Keratinization of outer root sheath cells is prevented by contact with inner root sheath of rat hair follicles

Abstract

The purpose of the present study was to elucidate why keratinocytes of the outer root sheath (ORS) do not keratinize in situ. Two possibilities were considered--inhibition of keratinization is caused by contact of ORS with inner root sheath (IRS) or insufficient supply of keratinization promoting factors from the surrounding tissues to the ORS. In order to distinguish between these possibilities mid-segments of hair follicles were liberated from the dermis by dissection followed by collagenase digestion. ORS cells were then either allowed to migrate from the mid-segments or were kept on the agarose layer which prevented cell spreading and preserved three dimensional structure of hair root. Cultures were stimulated with calcium or EGF, and studied morphologically at the light and transmission electron microscope level. The level of mRNA for differentiation cell markers was also studied by RealTime PCR. ORS cells growing in a medium with low Ca2+ content formed monolayers, which after elevation of Ca2+ produced multilayers with cells containing keratohyalin-like granules. Ca2+ or EGF treatment upregulated expression of involucrin, filaggrin and keratinocyte differentiation associated protein (Kdap). Culture of mid-segments of hair follicles in low calcium culture medium kept on agarose increased expression of filaggrin and Kdap, but downregulated expression of involucrin. Stimulation by Ca2+ further increased expression of filaggrin and Kdap, but had no effect on the level of involucrin expression. EGF stimulated expression of filaggrin only. It is concluded that IRS exerted an inhibitory effect on the expression of involucrin, an essential component of the cornified envelope, thus preventing keratinization of ORS cells in situ. On the other hand, improved access of nutrients or promoting factors of keratinization to the mid-segment of hair follicles augmented expression of filaggrin and Kdap, proteins engaged in the differentiation of keratinocytes but not involved in its terminal phase.

Epidermal Growth Factor Promotes Proliferation and Migration of Follicular Outer Root Sheath Cells via Wnt/β-Catenin Signaling

“…The ORS forms the outermost epithelial layer of the hair follicle and represents a direct continuation of the basal layer of the epidermis. EGF was previously demonstrated to promote keratinization of ORS cells [21], and it induces epidermal differentiation at the expense of hair follicle fate [22]. Thus, high concentrations of EGF may promote differentiation of ORS cells, rather than proliferation.…”

Histology and Ultrastructure of Transitional Changes in Skin Morphology in the Juvenile and Adult Four‐Striped Mouse (Rhabdomys pumilio)

“…Separating the inner and outer root sheath is a monolayer of flattened cells known as the companion layer [ 20 , 26 , 27 ] and identified in the juvenile four-striped mouse hair follicle. In our findings, the companion layer was associated with the Henle layer instead of the outer root sheath [ 20 , 22 , 27 ].…”

“…Hair bulb melanocytes were situated in the basal region of the hair matrix, resting on a basement membrane which separates it from the dermal papilla, as in a general description of the mouse by Boissy [ 8 ]. According to Niderla-Bielinska et al [ 22 ], mesenchymal cells of the dermal papilla and dermal fibroblasts secrete regulatory factors that control the proliferation and differentiation of hair follicles. Midway up the dermal papilla undifferentiated hair matrix cells start to form 6 concentric layers from where they rise in the follicle to undergo their individual fates [ 21 ].…”

“…Instead, the inner root sheath ends at the duct of the sebaceous gland, from where it joins the stratum corneum [ 20 , 25 ]. At this position, both the inner root sheath and stratum corneum are shed into the follicle lumen as the hair shaft exits through the skin surface [ 22 – 24 ] which happened to be the same in the juvenile four-striped mouse skin. Separating the inner and outer root sheath is a monolayer of flattened cells known as the companion layer [ 20 , 26 , 27 ] and identified in the juvenile four-striped mouse hair follicle.…”

“…At the level of the sebaceous gland, the outer root sheath is continuous with the basal layer of epithelium [ 20 , 22 , 26 , 28 ] as it is essentially an epidermal invagination into the dermis but cannot produce spinous, granular, or cornified cells such as the epidermis [ 20 , 22 ]. The presence of a specialised area of the outer root sheath known as the bulge area, located just below the sebaceous gland [ 22 , 29 ], has been reported by some investigators as not only providing attachment for the arrector pili muscle but also the location of hiding multipotent stem cells [ 20 , 22 , 28 , 29 ]. The basal layer of epithelium migrates to contribute to the maintenance of epithelium and epithelial derivatives including hair follicles and sebaceous glands [ 20 , 22 , 29 ].…”