Involucrin, but not filaggrin and Kdap mRNA, expression is downregulated in 3-D cultures of intact rat hair bulbs after calcium stimulation

The hair follicle consists of several distinctive epidermal cell layers. The hair root, which undergoes keratinization, is surrounded by two sheaths: the inner root sheath (IRS) and the outer root sheath (ORS). The ORS is continuous with the basal layer of the epidermis. Its cells do not keratinize in situ, unlike IRS. We have previously demonstrated that keratinization of the ORS was prevented by contact with the IRS in hair follicle mid-segments (i.e. fragments dissected from skin at the level above the hair bulb and below the opening of the sebaceous gland duct) cultured on agarose layer. The purpose of this study was to determine whether the same applies to the hair bulb. After isolation, intact bulbs or hair bulb-derived cells were incubated in suspension in a low or high calcium medium. The level of mRNA for differentiation markers: involucrin, filaggrin, keratinocyte differentiation associated protein and trichohyalin, was studied by RealTime PCR. We observed increased Ca(2+) upregulated expression of involucrin, filaggrin, trichohyalin and Kdap in cultures of bulb-derived cells, but in hair bulbs downregulation of involucrin and trichohyalin was observed. We concluded that the inner root sheath exerts an inhibitory effect on the expression of involucrin and trichohyalin already in the hair bulbs. The observation that downregulation of involucrin expression under Ca(2+) influence occurs both in hair bulb and midsegments could simplify future experiments, since their separation does not seem to be necessary.     

Hair cycle dynamics: the case of the human hair follicle

The existence of a growth and regeneration cycle makes the hair follicle a true paradigm of tissue homeostasis. Analysis of about 9000 cycles led us to propose a stochastic model of human hair dynamics. The existence of hair cycles implies that stem cells must be cyclically activated and hair melanin unit has to be renewed. Using different markers, we were able to identify two distinct epithelial stem cell reservoirs, located in the upper and lower thirds of the anagen hair follicle outer root sheath. These two reservoirs fuse during the regression phase and individualize again in the new forming anagen hair follicle. Using a set of antibodies specific of melanocyte lineage and melanogenesis, pigmentation unit turnover was followed throughout the entire hair cycle. In the terminal anagen hair, active melanocytes were localized on top of the dermal papilla, while amelanotic melanocytes were identified in the upper third of the outer root sheath (ORS). Those amelanotic melanocytes located in upper ORS probably represented a melanocyte reservoir for successive hair generation, since at the induction of anagen phase, some melanocytes were committed to cell division and melanogenesis was turned on, but only in the nascent hair bulb, close to the dermal papilla.     

Hair follicle stem cells are specified and function in early skin morphogenesis

In adult skin, epithelial hair follicle stem cells (SCs) reside in a quiescent niche and are essential for cyclic bouts of hair growth. Niche architecture becomes pronounced postnatally at the start of the first hair cycle. Whether SCs exist or function earlier is unknown. Here we show that slow-cycling cells appear early in skin development, express SC markers, and later give rise to the adult SC population. To test whether these early slow-cycling cells function as SCs, we use Sox9-Cre for genetic marking and K14-Cre to embryonically ablate Sox9, an essential adult SC gene. We find that the progeny of Sox9-expressing cells contribute to all skin epithelial lineages and Sox9 is required for SC specification. In the absence of early SCs, hair follicle and sebaceous gland morphogenesis is blocked, and epidermal wound repair is compromised. These findings establish the existence of early hair follicle SCs and reveal their physiological importance in tissue morphogenesis.     

Abnormal hair development and apparent follicular transformation to mammary gland in the absence of hedgehog signaling

We show that removing the Shh signal tranducer Smoothened from skin epithelium secondarily results in excess Shh levels in the mesenchyme. Moreover, the phenotypes we observe reflect decreased epithelial Shh signaling, yet increased mesenchymal Shh signaling. For example, the latter contributes to exuberant hair follicle (HF) induction, while the former depletes the resulting follicular stem cell niches. This disruption of the niche apparently also allows the remaining stem cells to initiate hair formation at inappropriate times. Thus, the temporal structure of the hair cycle may depend on the physical structure of the niche. Finally, we find that the ablation of epithelial Shh signaling results in unexpected transformations: the follicular outer root sheath takes on an epidermal character, and certain HFs disappear altogether, having adopted a strikingly mammary gland-like fate. Overall, our study uncovers a multifaceted function for Shh in sculpting and maintaining the integrity and identity of the developing HF.     

