Tissue engineering of human hair follicles using a biomimetic developmental approach

Human skin constructs (HSCs) have the potential to provide an effective therapy for patients with significant skin injuries and to enable human-relevant drug screening for skin diseases; however, the incorporation of engineered skin appendages, such as hair follicles (HFs), into HSCs remains a major challenge. Here, we demonstrate a biomimetic approach for generation of human HFs within HSCs by recapitulating the physiological 3D organization of cells in the HF microenvironment using 3D-printed molds. Overexpression of Lef-1 in dermal papilla cells (DPC) restores the intact DPC transcriptional signature and significantly enhances the efficiency of HF differentiation in HSCs. Furthermore, vascularization of hair-bearing HSCs prior to engraftment allows for efficient human hair growth in immunodeficient mice. The ability to regenerate an entire HF from cultured human cells will have a transformative impact on the medical management of different types of alopecia, as well as chronic wounds, which represent major unmet medical needs.

Human skin constructs hold potential for regenerative medicine, but the incorporation of hair follicles into such constructs is a challenge. Here, the authors use 3D printed molds to pattern hair follicle cell types in a physiological organization, and achieve human hair growth on the back of a mouse.

Cbx4 maintains the epithelial lineage identity and cell proliferation in the developing stratified epithelium

Polycomb complex member Cbx4 represses nonepidermal lineage and cell cycle inhibitor genes in the epidermal keratinocytes and operates as a direct p63 target, maintaining epithelial identity and proliferative activity in the developing epidermis.

During development, multipotent progenitor cells establish lineage-specific programmers of gene activation and silencing underlying their differentiation into specialized cell types. We show that the Polycomb component Cbx4 serves as a critical determinant that maintains the epithelial identity in the developing epidermis by repressing nonepidermal gene expression programs. Cbx4 ablation in mice results in a marked decrease of the epidermal thickness and keratinocyte (KC) proliferation associated with activation of numerous neuronal genes and genes encoding cyclin-dependent kinase inhibitors (p16/p19 and p57). Furthermore, the chromodomain- and SUMO E3 ligase–dependent Cbx4 activities differentially regulate proliferation, differentiation, and expression of nonepidermal genes in KCs. Finally, Cbx4 expression in KCs is directly regulated by p63 transcription factor, whereas Cbx4 overexpression is capable of partially rescuing the effects of p63 ablation on epidermal development. These data demonstrate that Cbx4 plays a crucial role in the p63-regulated program of epidermal differentiation, maintaining the epithelial identity and proliferative activity in KCs via repression of the selected nonepidermal lineage and cell cycle inhibitor genes.

Human hair follicle dermal sheath and papilla cells support keratinocyte growth in monolayer coculture

Traditional skin grafting techniques are effective but limited methods of skin replacement. Autologous transplantation of rapidly cultured keratinocytes is successful for epidermal regeneration, but the current gold-standard technique requires mouse fibroblast feeders and serum-rich media, with serum-free systems and dermal fibroblast (DF) feeders performing relatively poorly. Here, we investigated the capacity of human hair follicle dermal cells to act as alternative supports for keratinocyte growth. Dermal papilla (DP) dermal sheath (DS), DF and 3T3 cells were used as inactivated feeder cells for human keratinocyte coculture. Under conditions favouring dermal cells, proliferation of keratinocytes in the presence of either DS or DP cells was significantly enhanced compared with DF cells, at levels comparable to keratinocytes cultured under gold-standard conditions. Secreted protein acidic and rich in cysteine (SPARC) expression increased DS and DP cells relative to DFs; however, further experiments did not demonstrate a role in keratinocyte support.

Dermal stem cells can differentiate down an endothelial lineage

In this study, we have demonstrated that cells of neural crest origin located in the dermal papilla (DP) exhibit endothelial marker expression and a functional activity. When grown in endothelial growth media, DP primary cultures upregulate expression of vascular endothelial growth factor receptor 1 (FLT1) mRNA and downregulate expression of the dermal stem cell marker α-smooth muscle actin. DP cells have demonstrated functional characteristics of endothelial cells, including the ability to form capillary-like structures on Matrigel, increase uptake of low-density lipoprotein and upregulate ICAM1 (CD54) in response to tumour necrosis factor alpha (TNF-α) stimulation. We confirmed that these observations were not due to contaminating endothelial cells, by using DP clones. We have also used the WNT1cre/ROSA26R and WNT1cre/YFP lineage-tracing mouse models to identify a population of neural crest-derived cells in DP cultures that express the endothelial marker PECAM (CD31); these cells also form capillary-like structures on Matrigel. Importantly, cells of neural crest origin that express markers of endothelial and mesenchymal lineages exist within the dermal sheath of the vibrissae follicle.

Emulsion-derived foams (PolyHIPEs) containing poly(epsilon-caprolactone) as matrixes for tissue engineering

The preparation of PolyHIPE foams containing poly(epsilon-caprolactone) from macromonomers by free radical homo- or copolymerization is described. The macromonomers are synthesized from PCL diols and are polymerized in the continuous phase of high internal phase emulsions (HIPEs). Subsequent drying yields low-density foams with cell diameters of 5-100 microm. Foam morphology, as determined by scanning electron microscopy, depends on the type of diluent (styrene, methyl methacrylate, or toluene) added to the emulsion organic phase and on the PCL content. Increasing the latter increases the continuous phase viscosity to a point where emulsion formation is impeded. Foam swelling in toluene, 2-propanol, and water was investigated by solvent imbibition and increased with increasing solvent hydrophobicity. Furthermore, it was found generally to decrease with increasing PCL content, due to increasing cross-link density. Swelling generally increased when higher molar mass PCL macromonomer was used due to the formation of a less tightly cross-linked network. One type of foam sample was shown to support the growth of human fibroblasts over a period of 2.5 days.

