In I990, Cotsarelis et al. challenged the traditional view that stem cells of follicles reside in the matrix of a follicular bulb and suggested instead that cells which serve as the source of the inferior segment of a follicle reside in the bulge, that portion of the outer sheath that also functions as the site of attachment for a muscle of hair erection.¹ That proposal, which Cotsarelis and co-workers encapsulated in the phrase "bulge activation hypothesis," was founded on the presumption of a population of cells in the bulge of mouse follicles that seemed to be akin to stem cells of bone marrow and stratified squamous epithelium.^2-7 Among the features of bulge cells emphasized by them was convoluted nuclear membranes.

Until now, the debate about the validity of the bulge-activation hypothesis has focused on follicles of mice and men.^8-10 Observations of follicles in a diffusely hairy rodent may provide insight into the nature of follicles in a relatively hairless primate like man, and if the bulge-activation is valid it should be applicable to all species of mammals. Curiously, we have been unable to find a single reference to a "bulge" in mice prior to the advent of the bulge-activation hypothesis.^l1-13 That prompted us to undertake, ourselves, an examination by conventional microscopy of the site of attachment of the arrector pili muscle in eight different mammals (cats, cattle, dogs, horses, nice, pigs, rats, and sheep) for the purpose of testing the validity of the premise upon which the bulge-activation hypothesis is founded, namely, that follicles in nonhuman mammals sport a bulge.

Follicles of mammals are divided be veterinarians into two major types: simple and compound.¹⁴ Simple follicles, in which a single hair shaft exits through a single ostium, are typical for humans and nice. Compound follicles, in which many hair shafts emerge through one ostium, usually are found in mammals that are carnivorous, like dogs and cats. In most nonhuman mammals, "terminal follicles" and "vellus follicles" are referred to as "primary follicles" and "secondary follicles" respectively,¹⁴ but the classification does not apply to all species. In mice, for example, four types of follicles are recognized according to characteristics of the shaft that each produces.¹⁵ In the lines that follow, larger follicles in all species will be referred to as primary follicles and smaller ones as secondary follicles. The term "bulge" as employed in this article designates a distinct protrusion from the outer sheath at the lower part of the isthmus that serves as the site of attachment for a muscle of hair erection. The term pseudobulge is applied in this article to a discrete swelling of the outer sheath that represents the outline of the "club" of a telogen hair retained at that site. A true bulge and a pseudobulge are wholly unrelated to one another.