Segmental Igfbp5 expression is specifically associated with the bent structure of zigzag hairs

The murine hair coat consists of four different hair types that are characterised by hair length, the number of medulla columns, and the presence and number of bends. The molecular mechanisms underlying the establishment and maintenance of distinct hair follicle fates are unknown. We identify Igfbp5 as the first molecular marker that distinguishes among different hair follicle types. High-resolution expression analysis revealed that its expression in the medulla of hair shafts is associated with the bend-forming zones of zigzag hairs. To directly examine the functional importance of segmental gene expression in the hair follicle, we have generated transgenic mice expressing Igfbp5 in differentiating keratinocytes of the medulla and inner root sheath. Ectopic expression of Igfbp5 resulted in the appearance of remarkable curvatures and thinning of hair shafts, two hallmarks of hair bends. Both effects and the natural bending process are under negative control of IGF signalling. Thus, our data identify Igfbp5 as a central regulator of hair shaft differentiation and hair type determination.     

The role of activins and follistatins in skin and hair follicle development and function

Investigations of the signalling between epithelial and mesenchymal compartments of skin during hair follicle initiation in utero and hair cycling have revealed the importance of the TGFβ superfamily in ectodermal organogenesis and morphogenesis. In particular the activins, their receptors and binding proteins such as follistatin, have been shown to be important regulators of cell proliferation, differentiation and apoptosis in hair follicle initiation, hair cycling, normal skin homeostasis and wound healing. Transgenic mice lacking various components of the activin signalling pathways display varying ectodermal pathologies including altered pelage hair follicle initiation. This review summarises the activin signal transduction pathways and the interactions between activins and other TGFβ signalling systems during hair follicle formation, hair growth cycling, skin function and wound healing.     

Deregulated expression of Cdc6 in the skin facilitates papilloma formation and affects the hair growth cycle

Cdc6 encodes a key protein for DNA replication, responsible for the recruitment of the MCM helicase to replication origins during the G1 phase of the cell division cycle. The oncogenic potential of deregulated Cdc6 expression has been inferred from cellular studies, but no mouse models have been described to study its effects in mammalian tissues. Here we report the generation of K5-Cdc6, a transgenic mouse strain in which Cdc6 expression is deregulated in tissues with stratified epithelia. Higher levels of CDC6 protein enhanced the loading of MCM complexes to DNA in epidermal keratinocytes, without affecting their proliferation rate or inducing DNA damage. While Cdc6 overexpression did not promote skin tumors, it facilitated the formation of papillomas in cooperation with mutagenic agents such as DMBA. In addition, the elevated levels of CDC6 protein in the skin extended the resting stage of the hair growth cycle, leading to better fur preservation in older mice.     

Vital roles of stem cells and biomaterials in skin tissue engineering

Tissue engineering essentially refers to technology for growing new human tissue and is distinct from regenerative medicine. Currently, pieces of skin are already being fabricated for clinical use and many other tissue types may be fabricated in the future.Tissue engineering was first defined in 1987 by the United States National Science Foundation which critically discussed the future targets of bioengineering research and its consequences. The principles of tissue engineering are to initiate cell cultures in vitro, grow them on scaffolds in situ and transplant the composite into a recipient in vivo. From the beginning, scaffolds have been necessary in tissue engineering applications. Regardless, the latest technology has redirected established approaches by omitting scaffolds. Currently, scientists from diverse research institutes are engineering skin without scaffolds. Due to their advantageous properties, stem cells have robustly transformed the tissue engineering field as part of an engineered bilayered skin substitute that will later be discussed in detail. Additionally, utilizing biomaterials or skin replacement products in skin tissue engineering as strategy to successfully direct cell proliferation and differentiation as well as to optimize the safety of handling during grafting is beneficial. This approach has also led to the cells’ application in developing the novel skin substitute that will be briefly explained in this review.     

Functional role of beta 1 integrin-mediated signalling in the human hair follicle

Integrins are transmembrane adhesion proteins that convey critical topobiological information and exert crucial signalling functions. In skin and hair follicle biology, beta1 integrins and their ligands are of particular interest. It is not yet known whether beta1 integrins play any role in the regulation of human hair growth and the expression pattern of beta1 integrin in the human pilosebaceous unit remains ill-defined. Here, we show that pilosebaceous immunoreactivity for beta1 integrin is most prominent in the outermost layer of the outer root sheath and the surrounding connective tissue sheath of human scalp hair follicles in situ and in vitro. Sites of beta1 integrin immunoreactivity co-express fibronectin and tenascin-C. Contrary to previous reports, beta1 integrin immunoreactivity in situ was not significantly upregulated in the human bulge region. Functionally, two beta1 integrin-activating antibodies (12G10, TS2/16) and ligand-mimicking RGD peptides promoted the growth of microdissected, organ-cultured human scalp hair follicles in vitro and inhibited spontaneous hair follicle regression. This supports the concept that beta1 integrin-mediated signalling is also important in human hair growth control. The physiologically relevant organ culture assay employed here is a potential research tool for exploring whether targeted stimulation of beta1 integrin-mediated signalling is a suitable candidate for human hair loss management.     