Hair follicle dermal sheath cells: unsung participants in wound healing

The dermal sheath that surrounds the outside of the hair follicle contains progenitor cells that maintain and regenerate the dermal papilla, a key component for hair growth. Our contention is that dermal sheath cells have other roles. We believe that they can become wound healing fibroblasts and perform an important function in the repair of skin dermis after injury. The dermal sheath has close developmental and anatomical parallels with follicle outer root sheath, the epithelial component that contains the stem cells responsible for replacing skin epidermis. Dermal sheath cells also have a myofibroblast or wound healing phenotype, and in animals with high follicle densities differences in wound healing are observed in conjunction with changes in the hair growth cycle. Similarly, in human beings there are apparent differences in wound healing responses between hairy and non-hairy body sites. Moreover, clinical and experimental data suggest that the involvement of follicle-derived dermal cells results in qualitatively improved dermal repair. Therefore, in a therapeutic context, hair follicle dermal cells provide an accessible option for the creation of dermal or full skin equivalents that could both improve wound healing and reduce scarring. Indeed, given the inductive properties of adult hair follicle dermal cells, it is reasonable to envisage a tissue engineering approach for the production of a skin equivalent that will grow hair follicles when grafted.

Hair follicle predetermination

Recent genetic and molecular studies of hair follicle (HF) biology have provided substantial insight; however, the molecular data, including expression patterns, cannot be properly appreciated without an understanding of the basic cellular rearrangements and interactions that underpin HF cyclic transformations. We present a novel interpretation of the major cellular processes that take place during HF cycling – the hypothesis of hair follicle predetermination. This hypothesis is an extension of previous models of HF cellular kinetics but has two critical modifications: the dual origin of the cycling portion of the HF, and the timing of the recruitment of stem cells. A compilation of evidence suggests that the ascending portion of the HF (hair shaft and inner root sheath) arises not from bulge-located HF stem cells that contribute to the formation of only the outer root sheath (ORS), but instead from the germinative cells localized in the secondary hair germ. In middle anagen, upon completion of the downward growth of the HF, cells derived from the bulge region migrate downward along the ORS to reside at the periphery of the HF bulb as a distinct, inactive cell population that has specific patterns of gene expression - ‘the lateral disc’. These cells survive catagen-associated apoptosis and, under the direct influence of the follicular papilla (FP), transform into the hair germ and acquire the ability to respond to FP signaling and produce a new hair. Thus, we propose that the specific sensitivity of germ cells to FP signaling and their commitment to produce the ascending HF layers are predetermined by the previous hair cycle during the process of transformation of bulge-derived lateral disc cells into the secondary hair germ.

In vivo induction of hair growth by dermal cells isolated from hair follicles after extended organ culture

Successful hair follicle organ culture has been established for some time, but hair growth in vitro is limited and generally terminates prematurely in comparison with in vivo. The reasons why growth stops in culture are as yet unknown. In this investigation, adult rat vibrissa follicles for which growth in culture is limited to about 10 d, were maintained in vitro for a minimum of 20 d after the hair shaft stopped growing. The pattern of fiber growth and long-term follicle pathology reflected the initial hair cycle stage at the time of isolation. Furthermore, there was evidence that a group of follicles put into culture when in late anagen were attempting to cycle in vitro. Microscopy showed that, in spite of widespread pathologic changes to the follicle epithelium, dermal cells in the follicle showed remarkable resilience. Their viability was confirmed when primary cell cultures were established from isolated dermal tissue. These cells labeled positively for alpha-smooth muscle actin, an established marker of hair follicle dermal cell phenotype in vitro. Moreover, isolated dermal tissue induced hair growth when implanted into inactivated hair follicles in vivo. These data confirm that the cessation in hair growth is not due to a loss of the inductive capacity in the dermal component. Long-term organ culture may provide opportunities to investigate factors that are expressed or lost during hair growth cessation. In addition it may be possible to develop this method further to obtain a reliable and predictable model of hair follicle cycling in vitro.

Trans-species hair growth induction by human hair follicle dermal papillae

A series of experimental bioassays has shown that the dermal papilla of the adult rodent vibrissa hair follicle retains unique inductive properties. In view of the many phenotypic and functional differences between specific hair follicle types, and the growing interest in hair follicle biology and disease, it remains important to establish that the human hair follicle dermal papilla has equivalent capabilities. In this study we tested the ability of human hair follicle papillae to induce hair growth when implanted into transected, athymic mouse vibrissa follicles. The implanted papillae that interacted with mouse follicle epithelium created new fibre-producing follicle end bulbs. The origin of the papillae in the recombinant structures was confirmed using laser capture microdissection and human specific gender determination by PCR. The demonstration that intact adult human dermal papillae can induce hair growth has implications for molecular analysis of basic hair growth mechanisms, particularly since the study involved common epithelial-mesenchymal signalling and recognition properties across species. It also improves the prospects for a cell-based clinical approach to hair follicle disorders.

A correlation between versican and neurofilament expression patterns during the development and adult cycling of rat vibrissa follicles

Versican, a proteoglycan recently implicated in hair follicle induction, has been shown to influence axon outgrowth in vitro and in vivo. We used immunohistochemistry to study the relationship between versican expression and innervation, during rat vibrissa follicle development and the adult hair cycle. During development, nerve fibres were commonly associated with areas of weak versican expression, and the path of axons appeared to be delineated by sharp boundaries of versican expression. Versican expression changed in the lower follicle dermis during the adult hair follicle cycle but remained strong around the follicle neck reflecting the constant innervation. Our observations show a correlation between versican expression and peripheral innervation indicating that versican may have a dual role in hair follicle biology.