An in vivo comparative study of sonic, desert and Indian hedgehog reveals that hedgehog pathway activity regulates epidermal stem cell homeostasis

Despite the well-characterised role of sonic hedgehog (Shh) in promoting interfollicular basal cell proliferation and hair follicle downgrowth, the role of hedgehog signalling during epidermal stem cell fate remains largely uncharacterised. In order to determine whether the three vertebrate hedgehog molecules play a role in regulating epidermal renewal we overexpressed sonic (Shh), desert (Dhh) and Indian (Ihh) hedgehog in the basal cells of mouse skin under the control of the human keratin 14 promoter. We observed no overt epidermal morphogenesis phenotype in response to Ihh overexpression, however Dhh overexpression resulted in a range of embryonic and adult skin manifestations indistinguishable from Shh overexpression. Two distinct novel phenotypes were observed amongst Shh and Dhh transgenics, one exhibiting epidermal progenitor cell hyperplasia with the other displaying a complete loss of epidermal tissue renewal indicating deregulation of stem cell activity. These data suggest that correct temporal regulation of hedgehog activity is a key factor in ensuring epidermal stem cell maintenance. In addition, we observed Shh and Dhh transgenic skin from both phenotypes developed lesions reminiscent of human basal cell carcinoma (BCC), indicating that BCCs can be generated despite the loss of much of the proliferative (basal) compartment. These data suggest the intriguing possibility that BCC can arise outside the stem cell population. Thus the elucidation of Shh (and Dhh) target gene activation in the skin will likely identify those genes responsible for increasing the proliferative potential of epidermal basal cells and the mechanisms involved in regulating epidermal stem cell fate.     

Manipulation of outer root sheath cell survival perturbs the hair-growth cycle.

Transgenic mice that overexpress the anti-apoptotic gene bcl-xL under the control of the keratin 14 promoter have significantly shorter hair than non-transgenic littermates. The deficit in hair length correlated with a decrease in the duration of anagen, the growth phase of the hair cycle. A prolongation in telogen, the resting phase of the hair cycle, was also observed in adult animals. In the developing hair bulb, bcl-xL transgene expression was observed exclusively in the outer root sheath (ORS) cells. Bcl-xL expression enhanced the survival of ORS cells treated with apoptotic stimuli. The results suggest that preventing the apoptotic death of ORS cells during anagen leads to a more rapid termination of progenitor cell commitment/proliferation, while the increased survival of ORS cells during telogen delays the initiation of a new hair cycle. ORS cells produce fibroblast growth factor-5 (FGF-5), which acts in a paracrine fashion to terminate precursor cell division during anagen. The short hair phenotype of bcl-xL transgenic mice was substantially reversed in FGF-5-deficient mice. Thus, the production of growth inhibitory factors by ORS cells may provide a mechanism through which the hair-growth cycle is regulated by cell survival.     

Expression of keratin K14 in the epidermis and hair follicle: insights into complex programs of differentiation

Keratins K14 and K5 have long been considered to be biochemical markers of the stratified squamous epithelia, including epidermis (Moll, R., W. Franke, D. Schiller, B. Geiger, and R. Krepler. 1982. Cell. 31:11-24; Nelson, W., and T.-T. Sun. 1983. J. Cell Biol. 97:244-251). When cells of most stratified squamous epithelia differentiate, they downregulate expression of mRNAs encoding these two keratins and induce expression of new sets of keratins specific for individual programs of epithelial differentiation. Frequently, as in the case of epidermis, the expression of differentiation-specific keratins also leads to a reorganization of the keratin filament network, including denser bundling of the keratin fibers. We report here the use of monospecific antisera and cRNA probes to examine the differential expression of keratin K14 in the complex tissue of human skin. Using in situ hybridizations and immunoelectron microscopy, we find that the patterns of K14 expression and filament organization in the hair follicle are strikingly different from epidermis. Some of the mitotically active outer root sheath (ORS) cells, which give rise to ORS under normal circumstances and to epidermis during wound healing, produce only low levels of K14. These cells have fewer keratin filaments than basal epidermal cells, and the filaments are organized into looser, more delicate bundles than is typical for epidermis. As these cells differentiate, they elevate their expression of K14 and produce denser bundles of keratin filaments more typical of epidermis. In contrast to basal cells of epidermis and ORS, matrix cells, which are relatively undifferentiated and which can give rise to inner root sheath, cuticle and hair shaft, show no evidence of K14, K14 mRNA expression, or keratin filament formation. As matrix cells differentiate, they produce hair-specific keratins and dense bundles of keratin filaments but they do not induce K14 expression. Collectively, the patterns of K14 and K14 mRNA expression and filament organization in mitotically active epithelial cells of the skin correlate with their relative degree of pluripotency, and this suggests a possible basis for the deviation of hair follicle programs of differentiation from those of other stratified squamous epithelia.     

