the effect that they are able to identify a bulge in almost all follicles in rodents.
In our view, what they claim to be a bulge is not akin to the knobby structure that in humans serves as a site of attachment for a muscle of hair erection, but a pseudobulge caused by the club-like base of a follicle in late catagen or telogen as it presses upon the outer sheath and forces it to protrude in a manner that causes it to resemble an authentic bulge. Pseudobulges are found commonly in follicles of mice and rats, and are depicted, although not acknowledged, in the article in which the bulge-activation hypothesis was set forth.
The issue of whether the outer sheath that surrounds a retained club hair constitutes a "bulge" is not new. According to Pinkus: "Unna (1876) named the bulge area of the hair follicle the "hair bed" (Haarbeet) believing that the club hair became implanted there and derived additional growth from it. Stohr (1904) gave it the more neutral "Wulst" (bulge or swelling)."
All eight mammalian species examined by us had structures analogous to a "Haarbeet," but none had a "Wulst."
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