Adult corneal epithelium basal cells possess the capacity to activate epidermal, pilosebaceous and sweat gland genetic programs in response to embryonic dermal stimuli

Recent work has shown remarkable plasticity between neural and hematopoeitic, as well as between hematopoeitic and muscle stem cells, depending on environmental stimuli (Fuchs, E. and Segre, J. A. (2000) Cell 100, 143–155). Stem cells give rise to a proliferative transient amplifying population (TA), which is generally considered to be irreversibly committed. Corneal epithelium provides a particularly useful system for studying the ability of TA cells to activate different genetic programs in response to a change in their fibroblast environment. Indeed, corneal stem and TA cells occupy different localities - stem cells at the periphery, and TA cells more central (Lehrer, M. S., Sun, T. T. and Lavker, R. M. (1998) J. Cell Sci. 111, 2867–2875) - and thus can be discretely dissected from each other. It is well known that pluristratified epithelia of cornea and skin display distinct programs of differentiation: corneal keratinocytes express keratin pair K3/K12 and epidermal keratinocytes keratin pair K1-2/K10; moreover, the epidermis forms cutaneous appendages, which express their own set of keratins. In our experiments, central adult rabbit corneal epithelium was thus associated either with a mouse embryonic dorsal, upper-lip or plantar dermis before grafting onto nude mice. Complementary experiments were performed using adult mouse corneal epithelium from the Rosa 26 strain. The origin of the differentiated structures were identified in the first case by Hoechst staining and in the second by the detection of beta-galactosidase activity. The results show that adult central corneal cells are able to respond to specific information originating from embryonic dermis. They give rise first to a new basal stratum, which does not express anymore corneal-type keratins, then to pilosebaceous units, or sweat glands, depending of the dermis, and finally to upper layers expressing epidermal-type keratins. Our results provide the first evidence that a distinct TA cell population can be reprogrammed.

Localisation of members of the notch system and the differentiation of vibrissa hair follicles: receptors, ligands, and fringe modulators

Hair vibrissa follicle morphogenesis involves several cell segregation phases, in the dermis as well as in the epidermis. The expression of Notch-related genes, which are well established mediators of multiple cell segregation events in Drosophila development, was studied by in situ hybridisation during embryonic mouse vibrissa follicle morphogenesis and the first adult hair cycle. The results show that two receptors, Notch1 and -2, three ligands, Delta1, Serrate1, and -2, and the three Fringe regulators, Lunatic, Manic, and Radical, are expressed in different locations and morphogenetic stages. First, the appearance of hair vibrissa primordia involves the expression of complementary patterns of Notch2, Delta1, and Lunatic Fringe in the dermis and of Notch1, Serrate2, and Lunatic Fringe in the epidermis. Second, this expression pattern is no longer found after stage 3 in the dermis. Meanwhile, in the epidermis, the expression of Notch1, Serrate2, and Lunatic Fringe before the formation of the placode may be involved in determining two populations of epidermal cells in the developing follicle. Third, complementary expression patterns for Notch1, Manic, and Lunatic Fringe, as well as Serrate1 and -2 as previously shown (Powell et al., 1998), are progressively established from stage 4 of embryonic development both in the outer root sheath and in the hair matrix. These patterns are consistent with the one found in the adult anagen phase. During the hair vibrissa cycle, Notch1 and Manic Fringe display temporal and spatial changes of expression, suggesting that they may intervene as modulators of trichocyte activities.

Cellular and developmental aspects of androgenetic alopecia

The hair follicle is a highly complex system that can be investigated at many levels and from multiple perspectives. However, underlying the cyclic production of all hair fibres are a set of common developmental processes. Many current investigations of androgenetic alopecia concentrate on the direct influences of hormones on hair follicles at the cellular or intracellular level. This paper attempts to step back from this and consider the process of terminal to vellus transition in androgenetic alopecia in terms of basic cellular and developmental mechanisms. Ideas about the mechanism and timing of follicle size reduction are put forward, but the paper also tries to point out inherent difficulties in the investigation of androgenetic alopecia and important gaps in current knowledge.

Hair cycle stage of the mouse vibrissa follicle determines subsequent fiber growth and follicle behavior in vitro

The establishment of culture models representative of all aspects of in vivo hair follicle behavior is an important goal for theoretic and analytic studies. Rodent vibrissa follicles have regular, predictable, and relatively short growth cycles. In this investigation, we took advantage of these properties; we classified mouse vibrissa follicles according to different phases in the hair cycle and then compared fiber growth and morphologic changes in culture. Follicles isolated in the early phase of the growth cycle produced fine growing fibers with an average growth that exceeded 3 mm over 15 d. Even when hair growth had slowed or halted subsequently, histology showed that these follicles retained an anagen-like morphology. By contrast, follicles isolated toward the end of the growing cycle produced thicker fibers for much shorter periods, after which growth ceased and the fibers lifted up from the base of the follicle. Internally, these specimens resembled their telogen counterparts in situ. Follicles isolated in mid-growth demonstrated intermediate fiber growth characteristics. In organ culture, mouse vibrissa follicles therefore closely reflect their in vivo origin in growth characteristics and cycle timing. These data provide new opportunities for studying hair growth cycle mechanisms in vitro, but present a caveat for quantitative studies because there may be a greater growth cycle-related variation than has previously been assumed.

