From Telogen to Exogen: Mechanisms Underlying Formation and Subsequent Loss of the Hair Club Fiber

Abstract

The hair follicle has the unique capacity to undergo periods of growth, regression, and rest before regenerating itself to restart the cycle. This dynamic cycling capacity enables mammals to change their coats, and for hair length to be controlled on different body sites. More recently, the process of club fiber shedding has been described as a distinct cycle phase known as exogen, and proposed to be an active phase of the hair cycle. This review focuses on the importance of the shedding phase of the hair cycle and, in the context of current literature, analyzes the processes of club fiber formation, retention, and release, which may influence progression through exogen, particularly in relation to human hair.

FOXC1 maintains the hair follicle stem cell niche and governs stem cell quiescence to preserve long-term tissue-regenerating potential

“…Because it happens infrequently in WT pelage HFs, multiple bulges accumulate. When it does happen, it largely coincides with anagen (30,31). Interestingly, Foxc1-cKO HFs also lose their club hairs in late anagen.…”

Hopx expression defines a subset of multipotent hair follicle stem cells and a progenitor population primed to give rise to K6+ niche cells

“…The K6 + layer is thought to share similarities with segments of the companion layer of anagen HFs (Higgins et al, 2009); both are adjacent to the ORS and express K6 (Winter et al, 1998;Wang et al, 2003;Hsu et al, 2011), plasminogen activator inhibitor 2 (PAI2) (Lavker et al, 1998), calretinin (Poblet et al, 2005) and S100 calcium-binding protein A6 (S100A6) (Ito and Kizawa, 2001). However, the lineage relationship between the K6 + inner bulge and companion layers remains a mystery.…”

The Hair Follicle

“…The initiation of this phase coincides with the end of telogen and is completed when the club hair has been shed. Depending on the hair type and the species, the shedding of the hair fiber can occur at any time of the subsequent HC and thus is independent of the other cycle phases (Higgins, Westgate, and Jahoda 2009;Stenn 2005). Mice, for example, shed their overfur hair in the subsequent anagen, while their underfur hair is shed either in the following anagen or telogen phase.…”

“…Histological image depicting the infundibulum with the ostium to the outside (A), the transition between infundibulum and isthmus (B), the isthmus (C), the transition between isthmus and inferior portion (D), the inferior portion (E), and the bulbar region of the inferior portion (F). Note the cuticule (thin arrow), the cortex (star), the medulla (asterisk), and the infundibular keratinocytes (thick arrow) in A; HC terminology is ''kenogen'': It applies to HFs that have passed the telogen stage, lost their hair fiber (exogen), and remain empty for a certain time before a new anagen phase is initiated (Higgins, Westgate, and Jahoda 2009). Each of these HC phases has several subphases.…”