Improved isolation of outer root sheath cells from human hair follicles and their proliferation behavior under serum-free condition

Hair follicle is a small but very complex and dynamic miniorgan of the human body. It is easy to isolate and culture mesenchymal cells but not epithelial cells of hair follicle. It is necessary for intact and healthy outer root sheath (ORS) cells to be isolated and cultured. In this study we developed an appropriate isolation method to yield 6.4±0.75×10⁴ cells/hair follicle, which is about 9-fold comparing to our previous data. This yield was achieved by modifications such as different kinds of enzyme uses, fragmentation, and mechanical stimuli. Especially we detected that the different kinds of isolation enzyme could affect proliferation of ORS cells during primary culture. In addition, bovine pituitary extract (BPE) was needed for ORS cells to proliferate and to form colonies under serum-free, feeder layer-free culture condition, but type I collagen as a substratum did not have any positive effect. Moreover, ORS cells under BPE-added condition contained stem/progenitor cells expressing β1-integrin. CK19, and CD34. These results can provide useful cell culture information, not only in the study of hair biology but also in the field of tissue engineering and cell therapy for the treatment of alopecia.     

Migration of melanoblasts into the developing murine hair follicle is accompanied by transient c-Kit expression.

Disruption of the c-Kit/stem cell factor (SCF) signaling pathway interferes with the survival, migration, and differentiation of melanocytes during generation of the hair follicle pigmentary unit. We examined c-Kit, SCF, and S100 (a marker for precursor melanocytic cells) expression, as well as melanoblast/melanocyte ultrastructure, in perinatal C57BL/6 mouse skin. Before the onset of hair bulb melanogenesis (i.e., stages 0-4 of hair follicle morphogenesis), strong c-Kit immunoreactivity (IR) was seen in selected non-melanogenic cells in the developing hair placode and hair plug. Many of these cells were S100-IR and were ultrastructurally identified as melanoblasts with migratory appearance. During the subsequent stages (5 and 6), increasingly dendritic c-Kit-IR cells successively invaded the hair bulb, while S100-IR gradually disappeared from these cells. Towards the completion of hair follicle morphogenesis (stages 7 and 8), several distinct follicular melanocytic cell populations could be defined and consisted broadly of (a) undifferentiated, non-pigmented c-Kit-negative melanoblasts in the outer root sheath and bulge and (b) highly differentiated melanocytes adjacent to the hair follicle dermal papilla above Auber's line. Widespread epithelial SCF-IR was seen throughout hair follicle morphogenesis. These findings suggest that melanoblasts express c-Kit as a prerequisite for migration into the SCF-supplying hair follicle epithelium. In addition, differentiated c-Kit-IR melanocytes target the bulb, while non-c-Kit-IR melanoblasts invade the outer root sheath and bulge in fully developed hair follicles.     

Expression of E6 and E7 papillomavirus oncogenes in the outer root sheath of hair follicles extends the growth phase and bypasses resting at telogen.

Hair follicle growth cycle proceeds through a series of stages in which strict control of cell proliferation, differentiation, and cell death occurs. Transgenic mice expressing human papillomavirus type 16 E6/E7 papillomavirus oncogenes in the outer root sheath (ORS) display a fur phenotype characterized by lower hair density and the ability to regenerate hair much faster than wild-type mice. Regenerating hair follicles of transgenic mice show a longer growth phase (anagen), and although bulb regression (catagen) occurs, rest at telogen was not observed. No abnormalities were detected during the first cycle of hair follicle growth, but by the second cycle, initiation of catagen was delayed, and rest at telogen was again not attained, even in the presence of estradiol, a telogen resting signal. In conclusion, expression of E6/E7 in the ORS delays entrance to catagen and makes cells of the ORS insensitive to telogen resting signals bearing to a continuous hair follicle cycling in transgenic mice.     

