The bulge-activation hypothesis, as it pertains to the murine "hair cycle," depends upon four elements.

1. "The bulge" is activated by a "dermal papilla."

2. The mesenchymal papilla is activated by the growing matrix.

3. The proliferative potential of matrix cells is limited.

4. The "dermal papilla" migrates upwards.

The "hair cycle" is really a follicular cycle and the "dermal papilla" is actually a follicular papilla.

The bulge-activation hypothesis derives from studies performed in mice, and follicles of mice are different from follicles of humans, e.g., human follicles are characterized by bulges, mouse follicles are devoid of them.

The term "bulge stem cells," as employed by Sun, Cotsarelis, and Lavker, has no meaning in the context of murine follicles because mice have no bulges. Bulges in human follicles are protuberances of cells positioned in the lower part of the isthmus. Some cells at the base of the isthmus, therefore, are cells at the base of bulges.