Human hair follicle regeneration following amputation and grafting into the nude mouse

In this study we investigated the capacity of the human hair follicle to regenerate a fiber-forming bulb after its amputation. We removed the bases from terminal follicles from a variety of sites and transplanted the follicles onto athymic mice, either still attached to a skin graft or as subcutaneous implants of individual follicles. External hair growth was observed on the skin grafts, and histology of the follicles revealed restoration of dermal papillae and follicle bulb structures. This result suggests that the capacity of hair follicles to regenerate their lower structures after removal, which was first demonstrated on whisker follicles, may be a general phenomenon. It emphasizes the importance of specific cellular subpopulations within the follicle and the role of dermal-epidermal interactions in adult follicle activities.

Dermal-epidermal interactions. Adult follicle-derived cell populations and hair growth

Intrinsic dermal-epidermal interactions are central to the development and growth of hair. This article describes investigations into the inductive properties of specific dermal and epidermal cell populations from adult follicles by means of cell culture and in vivo implantation. It highlights the inductive powers of cultured dermal papilla cells and the more recent finding that the germinative epidermal cells of the lower follicle also can stimulate hair growth. How the reconstruction of a hair follicle from its constituent parts has been achieved is described. The significance of these findings is considered with reference to human hair growth, tissue engineering, and the prospects for elucidating the molecular signalling mechanisms that underpin dermal-epidermal interplay.

Hair matrix germinative epidermal cells confer follicle-inducing capabilities on dermal sheath and high passage papilla cells

Low passage cultured dermal papilla cells from adult rats stimulate complete hair follicle neogenesis when re-implanted into heterotypic skin. In contrast, cultured sheath cells are non-inductive despite sharing other behavioural characteristics (a common lineage and in situ proximity) with papilla cells. However, since sheath cells can behave inductively in amputated follicles after regenerating the papilla, this poses the question of what influences the sheath to papilla cell transition? During reciprocal tissue interactions specific epidermal cues are crucial to skin appendage development, and while in vivo assays to date have focussed on dermal interactive influence, our aim was to investigate epidermal potential. We have previously observed that hair follicle epidermal cells display exceptional interactive behaviour when combined with follicle dermal cells in vitro. Thus in the present study, hair follicle germinative, outer root sheath or skin basal epidermal cells were separately combined with each of three non-inductive dermal cell types (high passage papilla, low passage sheath or fibroblast) and then implanted into small ear skin wounds. The sheath/germinative and papilla/germinative cell implants repeatedly induced giant vibrissa-type follicles and fibres. In complete contrast, any single cell type and all other forms of recombination were consistently non-inductive. Hence, the adult germinative epidermal cells enable non-inductive adult dermal cells to stimulate hair follicle neogenesis, effectively, by altering their ‘status’, causing the sheath cells to ‘specialise’ and the ‘aged’ papilla cells to ‘rejuvenate’.

Hair follicle reconstruction in vitro

The in vitro creation of a follicular structure capable of hair growth from cultured adult cells has long been an aim of researchers in hair biology. Basal outer root sheath cells (ORS) attached to the hair follicle glassy membrane (GM) were isolated and cultured, where they revealed greater replicative potential and longevity than ORS cells from plucked fibres. Hair follicles were reconstructed in vitro using the collagenous shells of rat vibrissa follicles as natural containers for 4 hair follicle cell types. Basal ORS cells were initially seeded onto the residual vibrissa GM, and cultured dermal papilla, dermal sheath and germinative epidermal cells were then added. Histology revealed that after 2 or 3 weeks in combined culture, cell interactions and tissue morphogenesis had resulted in the formation of irregular but recognizable hair fibres, produced from unusual bulb structures. To our knowledge this represents the first example of adult cell populations instigating the de novo creation of hair fibres in a culture dish. While the usefulness of the current methodology relates immediately to hair growth research, the generation of hair follicles and fibres in vitro establishes the enormous potential of this type of interactive work for practical purposes.

Hair follicle stem cells: characteristics and possible significance

All four of the principle dermal and epidermal cell types from the adult hair follicle (dermal papilla and sheath, germinative epidermal and outer root sheath) can now be grown in culture. The germinative epidermal cells from the source of the hair fibre appear to be the most visually distinctive of these populations, but all four can be morphologically, synthetically and behaviourally distinguished from general interfollicular skin cells. The germinative population also most obviously exhibit many classical stem cell attributes, but the interactive and inductive capabilities of all of the cell types, in addition to their multipotential natures, highlights that they all share an intriguing level of developmental flexibility.

Induction of hair growth in ear wounds by cultured dermal papilla cells

In the adult hair follicle the dermal papilla plays a crucial role in the dermal-epidermal interactions that control hair production and events of the growth cycle. It has previously been shown that cultured cells from rat vibrissa follicle dermal papillae can stimulate hair growth when implanted into amputated follicles. This study investigated the effects of implanting low-passage cultured papilla cells into small incisional wounds in the rat ear pinna. The groups of fibers that emerged from wound sites were much larger than local hairs, and often had vibrissa-type characteristics. Later-passage papilla cells or cultured skin fibroblasts failed to elicit the same response. Histology revealed that big follicles were formed when papilla cells were trapped between the cut edges of the epidermis. Abnormally large follicles were seen at wound sites many months post-operatively. Independent of epidermal influence, cultured papilla cells in the wound dermis formed rounded papilla-like aggregates that also persisted until biopsy. A previously described method of wrapping papilla cells in glabrous epidermis was less successful in percentage terms but resulted in the production of one very large vibrissa-type follicle and fiber. These results further illustrate that the inductive powers and developmental information retained by cultured dermal papilla cells parallel the properties of their embryonic precursors; the findings may have implications for human hair growth.

