The hair follicle is a complex structure responsible for hair production. Within this structure is a small, yet significant, component known as the hair bulge, which is involved in hair growth and regeneration. Its function is integral to the cyclical nature of hair production that occurs throughout a person’s life. Understanding the hair bulge provides insight into the fundamental biology of hair.
Locating the Hair Bulge
The hair bulge is a specific region within the permanent, upper portion of the hair follicle. Unlike the lower segment, this area does not disappear during the hair’s regression phase. It is located about midway down the follicle at the insertion point for the arrector pili muscle, the tiny muscle that causes hair to stand on end. This position places it just below the sebaceous gland, which secretes oil onto the hair.
This structure is a component of the outer root sheath (ORS), the external layer of the follicle that encases the growing hair shaft. While it can be inconspicuous in adult human hair follicles, its presence is consistent. The bulge’s distinct location and cellular arrangement are directly related to its function in hair follicle development and maintenance.
The Stem Cell Reservoir
The primary function of the hair bulge is to act as a reservoir for adult stem cells, known as hair follicle stem cells (HFSCs). This area is a “stem cell niche,” a specialized microenvironment that protects and maintains the cells. Within this niche, HFSCs are kept in a relatively inactive or quiescent state, which preserves their long-term potential for regeneration.
These resident stem cells are self-renewing and multipotent, meaning they can create more stem cells and differentiate into the cell types needed to rebuild the hair follicle. While their main role is to regenerate the follicle’s epithelial layers, they can also migrate during wound healing to help repair the epidermis. This regenerative capacity is central to the continuous renewal of hair.
Function in the Hair Growth Cycle
The hair follicle undergoes a perpetual cycle of growth (anagen), regression (catagen), and rest (telogen). The stem cells in the hair bulge are fundamental to this process. During the telogen, or rest, phase, the bulge stem cells remain largely dormant. The transition to the anagen phase begins when these stem cells receive activation signals from the dermal papilla at the base of the follicle.
In response, the bulge stem cells proliferate and migrate down to form a new lower follicle structure, including the hair bulb. This new structure produces the hair shaft, initiating the anagen phase. As the follicle later enters the catagen phase, the lower part regresses, but the bulge remains intact, ready to start the cycle anew.
Relevance in Hair Loss and Regeneration
The health of the hair bulge is directly related to hair loss. In certain types of scarring alopecias, inflammation targets and damages the bulge region, leading to permanent hair loss. In androgenetic alopecia, or pattern baldness, the hair follicle miniaturizes over time. While the stem cells in the bulge may still be present, their activation and regenerative capacity are diminished.
This connection makes the hair bulge a primary focus for hair regeneration therapies. Scientists are exploring methods to stimulate quiescent stem cells to trigger new hair growth. Research suggests that even in bald scalps, a population of these stem cells remains in the upper follicle. Reactivating them could potentially regrow hair, so treatments could involve stem cell therapies or molecules that awaken these dormant cells.