The bulge area is the origin of nestin-expressing pluripotent stem cells of the hair follicle

Nestin-expressing pluripotent stem cells have been found both in the bulge area (BA) as well as the dermal papilla (DP). Nestin-expressing stem cells of both the BA and DP have been previously shown to be able to form neurons and other non-follicle cell types. The nestin-expressing stem cells from the DP have been termed skin precursor or SKP cells. Both nestin-expressing DP and BA cells have been previously shown to effect repair of the injured spinal cord and peripheral nerve, with the BA being the greater and more constant source of the stem cells. The BA contains nestin-expressing stem cells throughout the hair cycle, whereas nestin-expressing dermal papillae stem cells were found in early and mid-anagen only. Our previous studies have shown that the nestin-expressing stem cells in the BA and DP have similar morphological features. The cells from both regions have a small body diameter of approximately 7 μm with long extrusions, as shown by 2-photon imaging. In the present study, using 2-photon imaging of whisker follicles from transgenic mice expressing nestin-driven green fluorescent protein (ND-GFP), we demonstrate that the BA is the source of the nestin-expressing stem cells of the hair follicle. The nestin-expressing stem cells migrate from the BA to the DP as well as into the surrounding skin tissues including the epidermis, and during wound healing, suggesting that the BA may be the source of the stem cells of the skin itself.     

Dual origin of melanocytes defined by Sox1 expression and their region‐specific distribution in mammalian skin

Melanocytes are pigment‐producing cells generated from neural crest cells (NCCs) that delaminate from the dorsal neural tube. The widely accepted premise that NCCs migrating along the dorsolateral pathway are the main source of melanocytes in the skin was recently challenged by the finding that Schwann cell precursors are the major cellular source of melanocytes in the skin. Still, in a wide variety of vertebrate embryos, melanocytes are exclusively derived from NCCs. In this study, we show that a NCC population that is not derived from Sox1⁺ dorsal neuroepithelial cells but are derived from Sox1^? cells differentiate into a significant population of melanocytes in the skin of mice. Later, these Sox1^? cells clearly segregate from cells that originated from Sox1⁺ dorsal neuroepithelial cell‐derived NCCs. The possible derivation of Sox1^? cells from epidermal cells also strengthens their non‐neuroepithelial origin.     

Dynamic regulation of retinoic acid-binding proteins in developing, adult and neoplastic skin reveals roles for beta-catenin and Notch signalling

Retinoic acid (RA) signalling is essential for epidermal differentiation; however, the mechanisms by which it acts are largely unexplored. Partitioning of RA between different nuclear receptors is regulated by RA-binding proteins. We show that cellular RA-binding proteins CRABP1 and CRABP2 and the fatty acid-binding protein FABP5 are dynamically expressed during skin development and in adult tissue. CRABP1 is expressed in embryonic dermis and in the stroma of skin tumours, but confined to the hair follicle dermal papilla in normal postnatal skin. CRABP2 and FABP5 are expressed in the differentiating cells of sebaceous gland, interfollicular epidermis and hair follicles, with FABP5 being a prominent marker of sebaceous glands and anagen follicle bulbs. All three proteins are upregulated in response to RA treatment or Notch activation and are negatively regulated by Wnt/β-catenin signalling. Ectopic follicles induced by β-catenin arise from areas of the sebaceous gland that have lost CRABP2 and FABP5; conversely, inhibition of hair follicle formation by N-terminally truncated Lef1 results in upregulation of CRABP2 and FABP5. Our findings demonstrate that there is dynamic regulation of RA signalling in different regions of the skin and provide evidence for interactions between the RA, β-catenin and Notch pathways.     

Corneal epithelial-like transdifferentiation of hair follicle stem cells is mediated by pax6 and β-catenin/Lef-1

Several types of adult stem cells are capable of transdifferentiaton into other types of tissues. The hair follicle bulge area is an abundant and easily accessible source of pluripotent adult stem cells. We demonstrate that the bulge KSCs have the potential for transdifferentiation into corneal epithelial-like cells. Bulge KSCs isolated by collagen type IV adhesiveness possessed the highest colony formation efficiency (CFE), and expressed specific markers (CD34 and α6-integrin). The isolated cells transdifferentiate into corneal epithelial-like cells in conditioned medium containing corneal limbus soluble factors, including their specific marker, keratin12. The transdifferentiation depends on upregulation of pax6 and downregulation of β-catenin and Lef-1. Furthermore, overexpression of pax6 in bulge KSCs induced their expression of k12. The expressions of β-catenin and Lef-1 were not suppressed in the pax6-transfected bulge KSCs, but which were downregulated pax6-transfected cells cultured in the conditioned medium. Bulge KSCs may have potential therapeutic application as cell source for the construction of bioengineered corneas.