A quantitative study of the differential expression of alpha-smooth muscle actin in cell populations of follicular and non-follicular origin

Alpha-smooth muscle actin (ASMA) is an actin isoform present in the filaments of smooth muscle cells, myofibroblasts, and a specific region of hair follicle dermal sheath in vivo. We employed double immunofluorescence, two-dimensional electrophoresis, Western blots and DNA, protein, and actin isoform determinations to quantify the relative levels of ASMA in four populations of cultured hair follicle dermal cells, and fibroblasts derived from three regions of adult and comparable areas of 4-d rat skin. Although follicle sheath populations were morphologically similar, they contained variable proportions of cells that expressed ASMA. Tissue from the most positive region in situ, the lower/mid sheath, also gave rise to the most positive cells in culture (98%), followed by the end bulb (85%) and then upper sheath (50%). The follicle dermal cells (including papilla 81%) displayed and maintained levels of expression well above those obtained for adult (below 10%) or 4-d (9-40%) fibroblasts, and even cultured smooth muscle cells. It was also confirmed that levels of expression in adult fibroblasts could be positively correlated with hair follicle density in the biopsies from which they were initiated. Differential expression of ASMA in follicle subpopulations provides an insight into how their behavior may be linked to their specialized functions, for example, their likely involvement in the mechanics of the hair cycle. Moreover, the proposition that hair follicle dermal cells represent unappreciated constituents of general skin fibroblast cultures has substantial implications.

Human hair follicle germinative epidermal cell culture

Isolated human hair follicle germinative epidermal cells were observed in vitro for the first time. When cultured alone, this small, round, novel cell type did not grow, divide, take on an outer root sheath-type appearance, or display any obvious signs of epidermal differentiation. We have previously described comparable cells from rat vibrissa follicles. However, in combination with human hair follicle dermal papilla populations, the germinative epidermal cells were stimulated into proliferative and complex interactive behaviors. This included the formation of composite organotypic structures containing not only impressively intact basement membrane, but also the hair-specific form, glassy membrane.

Dermal-epidermal interactions--follicle-derived cell populations in the study of hair-growth mechanisms

All skin appendage development is initiated by a series of dermal-epidermal interactions. These continue to underpin adult hair follicle activities through the specialized follicular cell populations--indeed the inductive properties of isolated dermal papillae from adult vibrissa follicles are well established. Far less is known about the influence of adult follicle epidermis on dermal cells, or inductive properties of papilla cells from other follicle types. Cultured papilla cells, unusually, are able to support the proliferation of skin epidermal cells during simple association in culture, but do not produce more elaborate organization or differentiation. However, germinative epidermal cells from the follicle base are morphologically and behaviorally distinct from other epidermal populations, and in simple association with papilla cells interact to form complex structures with a distinct basal lamina. That hair follicle germinative cells have an important influence on dermal cells is further demonstrated by in vivo recombinations, where germinative cells interact with otherwise non-inductive follicle dermal sheath cells to initiate follicle formation and hair growth. In vitro, several follicle cell populations assembled within the capsule of a vibrissa follicle and grown in a three-dimensional culture system produce hair-type fibers. When cultured pelage follicle dermal papilla cells are implanted alone into footpad skin under controlled conditions, new pelage-type follicles and fibers are induced. This emphasizes the power and universal nature of inductive influences from papilla cells, and underlines the dermatologic potential of cell manipulations. The transdifferentiation of the footpad epidermis is a powerful biologic phenomenon normally only seen in embryonic-type association experiments.

Restoration of hair growth by surgical implantation of follicular dermal sheath

The capacity of lower follicle dermal sheath to restore hair growth was tested by removing the lower halves of follicles, and then immediately implanting material containing dermal sheath cells from these bases, into the remaining upper epidermal follicle cavity. Over 60% of recipient follicles produced stout emergent vibrissa fibres and some operations resulted in multiple hair production from a single follicle. Histological examination revealed new dermal papillae within large bulb structures which were sited below the level of amputation--a feature that indicated that the new dermal papilla was derived from implanted material. For many follicles, the failure to produce emergent fibres could be accounted for after histological examination. These results provide clear evidence that lower follicle dermal sheath cells are capable of replacing those of the dermal papilla and it shows that they can do so in the context of the upper follicle. However, because elements of lower follicle epidermis were present in the implant material, the interactive sequence of events cannot be established. Dermal sheath cells have immense potential for papilla cell replacement: questions remain as to whether the distinction between sheath and papilla cells is one of context, or whether the transition requires specific external influences.

Induction of follicle formation and hair growth by vibrissa dermal papillae implanted into rat ear wounds: vibrissa-type fibres are specified

Adult vibrissa follicle dermal papillae have the capacity to induce hair growth and follicle formation when associated with epidermis from various sources. However, the range of conditions under which hair follicle induction will take place has not been established. The question of whether or not the adult papilla carries information to impose fibre-type specificity has also not been fully answered. This study describes how the implantation of isolated papillae into small incisional cuts on the rat ear pinna resulted in the subsequent emergence of abnormally large hair fibres from the wound sites. Many of these hairs were found to display vibrissa-type characteristics. Histological observations indicated that the papillae had interacted with the edges of the wound epidermis to produce new, and particularly large follicles, while immunohistochemical staining revealed that early follicle construction was accompanied by a profusion of the basement membrane constituents laminin and type IV collagen in the subjacent dermis. These findings show that adult rat papillae retain the capacity, as displayed by embryonic dermis, to determine vibrissa specificity in induced follicles.

