Changes in gap junction distribution and connexin expression pattern during human fetal skin development

Cell adhesion and junctional proteins in the developing skin of snakes indicate they coordinate the differentiation of the epidermis

The development of scales and the sequence of epidermal layers during snake embryogenesis has been studied by immunofluorescence for the localization of cell adhesion, adherens, and communicating cell junctional proteins. At about 2nd/3rd of embryonic development in snakes the epidermis forms symmetric bumps at the beginning of scale formation, and they rapidly become asymmetric and elongate forming outer and inner surfaces of the very overlapped scales seen at hatching. The dermis separates a superficial loose from a deeper dense part; the latter is joined to segmental muscles and nerves, likely acting on scale orientation during snake movements. N-cam is present in the differentiating epidermis and mesenchyme of forming scales while L-cam is only/mainly detected in the periderm and epidermis. Mesenchymal N-cam is associated with the epidermis of the elongating dorsal scale surface and with the beta-differentiation that occurs in the overlapping outer surface of scales. Beta-catenin and Connexin-43 show a similar distribution, and they are mainly present in the periderm and differentiating suprabasal keratinocytes likely forming an intense connectivity during epidermal differentiation. Beta-catenin also shows nuclear localization in differentiating cells of the shedding and beta-layers at late stages of scale morphogenesis, before hatching. The study suggests that intensification of adhesion and gap junctions allows synchronization of the differentiation of suprabasal cells to produce the ordered sequence of epidermal layers of snake scales, starting from the shedding complex and the beta-layer.

Germline soma communication mediated by gap junction proteins regulates epithelial morphogenesis

Gap junction (GJ) proteins, the primary constituents of GJ channels, are conserved determinants of patterning. Canonically, a GJ channel, made up of two hemi-channels contributed by the neighboring cells, facilitates transport of metabolites/ions. Here we demonstrate the involvement of GJ proteins during cuboidal to squamous epithelial transition displayed by the anterior follicle cells (AFCs) from Drosophila ovaries. Somatically derived AFCs stretch and flatten when the adjacent germline cells start increasing in size. GJ proteins, Innexin2 (Inx2) and Innexin4 (Inx4), functioning in the AFCs and germline respectively, promote the shape transformation by modulating calcium levels in the AFCs. Our observations suggest that alterations in calcium flux potentiate STAT activity to influence actomyosin-based cytoskeleton, possibly resulting in disassembly of adherens junctions. Our data have uncovered sequential molecular events underlying the cuboidal to squamous shape transition and offer unique insight into how GJ proteins expressed in the neighboring cells contribute to morphogenetic processes.

Shape transitions between different subtypes of epithelial cells i.e., cuboidal, squamous and columnar are ubiquitous and are essential during organogenesis across animal kingdom. We demonstrate that heteromeric combination of gap junction proteins, Drosophila Innexin2 and Drosophila Innexin 4 (also known as Zero population growth or Zpg), expressed in the soma and germline of fly egg respectively, mediates the shape transition of cuboidal follicle cells to squamous fate. Interestingly, the two gap junction proteins likely participate as constituents of a calcium channel. Further, we show that somatic follicle cells and germline nurse cells communicate through calcium fluxes that activates STAT in the follicle cells. Activated STAT modulates the levels/ activity of junctional complexes thus aiding shape transition of cuboidal cells to squamous fate. These findings provide novel insights into how communication between different cell types with distinct origins achieve shape transitions essential for proper organ assemblies.

Skin organoids: A new human model for developmental and translational research

Culturing skin cells outside of the body has been a cornerstone of dermatological investigation for many years; however, human skin equivalent systems typically lack the full complexity of native skin. Notably, skin appendages, such as hair follicles and sweat glands, remain a challenge to generate or maintain in cell cultures and reconstruct in damaged skin. Recent work from our lab has demonstrated methods for generating appendage-bearing skin tissue-known as skin organoids-from pluripotent stem cells. Here, we will summarize this work and other related works, and then discuss the potential future applications of skin organoids in dermatological research.

