The growth of hair proceeds in cycles, and at times, we may have episodes of hair fall for various factors. Yet, in a remarkable display of evolution, our bodies can regenerate hair follicles most effectively. In this article, we explore the scientific basis of both how and why this occurs. Understanding this process can provide valuable insights into treating hair loss.
Hair Follicle Anatomy and Structure
The tiny structures found in the skin that produce hair are called hair follicles. Every hair grows from its own follicle which is pre-existing, highly intricate, and has a growth cycle regulated by hormones. There is a dermal papilla at the base of follicles which dips into the dermis layer of the skin, and it holds signals that are essential for the control of hair growth.
The hair shaft extends down from the infundibulum and is associated with sebaceous glands and arrector pili muscles. Blood vessels and nerves are responsible for their nutrition and sensation respectively. When stimulated in various growth phases, stem cells within the bulge region replenish the lower follicles to maintain the cycle.
The Hair Growth Cycle
Hair follicles have distinct growth (anagen), regression (catagen), and resting (telogen) phases that last different amounts of time and regulate visible hair length. During anagen, which lasts 2-6 years for scalp hair, stem cells rapidly divide to grow the hair shaft upwards at great speed. As this phase ends, the follicle enters a short and abrupt catagen phase where growth stops.
Finally, in the telogen phase that lasts around 3 months, the hair rests and detaches from its follicle bulb, making room for a new hair to regrow in the next cycle. Hair loss occurs when follicles stay in telogen for too long before preparing for the next cycle.
How Hair Follicle Regeneration Works
Even after a hair falls out naturally or due to balding, its follicle stays alive and embedded in the dermis. During normal cycling, follicle stem cells lay dormant in the bulge region until anagen starts again. However, after significant hair loss or damage, dormant cells are stimulated to proliferate and develop a new hair shaft and surrounding structures from scratch.
Growth factors, chemical signals and the dermal papilla facilitate structural reconstitution in a carefully orchestrated manner that mirrors embryonic hair development. The newly formed, fully functioning follicle is now capable of undergoing its cyclic pattern and growing new hair strands from the surface of the skin.
Molecular Mechanisms Stimulating Regeneration
Many inductive signals, secreted growth factors and proteins are behind starting the regeneration process within follicles:
- Want proteins work through the β-catenin pathway and are vital for developing follicles from progenitor cells during embryogenesis. Studies show they have a similar role in recruiting adult progenitor cells.
- Dermal papilla cells secrete fibroblast growth factors (FGF) like FGF-7 and -10 that stimulate neighbouring follicle stem cells to differentiate into new epithelial lineages and hair germ formation.
- Transforming growth factor beta (TGF-β) superfamily molecules fine-tune progenitor cell activities through SMAD pathways and help organise the follicle structures.
- Bone morphogenic proteins (BMP) maintain an appropriate balance between stem cell self-renewal and lineage commitment during new bulb organisation.
- Extracellular matrix remodelling via matrix metalloproteinases helps with tissue interactions and cellular movement as regeneration progresses.
- Other lesser-known signals like neuregulins, sonic hedgehog proteins and WNT-inhibitory factors also coordinate aspects of follicle induction from dermal compartments.
Stem Cells Driving Regeneration
Adult epithelial follicle stem cells located in the bulge region and dermal papilla fibroblasts have tremendous plasticity to recreate entire follicles when required.
Key stem cell types and their roles include:
- Epithelial follicle stem cells in the bulge region rapidly proliferate upon stimulation to self-renew and give rise to all epithelial lineages of the new follicle (sheath, cortex and hair shaft matrix).
- Dermal sheath cup cells at the base also produce transit-amplifying progenitors that help reconstruct epithelial components under guidance from the dermal papilla.
- Dermal papilla fibroblasts are the primary inductive lineage responsible for reactivating other stem/progenitor types and orchestrating morphogenesis of hair germ and follicle patterns from below the epidermis.
- Interfollicular epidermal stem cells situated between follicles contribute to regenerating the epidermis and connecting skin tissues as follicles replenish.
- Additional stem cell niches higher up in the sebaceous glands and infundibulum also participate in refashioning affiliated structures when follicles regenerate.
Conclusion
Hair follicles have an amazing ability for self-regeneration through carefully regulated molecular signals, stem/progenitor cells and lineage interactions. Understanding these mechanisms provides valuable insights for developing adjunct therapies that aid natural regeneration following hair loss from diseases, injuries or ageing.
More research in this field promises hope for treating various conditions related to abnormal cycling or ineffective regenerative potential of follicles. Harnessing our innate regenerative machinery paves the way for improving outcomes in hair transplants as well as solutions for androgenic alopecia.