Changes in fibronectin, laminin and type IV collagen distribution relate to basement membrane restructuring during the rat vibrissa follicle hair growth cycle

Hair growth in adult mammals involves continuous dermal-epidermal interaction across the follicular basement membrane, and repeated reorganisation of lower follicle structure during the hair growth cycle. The immunolocalisation of 3 extracellular matrix components, fibronectin, laminin and type IV collagen was investigated during the course of the rat vibrissa follicle growth cycle, and their distribution correlated with changes in cellular and extracellular ultrastructure, particularly around the basement membrane zone. Laminin and type IV collagen were omnipresent at the follicular dermal-epidermal junction, but were also seen in granular extracellular form within the inner dermal component of the follicle, the dermal papilla. Both the inner papilla-epidermal junction and the thick specialised outer basement membrane (the glassy membrane) revealed labelling by these 2 antibodies around telogen (the period of nonfibre production). By contrast, fibronectin was abundant within the anagen dermal papilla but at telogen stained the dermal-epidermal junction heterogeneously, when it disappeared from the inner papilla-epidermal interface but intensified externally. These changes to extracellular matrix distribution coincided with a modification of basement membrane ultrastructure from a relatively uniform line at anagen, to one which became much broader and multilayered at telogen with a loss of definite structure within the papilla. This shows that the lower part of the vibrissa follicle retains the capacity for very rapid basement membrane modification and remodelling, and implies that it is part of the biological process which enables dermal-epidermal signalling, rather than a secondary product of physical changes to the appendage. The work supports the idea that dermal papilla cells could contribute to basement membrane formation, and also that fibronectin may be involved in regulating cellular activities within the follicle. In the vibrissa follicle, dynamic cellular activity clearly takes place throughout the duration of the hair cycle.

Cultured dermal papilla cells induce follicle formation and hair growth by transdifferentiation of an adult epidermis

Adult rat pelage follicle dermal papilla cells induced follicle neogenesis and external hair growth when associated with adult footpad skin epidermis. They thus demonstrated a capacity to completely change the structural arrangement and gene expression of adult epidermis—an ability previously undocumented for cultured adult cells. Isolation chambers ensured that de novo follicle formation must have occurred by eliminating the possibility of cellular contributions, and/or inductive influences, from local skin follicles. These findings argue against previous suggestions of vibrissa follicle specificity, and imply that the potential for hair follicle induction may be common to all adult papilla cells.

Cellular and extracellular involvement in the regeneration of the rat lower vibrissa follicle

The sequence of events leading to the reconstruction of a fibre-producing hair follicle, after microsurgical amputation of the lower follicle bulb, has been detailed by immunohistology and electron microscopy. The initial response was essentially found to be a wound reaction, in that hyperproliferative follicle epidermis quickly spread to below the level of amputation—associated with downward movement of mesenchymal (or dermal) sheath cells. Fibronectin was prominent in both dermis and epidermis at this stage and, as in wound repair, preceded laminin and type IV collagen in covering the lower dermal-epidermal junction. Once a new basal line of epidermis and a complete basement membrane were established, laminin and type IV collagen were detected below this junction and within the prospective papilla-forming mesenchyme. This coincided with ultrastructural observations of profuse sub-basement membrane extracellular material in the region of new papilla formation. The glassy membrane displayed extensive ultrastructural modifications at its lower level, and these corresponded with localized variations in staining intensities for all three antibodies over time. The membrane hung below the level of the epidermis, and was crossed by migrating cells from the mesenchymal dermal sheath of the follicle - it acted to segregate the inner group of follicular dermal cells from wound fibroblasts. Extracellular matrix may be a mediator of the dermal-epidermal interactions associated with this hair follicle regeneration phenomenon.

The DEBR rat: an animal model of human alopecia areata

The Dundee experimental bald rat (DEBR) is reported as a model for human alopecia areata. Parallels with human alopecia areata were observed in relation to the gradual and patchy loss of hair and the penetration of both pelage and vibrissa follicles by mononuclear cells. In particular, the apparent disruption of the follicles within the precortical region of the epidermal component and consequent alterations of normal geometrical relationships between dermal and epidermal components relate directly to similar studies on human alopecia areata. In comparison with other previously described hypotrichotic rodent mutants, the DEBR rat exhibits a unique mechanism of hair loss which may provide important information regarding the pathomechanism of human alopecia areata.

Smooth muscle alpha-actin is a marker for hair follicle dermis in vivo and in vitro

We have examined the expression of smooth muscle alpha-actin in hair follicles in situ, and in hair follicle dermal cells in culture by means of immunohistochemistry. Smooth muscle alpha-actin was present in the dermal sheath component of rat vibrissa, rat pelage and human follicles. Dermal papilla cells within all types of follicles did not express the antigen. However, in culture a large percentage of both hair dermal papilla and dermal sheath cells were stained by this antibody. The same cells were negative when tested with an antibody to desmin. Overall, explant-derived skin fibroblasts had relatively low numbers of positively marked cells, but those from skin regions of high hair-follicle density displayed more smooth muscle alpha-actin expression than fibroblasts from areas with fewer follicles. 2-D SDS-PAGE confirmed that, unlike fibroblasts, cultured papilla cells contained significant quantities of the alpha-actin isoform. The rapid switching on of smooth muscle alpha-actin expression by dermal papilla cells in early culture, contrasts with the behaviour of smooth muscle cells in vitro, and has implications for control of expression of the antigen in normal adult systems. The very high percentage of positively marked cultured papilla and sheath cells also provides a novel marker of cells from follicle dermis, and reinforces the idea that they represent a specialized cell population, contributing to the heterogeneity of fibroblast cell types in the skin dermis, and possibly acting as a source of myofibroblasts during wound healing.

