Keratin Expression Provides Novel Insight into the Morphogenesis and Function of the Companion Layer in Hair Follicles

Abstract

Hair follicles cycle between stages of growth (anagen) and metabolic quiescence (telogen) throughout life. In mature follicles, transition from telogen back into anagen involves the activation, proliferation, and differentiation of epithelial stem cells located in the bulge, a specialization of the outer root sheath. Recent studies identified keratin 6a (K6a) transcripts as enriched in bulge epithelial stem cells in mouse skin. We used messenger RNA probes, antibodies, a LacZ reporter mouse model, and whole-mount staining assays to investigate the regulation of mK6a during mouse postnatal hair cycling, and compare it to mK75, a companion layer (Cl) marker. We find that mK75 regulation parallels that of inner root sheath (IRS) markers, with expression onset at anagen IIIa above the new hair bulb and subsequent spreading towards the bulge. Although also occurring in the Cl, mK6a expression begins at anagen IIIb in differentiating cells located proximal to the bulge, and subsequently spreads towards the hair bulb. mK6a and mK75 thus exhibit temporally distinct, and spatially opposed, expression patterns in the Cl during postnatal anagen. These findings provide novel insight into the morphogenesis and properties of the Cl, and raise the distinct possibility that it is an integral part of the IRS compartment.

Redefining the structure of the hair follicle by 3D clonal analysis

“…The Ki67 + dividing cells corresponded only to the basal ORS (Fig. 6Dd, arrows), confirming that the Cp does not originate from the basal ORS layer, as they do not proliferate (these results) (Gu and Coulombe, 2007). Clearly, the basal ORS progenitors are located in the PPZ (Fig.…”

“…These features certainly also optimise the establishment and maintenance of desmosomal junctions between the Cp and Henle cells (Gu and Coulombe, 2007;Ito, 1986;Orwin, 1971;Rogers, 1964).…”

“…The Cp cells have been considered by several researchers as belonging to the ORS (Ito, 1986;Ito, 1988;Pinkus et al, 1981) and by others as belonging to the IRS (Gu and Coulombe, 2007;Zhang et al, 2009). Our results demonstrate that they are not derived from the ORS or IRS but from unipotent progenitors in the bulb, more specifically from the cells proximal to the germinative layer stem cells (Legué and Nicolas, 2005) in positions 4-5 (Fig.…”

“…For others, it consists only of the K14 + ORS cell layer. In this model, the K75 + Cp cells either belong to the IRS compartment (Gu and Coulombe, 2007;Zhang et al, 2009) or constitute a separate HF structure (Orwin, 1971;Rothnagel and Roop, 1995;Wang et al, 2003;Winter et al, 1998). Related to these issues, there are several possibilities as to the origin of the Cp cells during anagen.…”

Hedgehog Signaling, Keratin 6 Induction, and Sebaceous Gland Morphogenesis

“…Genotyping for hK6a-lacZ and Ptch-lacZ were performed using ␤-galactosidase assay in situ. 31 All whole mount epidermal sheets were prepared from the upper onethird region on the dorsal side of male mouse tail skin, as described. 31,32 All protocols involving mice were reviewed and approved by the Johns Hopkins University Animal Care Use Committee.…”