Telocytes constitute the hair follicle stem cell niche

Telocytes are mesenchymal cells with long extensions separated from the
epithelium by sub-micrometre distances. Intestinal subepithelial FOXL1+
Telocytes provide crucial support for intestinal stem cell renewal by secreting
Wnt. We address the question of whether Telocytes are also an important
hair follicle stem cell niche component. We first characterized the distribution
and structure of FOXL1+ cells during murine hair cycle by using the
FOXL1Cre; Rosa-mTmG mouse model. We observed GFP signal in mesenchymal
population of cells characterized by long extensions forming a
continuous network along all the hair follicle concentric layers. Consequently,
to understand the importance of FOXL1+ Telocytes to hair follicle
regeneration, we used the FOXL1Cre-mTmG mouse line crossed to Rosa–
inducible diphtheria toxin receptor (iDTR), to ablate Telocytes at
anagen onset. Stem and progenitor cell proliferation, hair follicle depth and
diameter were significantly reduced. To understand whether hair follicle regeneraiton
directly depends on Wnt lignads secreted from FOXL1+ Telocytes
we induced recombinatoin in the FOXL1 CreERT2-PorcnΔ/Δ mouse
model at anagen onset. Strikingly, anagen initiation was arrested, as well as
stem and progenitor cells prolifeartion. Taken together these data point at
FOXL1+ Telocytes as the source of Wnt proteins essential for hair follicle
stem cells function.