Hair follicle stem cells? A distinct germinative epidermal cell population is activated in vitro by the presence of hair dermal papilla cells

Germinative epidermal cells in the lower end bulb region of anagen hair follicles are highly active, and give rise to hair fibres through rapid proliferation and complex differentiation. They have often been termed hair follicle stem cells, but owing to difficulties in isolation and identification their properties have previously only been clearly documented in vivo. We aimed to isolate and culture germinative cells in vitro, and used microdissection methods to dissect a small but identifiable group of cells from complete follicles. Transmission electron microscopy confirmed that the isolated cells were identical to germinative epidermal cells in situ. SDS-PAGE was used to show that they did not have the same protein composition as epidermis from their immediate proximity (overlying hair matrix), or from other follicular (outer root sheath) and interfollicular (skin basal) regions. Moreover, the germinative cells were found to display morphology and in vitro behaviour that distinguished them from comparative epidermal cells. When cultured in media and on substrata normally conducive to epidermal cell growth they remained in a quiescent state, and did not divide or differentiate. In contrast to other epidermal cells that formed typical pavement-like arrangements, germinative cells remained uniformly small, round and closely packed. However, when cultured in association with hair follicle dermal papilla cells they were radically stimulated into proliferative and aggregative behaviour. Furthermore, they were able to form organotypic-like structures, and exceptionally for skin-derived cell recombinations, a distinct basal lamina at the papilla-germinative cell junction. These results provide evidence that hair follicle germinative cells have intriguing properties that distinguish them from other follicular epidermis. The finding that they can be activated by dermal papilla cells reflects the intimate nature of the papilla-germinative cell relationship in situ, and should facilitate research into hair growth control mechanisms. The nature of germinative cells is discussed in the wider context of hair follicle stem-cell terminology.

Dermal cell populations show variable competence in epidermal cell support: stimulatory effects of hair papilla cells

A study was made of the comparative abilities of adult rat vibrissa dermal papilla cells, skin fibroblasts and 3T3 cells to support the initial attachment and subsequent growth and division of directly associated newborn rat skin basal epidermal cells. These associations were made under sub-optimal conditions; that is, in the absence of specific epidermal growth-promoting supplements, in order to assess more accurately the epidermal sustaining capacities of each dermal support. Analysis of epidermal cell counts and close photographic scrutiny revealed that low-passage dermal papilla cells, closely followed by transformed dermal papilla cells, were conducive to the successful attachment and subsequent proliferation of epidermal cell populations under three different experimental protocols. In contrast, skin fibroblasts did not support epidermal cell growth under any circumstances. These findings are particularly interesting in that they constitute a rare in vitro example of epidermal cells that are not only supported, but also encouraged to proliferate, by an actively dividing adult skin-derived dermal cell population.

Immunobiological studies on the alopecic (DEBR) rat

The Dundee experimental bald rat (DEBR) has been proposed as an animal model of human alopecia areata, which is suspected of being an autoimmune disease. This study was carried out to establish whether the immunological changes observed in the lesional DEBR rat correlated with studies of human alopecia areata. The immune infiltrate was characterized using immunoperoxidase techniques on cryostat sections of vibrissa follicles. Indirect immunofluorescence was used to quantify the peripheral blood leucocytes. Some parallels were observed in the infiltration of human and DEBR rat follicles by T lymphocytes. In contrast, pre-lesional DEBR rat follicles, which are not available for investigation in human alopecia areata, were not penetrated by leucocytes and MHC class II antigens were expressed in the precortical region of the epidermal component of these follicles. Quantification of peripheral blood leucocytes showed significant increases in both T-lymphocyte subsets during lesional expression. We consider that the pre-lesional form of the rat may provide important information as a model for the pre-lesional and uninvestigated form of alopecia areata in man.

Dermal-epidermal interactions

The dermal papilla is established as a permanent and stable population of specialized fibroblasts that first appear as a cellular aggregate that interacts with the epidermis to ensure follicle development. Our experimental findings strongly suggest that thereafter the papilla, in association with the confluent lower dermal sheath, continues to interact with the follicular epidermis with the papilla cells, which retain their aggregative property, undergoing cyclic changes in size and synthetic activities in phase with the hair cycle. In these activities, the lower follicle dermis appears to act as a functional unit that retains key embryonic characteristics throughout the lifetime of the follicle, re-enacting its inductive influence over follicular epidermis to regulate the profound morphogenetic changes that occur during successive hair cycles and to determine the physical characteristics of the fibers produced. While epidermal mitotic inhibitors have been suggested as a controlling mechanism in the hair cycle, we have argued that the papilla provides potent factors that stimulate epidermal proliferation in the hair germ to initiate, and then sustain, anagen and also follicle morphogenesis. Our recent findings with cocultures of dermal papilla and epidermal cells, which demonstrated that papilla cells enhance epidermal cell attachment and proliferative activity, reinforces this supposition. Thus, it may prove that the intrinsically determined aspect of the hair cycle reflects and is dependent on an intrapapillary cycle of events. Furthermore, we have suggested that at another level of interaction the dermal component of the follicle may mediate the influence of systemic factors, which are known to modify this innately programmed pattern of follicle behavior.(ABSTRACT TRUNCATED AT 250 WORDS)

Whisker growth induced by implantation of cultured vibrissa dermal papilla cells in the adult rat

Retention of the capacity to induce the growth of hair by cultured adult rat vibrissa dermal papilla cells has been investigated. Small pellets of serially cultured papilla cells were implanted into the bases of the exposed follicular epidermis of amputated adult rat vibrissa follicles. Amputated follicles that received no cell implants or implants of cultured dorsal skin fibroblasts were used as controls. Over 50% of follicles implanted with cultured papilla cells in the passage range 1-3 grew hairs. In contrast none of the follicles that received late passage cells (range 6-15) produced hairs; and spontaneous regeneration of hair occurred in only 3% of the control follicles. These results demonstrate that cultured papilla cells of early passage numbers retain their ability to induce hair growth. Histological examination confirmed that the implanted papilla cells interacted with follicular epidermis to organize the development of new, hair-producing bulbs, each containing a discrete dermal papilla. An important observation was that aggregative behaviour leading to papilla formation was only manifested by early passage papilla cell implants. This persisting embryonic characteristic appears to be an essential functional component of papilla cell activity which operates to regulate the profound morphogenetic changes that occur during the hair growth cycle.

