ABSTRACT
Hair follicles develop embryologically through a complex series of interactions between the epithelial components of the epidermis and the mesenchymal populations within the dermis. They maintain this essential and as yet largely mysterious and intricate relationship throughout life. The hair follicle has traditionally been described in terms of three distinct anatomic units: the infundibulum, the isthmus, and the inferior segment that includes the hair bulb. Unlike the considerable morphologic changes that the inferior aspect of the follicle undergoes during the course of the hair cycle, the upper permanent region, comprised of the infundibulum and the isthmus, maintains relative stability above the level of the insertion of the arrector pili muscle. Located in close proximity and largely enveloped by the hair bulb is the dermal, or follicular, papilla. Although not technically part of the follicle, this distinct dermal component holds great importance in the proper morphogenesis and cycling of the hair follicle.1