IL-22 Down-Regulates Cx43 Expression and Decreases Gap Junctional Intercellular Communication by Activating the JNK Pathway in Psoriasis

The roles of IL-22 in the pathomechanisms of psoriasis have been well demonstrated. Gap junctional intercellular communication (GJIC) is widely known for its involvement in multiple biological and pathological processes such as growth-related events, cell differentiation, and inflammation. Here, we show that IL-22 significantly decreased GJIC and down-regulated Cx43 expression in HaCaT cells. Cx43 overexpression markedly inhibited the proliferation of and increased GJIC in HaCaT cells, but the silencing of Cx43 exerted the opposite effects. Additionally, Cx43 overexpression effectively rescued the IL-22-induced decrease in GJIC in HaCaT cells. The IL-22-induced down-regulation of Cx43 expression and decrease in GJIC can be significantly blocked by the JNK inhibitor SP600125 and by the overexpression of IL-22RA2 (which specifically binds to IL-22 and inhibits its activity), but not by the NF-κB inhibitor BAY11-7082, in HaCaT cells. Furthermore, the IL-22-induced down-regulation of Cx43 expression mediated by the JNK signaling pathway was confirmed in a mouse model of IL-22-induced psoriasis-like dermatitis. Similarly, Cx43 expression was significantly lower in the lesional skin than in the nonlesional skin of patients with psoriasis. These results suggest that IL-22 decreases GJIC by activating the JNK signaling pathway, which down-regulates Cx43 expression; this process is a possible pathomechanism of keratinocyte hyperproliferation in psoriasis.

Connexin 43 regulates the expression of wound healing-related genes in human gingival and skin fibroblasts

Fibroblasts are the most abundant connective tissue cells and play an important role in wound healing. It is possible that faster and scarless wound healing in oral mucosal gingiva relative to skin may relate to the distinct phenotype of the fibroblasts residing in these tissues. Connexin 43 (Cx43) is the most ubiquitous Cx in skin (SFBLs) and gingival fibroblasts (GFBLs), and assembles into hemichannels (HCs) and gap junctions (GJs) on the cell membrane. We hypothesized that SFBLs and GFBLs display distinct expression or function of Cx43, and that this may partly underlie the different wound healing outcomes in skin and gingiva. Here we show that Cx43 distinctly formed Cx43 GJs and HCs in human skin and gingiva in vivo. However, in SFBLs, in contrast to GFBLs, only a small proportion of total Cx43 assembled into HC plaques. Using an in vivo-like 3D culture model, we further show that the GJ, HC, and channel-independent functions of Cx43 distinctly regulated wound healing-related gene expression in GFBLs and SFBLs. Therefore, the distinct wound healing outcomes in skin and gingiva may partly relate to the inherently different assembly and function of Cx43 in the resident fibroblasts.

Connexin 43: Key roles in the skin

Gap junctions are tightly packed intercellular channels that serve a common purpose of allowing the intercellular exchange of small metabolites, second messengers and electrical signals. Connexins (Cxs) are gap junction proteins. Currently, 20 and 21 members of Cxs have been characterized in mice and humans, respectively. Connexin 43 (Cx43) is the most ubiquitously expressed type of Cx in the skin. It is produced by various different types of skin cell, such as keratinocytes, fibroblasts, endothelial and basal cells, melanocytes and dermal papilla cells. At present, more evidence indicates that Cx43 has an important role in skin repair and skin tumor development, as well as in skin cell invasion and metastasis. In this review, current knowledge regarding the regulation and function of Cx43 is summarized and the therapeutic potential of regulating Cx43 activity is discussed.

Pediatric dermatohistopathology--histopathology of skin diseases in newborns and infants

While neonatal skin physiology has been thoroughly examined using non-invasive techniques in recent years, only few systematic studies and review articles addressing the histopathology of neonatal skin have been published thus far. In most cases, histopathological findings of dermatoses in neonatal skin do not significantly differ from those seen in adult skin. Nevertheless, a comprehensive knowledge of embryonic and fetal skin development as well as the microanatomical structure of neonatal skin can contribute to a better understanding of various dermatoses of infancy. In the first part of this review article, we present the histopathological features of such skin diseases, which, though generally rare, almost exclusively appear during the first weeks of life due to distinctive structural and functional features of neonatal skin. The second part is dedicated to classic dermatoses of infancy and their histopathological features.

