Answer:
The central region of the hair shaft that divides the permanent section of the hair (infundibulum and isthmus) from the lower non-permanent region (suprabulbar and bulbar) and is marked by the attachment of the arrector pili muscle. It is thought that the bulge area contains the stem cells that help regenerate the hair shaft. The bulge area like the upper hair shaft is also considered a permanent part of the hair shaft.
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