Induction of hair growth by implantation of cultured dermal papilla cells

Mammalian hairs are formed by differentiation and keratinization of cells produced in the epidermal matrix (Figs 3, 4). Using the rodent vibrissa follicle as a model, transplantation studies have shown that the dermal papilla, a discrete population of specialized fibroblasts, is of prime importance in the growth of hair. Papillae induce hair growth when implanted into follicles and can interact with skin epidermis to form new hair follicles. When grown in culture, papilla cells display singular morphological and behavioural characteristics compared with connective tissue cells from other skin sources. We report here that serially cultured adult papilla cells can induce the growth of hair when implanted into follicles which otherwise would not grow hairs. This finding presents an opportunity to characterize properties distinguishing the papilla cell population from other skin fibroblasts, and, more specifically, those which control hair growth. The eventual application of this work to human hair replacement techniques can also be envisaged.

Changes in hair growth characteristics following the wounding of vibrissa follicles in the hooded rat

The effect on hair growth of wounding the lower region of whisker follicles, and in particular the dermal papilla, with sharply pointed tungsten needles was studied in adult hooded rats. Following injury hair growth ceased, but was subsequently resumed. While it might have been anticipated that follicle wounding would have a negative effect on whisker length, regular postoperative length measurements revealed that in follicles where cellular material was not displaced from the follicle by the original manipulation, 50% of the subsequent hairs produced were longer than their counterparts on the opposite side of the face, with 25% shorter and 25% with their length unchanged. In every case increased hair length was achieved by a prolongation of the growing period of the hair. Growth rate, when altered, was reduced. These results suggest that the factors which control the duration of the hair cycle and fibre growth rate are independent in vibrissa follicles. Since removal of most of the epidermal component by plucking of the hair just prior to injury produced equivalent hair length increases, this implicated the proximal dermal components as being mainly responsible for the observed changes.

Histological studies of the effects of wounding vibrissa follicles in the hooded rat

The effects of wounding the lower region of rat vibrissa follicles with a sharp tungsten needle were examined histologically, both shortly after injury and up to one year postoperatively. Following cell damage in the dermal papilla component hair growth ceased, and resumption of fibre production was always preceded by dermal papilla reformation. This papilla healing and regeneration was not associated with the production of scar tissue. In follicles undergoing no cell displacement during wounding (an effect associated with the growth of longer than normal hairs) dermal papillae were reformed from the residual papilla cell population, with recruitment of cells from surrounding mesenchyme. Follicles plucked just prior to wounding revealed little or no original epidermal matrix three days later, confirming that dermal components were primarily affected. Papilla cell counts performed on follicles which had consistently produced longer hairs gave no indication of increased papilla cell numbers. Follicles which underwent displacement of cellular material and displayed distortion of normal follicle morphology shortly after wounding (effects associated with the production of shorter than normal hairs) also revealed abnormalities at long-term biopsy. Moreover these follicles often had a history of altered fibre characteristics from one postoperative generation to the next. It is concluded that gross morphological disruption of the normal cellular relationships in the lower follicle results in a series of reorganizational difficulties with each recurring phase of the hair cycle.

Observations on the relationship between nerve supply and hair positioning in the rat vibrissa follicle

The present study has demonstrated an anatomical relationship between the position of the growing and non-growing hairs of the rat vibrissa follicle and the point where the follicular nerve supply penetrates the follicle capsule circumference. This finding may be significant in relation to the sensory function of the follicle, as physiological data have shown that the degree of stimulation of brain cells following mechanical displacement of the whisker is direction dependent. The observed relationship between the nerve entry point and the hairs, therefore, may reflect an uneven distribution of nerve endings around the upper follicle circumference. In relation to hair growth, the above observation requires that the non-growing club hair must consistently move up one side of the follicle at the end of each growth cycle. A possible clue as to the mechanism by which club hair movement is directed can be derived from observation of asymmetry, both at the base of the club hair, and in the epidermal matrix near the end of the growth cycle.

Vibrissa dermal papilla cell aggregative behaviour in vivo and in vitro

Parallel cultures of adult rat vibrissa dermal papilla cells and skin fibroblasts revealed differences between the two cell types with respect to a number of criteria. In particular the dermal papilla cells demonstrated a distinctive single cell morphology, and at confluence formed cell aggregates radically different from regular fibroblast multilayering and patterning. This finding confirmed repeated observations of papilla cell clumping in short-term culture. The dermal papilla cells which are mitotically quiescent in situ were also shown to have a lower proliferative capacity than the skin fibroblasts. The affinity shown by papilla cells towards each other in culture reflected the behaviour demonstrated by isolated dermal papillae transplanted into ear dermis and into the collagenous capsule of the vibrissa follicle. In the absence of epidermal contact the papilla cells remained as recognizable rounded aggregates for the experimental period of up to nine months. Synthesis of extracellular material typical of that seen in situ was observed, particularly during the first weeks following transplantation. The collective behaviour of the dermal papilla cells revealed in this study may be significant for the morphogenetic activity of the papilla, and for papilla size during the hair cycle. It may also reflect the retention of embryonic-like properties by the dermal component of adult hair follicles.