Immunohistochemical demonstration of connexins in the developing feather follicle of the chicken

Based on immunohistochemistry, the study demonstrates the varying distribution and reaction intensity of connexins (Cx26 [chicken 31sim], 30 [chicken 31], 31, 32, 43, 45) in the developing feather follicle of the chicken (White Leghorn). The different embryonal stages were identified according to the normal table of Hamburger and Hamilton (1951). The development of the feather follicle complex is closely related to skin layer development, making use of the controlling function of connexins. This was evident during feather follicle differentiation, based on communication between ectomesodermal (fibroblasts) and ectodermal cells (developing epidermis), but also by the subsequent separation of the two cell line types related to their connexin-dependent differentiation degree. With the increase in mesenchymal cell numbers during feather placode development, the multiple connexins Cx26 [chicken 31sim] and 43, supported by Cx30 [chicken 31], 31 and 32, were increasingly activating the fibroblast concentrations as related to epidermal follicle buds, the specific follicle structure, the endothelial cells of capillaries and larger blood vessels, as well as the collagen fiber production and the growing feather musculature shortly before hatching; Cx45 could not be demonstrated. In conclusion, it seems that connexin expression is not only coupled to the origin of embryonic cells, but also connected with tissue formation before the follicle system can be formed.

Immunohistochemical and biochemical studies with region-specific antibodies to chromogranins A and B and secretogranins II and III in neuroendocrine tumors

This short review deals with our investigations in neuroendocrine tumors (NETs) with antibodies against defined epitopes of chromogranins (Cgs) A and B and secretogranins (Sgs) II and III. The immunohistochemical expression of different epitopes of the granin family of proteins varies in NE cells in normal human endocrine and non-endocrine organs and in NETs, suggesting post-translational processing. In most NETs one or more epitopes of the granins were lacking, but variations in the expression pattern occurred both in benign and malignant NETs. A few epitopes displayed patterns that may be valuable in differentiating between benign and malignant NET types, e.g., well-differentiated NET types expressed more CgA epitopes than the poorly differentiated ones and C-terminal secretoneurin visualized a cell type related to malignancy in pheochromocytomas. Plasma concentrations of different epitopes of CgA and CgB varied. In patients suffering from carcinoid tumors or endocrine pancreatic tumors the highest concentrations were found with epitopes from the mid-portion of CgA. For CgB the highest plasma concentrations were recorded for the epitope 439-451. Measurements of SgII showed that patients with endocrine pancreatic tumors had higher concentrations than patients with carcinoid tumors or pheochromocytomas. SgIII was not detectable in patients with NETs.

Gap and tight junctions in the formation of feather branches: A descriptive ultrastructural study

The present study has focused on the distribution and ultrastructure of gap and tight junctions responsible for the formation of the barb/barbule branching in developing feathers using immunocytochemical detection. Apart from desmosomes, both tight and gap junctions are present between differentiating barb/barbule cells and during keratinization. While gap junctions are rare along the perimeter of these cells, tight junctions tend to remain localized in nodes joining barbule cells and between barb cells of the ramus. Occludin and connexin-26 but not connexin-43 have been detected between barb medullary, barb cortical and barbule cells during formation of barbs. Gap junctions are present in supportive cells located in the vicinity of barbule cells and destined to degenerate, but no close junctions are present between supportive and barb/barbule cells. Close junctions mature into penta-laminar junctions that are present between mature barb/barbule cells. Immunolabeling for occludin and Cx26 is rare along these cornified junctions. The junctions allow barb/barbule cells to remain connected until feather-keratin form the mature corneous syncytium that constitutes the barbs. A discussion of the role of gap and tight junctions during feather morphogenesis is presented.

Desmosomes in developing human epidermis

Desmosomes play important roles in the cell differentiation and morphogenesis of tissues. Studies on animal models have greatly increased our knowledge on epidermal development while reports on human developing skin are rare due to the difficult accessibility to the samples. Although the morphology of periderm cells and the process how the epidermis develops very much resemble each other, the timetable and the final outcome of a mature human epidermis markedly differ from those of murine skin. Even the genetic basis of the junctional components may have profound differences between the species, which might affect the implementation of the data from animal models in human studies. The aim of this review is to focus on the development of human skin with special emphasis on desmosomes. Desmosomal development is mirrored in perspective with other simultaneous events, such as maturation of adherens, tight and gap junctions, and the basement membrane zone.