Expression of galectin-1 and -3 and of accessible binding sites during murine hair cycle

Although protein-carbohydrate interactions are supposed to play key roles in cell adhesion, signalling and growth control. Their exact role in skin physiology has only recently been investigated. The endogenous lectins galectin-1 and galectin-3 have been identified in skin including hair follicles. Here, we analyzed the expression and distribution of these galectins and their binding sites in C57BL/6 mice during hair cycle. The expression of galectin-1 and galectin-3 binding sites was found to be predominantly hair cycle-dependent showing some overlapping to the expression of galectin-1 and -3. The outer root sheath (ORS) expressed galectin-1 binding sites during anagen IV to VI and in early catagen, whereas galectin-1 was expressed from early anagen to late catagen. The ORS expressed galectin-3 binding sites during catagen transition corresponding to a galectin-3 expression during anagen V and catagen. The innermost layer of the ORS expressed galectin-3 binding sites during anagen VI until catagen VIII, but galectin-3 during anagen III to IV and catagen. The inner root sheath (IRS) expressed galectin-3 binding sites only in anagen IV but missed expression of any of the two galectins. The matrix cells expressed galectin-3 binding sites in catagen II-III as well as galectin-3 during anagen V to catagen IV. The present study provides the first evidence for a cycle-related expression of both galectin-1 and -3 and their binding sites during murine hair cycle.     

BMPR1A signaling is necessary for hair follicle cycling and hair shaft differentiation in mice

Interactions between ectodermal and mesenchymal extracellular signaling pathways regulate hair follicle (HF) morphogenesis and hair cycling. Bone morphogenetic proteins (BMPs) are known to be important in hair follicle development by affecting the local cell fate modulation. To study the role of BMP signaling in the HF, we disrupted Bmpr1a, which encodes the BMP receptor type IA (BMPR1A) in an HF cell-specific manner, using the Cre/loxP system. We found that the differentiation of inner root sheath, but not outer root sheath, was severely impaired in mutant mice. The number of HFs was reduced in the dermis and subcutaneous tissue, and cycling epithelial cells were reduced in mutant mice HFs. Our results strongly suggest that BMPR1A signaling is essential for inner root sheath differentiation and is indispensable for HF renewal in adult skin.     

CCN2 modulates hair follicle cycling in mice

It is critical to understand how stem cell activity is regulated during regeneration. Hair follicles constitute an important model for organ regeneration because, throughout adult life, they undergo cyclical regeneration. Hair follicle stem cells—epithelial cells located in the follicle bulge—are activated by periodic β-catenin activity, which is regulated not only by epithelial-derived Wnt, but also, through as-yet-undefined mechanisms, the surrounding dermal microenvironment. The matricellular protein connective tissue growth factor (CCN2) is secreted into the microenvironment and acts as a multifunctional signaling modifier. In adult skin, CCN2 is largely absent but is unexpectedly restricted to the dermal papillae and outer root sheath. Deletion of CCN2 in dermal papillae and the outer root sheath results in a shortened telogen-phase length and elevated number of hair follicles. Recombinant CCN2 causes decreased β-catenin stability in keratinocytes. In vivo, loss of CCN2 results in elevated numbers of K15-positive epidermal stem cells that possess elevated β-catenin levels and β-catenin–dependent reporter gene expression. These results indicate that CCN2 expression by dermal papillae cells is a physiologically relevant suppressor of hair follicle formation by destabilization of β-catenin and suggest that CCN2 normally acts to maintain stem cell quiescence.     

Establishment and characterization of outer root sheath (ORS) cell line from Jining grey goat

A new line of outer root sheath (ORS) cells was established from hair follicles of Jining grey goat by using a mechanical separation combined with enzyme digestion. Cell morphology is described at different phases. The chromosome analysis of ORS cells, identification of the ORS cells and morphological reversion test were detected at the 4th and 40th passages. The ORS cells were healthy and the growth characteristics were stable with a population doubling time of 52 h. Chromosome analysis showed that >58% of cells were diploid. Test for ORS cell line CK19 expression was positive. This newly established ORS cell line not only lays the foundation for further studying on the growth, regeneration, development law of goat hair follicle but also provides a mirror for the research of human hair in medical field.     

Hair follicle stem cells

The increasing use of the hair follicle as a stem cell paradigm is due in part to the complex interplay between epithelial, dermal and other cell types, each with interesting differentiation potential and prospective therapeutic applications. This review focuses on research into the environmental niche, gene expression profiles and plasticity of hair follicle stem cell populations, where many recent advances have come about through novel technological and experimental approaches. We discuss major developmental pathways involved in the establishment and control of the epithelial stem cell niche, and evidence of plasticity between stem and transit amplifying cell populations.