The Hair Follicle

Abstract

The hair follicle (HF) has a wide range of functions including thermoregulation, physical and immunological protection against external insults, sensory perception, social interactions, and camouflage. One of the most characteristic features of HFs is that they self-renew during hair cycle (HC) throughout the entire life of an individual to continuously produce new hair. HC disturbances are common in humans and comparable to some alopecic disorders in dogs. A normal HC is maintained by follicular stem cells (SCs), which are predominately found in an area known as the bulge. Due to similar morphological characteristics of the human and canine bulge area, the particularity of compound HFs in humans and dogs as well as similarities in follicular biomarker expression, the dog might be a promising model to study human HC and SC disorders. In this review, we give an overview of normal follicular anatomy, the HC, and follicular SCs and discuss the possible pathogenetic mechanisms of noninflammatory alopecia.

Hair of the Dog: Identification of a Cis-Regulatory Module Predicted to Influence Canine Coat Composition

“…In particular, thinning or loss of hair (alopecia) has a large psychological impact on both men and women, potentially increasing anxiety and affecting body image [17,18], thus highlighting the need to better understand features of hair composition and growth. Hair follicle composition in dogs is similar in anatomical structure to that observed on the human scalp as both are composed of a compound structure with multiple hair shafts extending from each follicle [19,20,21].…”

Efficacy and Function of Feathers, Hair, and Glabrous Skin in the Thermoregulation Strategies of Domestic Animals

“…Hair follicles are critical for thermoregulation because their vasculature comes from the deep dermal vascular plexus ( Figure 4 ) [ 66 ]. The disposition of primary hair and secondary fibers are factors that affect the thermal insulation provided by hair or fur [ 67 ].…”

Epidermal Stem Cells in Hair Follicle Cycling and Skin Regeneration: A View From the Perspective of Inflammation

“…Upon completion of anagen, a controlled involution of HF occurs denoting onset of catagen, when most follicular keratinocytes undergo apoptosis (Paus and Cotsarelis, 1999;Krause and Foitzik, 2006;Welle and Wiener, 2016). At this stage, poorly dividing ORS cells create a new bulge, while slightly faster dividing cells form the secondary hair germ (Welle and Wiener, 2016). Unlike ORS and the hair matrix, the cells of DP do not die, though they express the suppressor of apoptosis bcl-2 (Krause and Foitzik, 2006).…”

“…Hair germ HFSCs give rise to transit-amplifying cells that populate the germinative layer of the matrix. Outer root sheath (ORS) is formed by cells of the lower bulge ( Welle and Wiener, 2016 ). It is well known that the key factors that induce anagen are the Wnt proteins, while the level of BMP signaling is downregulated by expression of its antagonist noggin ( Botchkarev and Sharov, 2004 ).…”