Hair growth is a dynamic process that relies on specific cycles: anagen (growth), catagen (regression), and telogen (rest). Here's a deep dive into the fascinating journey of hair growth:

 

Anagen Phase (Growth Phase)

This is the active growth phase where the hair shaft is produced within the follicle.

Anagen follicles are long, straight, and positioned in a way that allows hair to lie flat against the skin. Over 18 hours, cells move upward, committing to either the inner root sheath or the hair shaft itself. As these cells differentiate, they fill with hair keratins that provide strength and flexibility to the hair. Ultimately, the inner root sheath guides the hair shaft and releases it to continue through the skin's surface. The length of hair depends on the duration of this phase and the constant proliferation and differentiation of cells at the follicle's base.

 

Catagen Phase (Transitional Phase)

 

As anagen concludes, the follicle enters catagen. Here, the growth of the hair shaft slows, and the lower part of the follicle undergoes regression. Molecules influencing this transition include FGF5, EGF, BDNF, p53, and members of the TGF-family like TGF1.

During this phase, epithelial cells in the bulb and outer root sheath experience apoptosis, leading the hair follicle to form a structure known as the "club." Another temporary structure, the epithelial strand, connects the dermal papilla to the upper hair follicle during catagen.

 

Telogen Phase (Resting Phase)

 

After catagen, the follicles enter a resting phase known as telogen. Initially, this phase lasts only a day or two, but subsequent telogen phases can extend beyond two weeks.

Follicle Stem Cell Compartment: Although hair follicles don't form after birth, they regenerate to produce new hair. This regeneration relies on a reservoir of multipotent epithelial stem cells found in the hair follicle's "bulge" area. These stem cells activate during the transition from telogen to anagen, initiating a new hair growth cycle.

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Transition from Telogen to Anagen

This transition kickstarts when stem cells at the base of the telogen follicle near the dermal papilla begin to proliferate, leading to the creation of a new hair follicle. This new follicle grows beside the old one, and over time, the old hair (or club hair) sheds.

Notably, signaling pathways, including Wnts and Shh, are crucial for initiating the new anagen phase. Understanding these pathways could offer insights into stem cell activation and specification.

 

Embryonic Origins In embryos, skin starts as a layer of epidermal stem cells. These cells form the dermis below and stimulate the formation of hair follicles. These follicles grow downward, eventually forming the hair shaft and the inner root sheath. The follicle's base, the mesenchymal dermal papilla, gets enveloped by the hair bulb. As the follicle matures, cells at its base continue dividing, which supports the hair's growth.

The intricacy of these cycles and the cellular dynamics involved underline the marvel of hair growth, regeneration, and the underlying genetic and molecular mechanisms at play.