Gap junctions in inherited human disease

Gap junctions (GJ) provide direct intercellular communication. The structures underlying these cell junctions are membrane-associated channels composed of six integral membrane connexin (Cx) proteins, which can form communicating channels connecting the cytoplasms of adjacent cells. This provides coupled cells with a direct pathway for sharing ions, nutrients, or small metabolites to establish electrical coupling or balancing metabolites in various tissues. Genetic approaches have uncovered a still growing number of mutations in Cxs related to human diseases including deafness, skin disease, peripheral and central neuropathies, cataracts, or cardiovascular dysfunctions. The discovery of a growing number of inherited human disorders provides an unequivocal demonstration that gap junctional communication is crucial for diverse physiological processes.

A versatile murine 3D organotypic model to evaluate aspects of wound healing and epidermal organization

Three-dimensional (3D) organotypic models are increasingly being used to study aspects of epidermal organisation and cutaneous wound-healing events. These are largely dependent on laborious histological analysis and immunohistochemical approaches. Here we outline a method for establishment of a versatile in vitro 3D organotypic skin equivalent that reflects murine epidermal organisation in vivo. The system is optically transparent and ideally suited to real-time analysis and integrated in situ imaging techniques. Moreover, the model permits the visualisation of epidermal regeneration following injury in real time, thereby facilitating avenues to explore distinctive modes of wound re-epithelialisation. The versatility of the model could help unravel molecular mechanisms underlying epidermal morphogenesis, assess novel therapeutic strategies and reduce animal experimentation in a non-invasive manner.

Connexin43 phosphorylation: structural changes and biological effects

Vertebrate gap junctions, composed of proteins from the connexin gene family, play critical roles in embryonic development, co-ordinated contraction of excitable cells, tissue homoeostasis, normal cell growth and differentiation. Phosphorylation of connexin43, the most abundant and ubiquitously expressed connexin, has been implicated in the regulation of gap junctional communication at several stages of the connexin 'life cycle', including hemichannel oligomerization, export of the protein to the plasma membrane, hemichannel activity, gap junction assembly, gap junction channel gating and connexin degradation. Consistent with a short (1-5 h) protein half-life, connexin43 phosphorylation is dynamic and changes in response to activation of many different kinases. The present review assesses our current understanding of the effects of phosphorylation on connexin43 structure and function that in turn regulate gap junction biology, with an emphasis on events occurring in heart and skin.

A murine living skin equivalent amenable to live-cell imaging: analysis of the roles of connexins in the epidermis

Three-dimensional (3D) organotypic models are increasingly used to study the aspects of epidermal organisation and cutaneous wound-healing events. However, these are largely dependent on laborious histological analysis and immunohistochemical approaches. Despite the large resource of transgenic and knockout mice harboring mutations relevant to skin disorders, few organotypic mouse skin models are available. We have developed a versatile in vitro 3D organotypic mouse skin equivalent that reflects epidermal organisation in vivo. The system is optically transparent and ideally suited to real-time analysis using a variety of integrated in situ imaging techniques. As a paradigm for coordination of cellular events, the epidermal gap junction network was investigated and the model displayed predominant connexin 43 (Cx43) expression in basal proliferating cells and Cx26 and Cx30 expression in differentiated keratinocytes. We show that attenuation of Cx43-mediated communication by a Cx mimetic peptide enhanced wound closure rates in keratinocyte monocultures and in the living skin equivalent system, emphasising the utility of the model to systematically unravel the molecular mechanisms underlying epidermal morphogenesis, assess promising therapeutic strategies, and reduce animal experimentation. Furthermore, we visualise epidermal regeneration following injury in real time, thereby facilitating avenues to explore distinctive modes of wound re-epithelialisation in a non-invasive manner.

Gap junction protein expression and cellularity: comparison of immature and adult equine digital tendons

Injury to the energy-storing superficial digital flexor tendon is common in equine athletes and is age-related. Tenocytes in the superficial digital flexor tendon of adult horses appear to have limited ability to respond adaptively to exercise or prevent the accumulation of strain-induced microdamage. It has been suggested that conditioning exercise should be introduced during the growth period, when tenocytes may be more responsive to increased quantities or intensities of mechanical strain. Tenocytes are linked into networks by gap junctions that allow coordination of synthetic activity and facilitate strain-induced collagen synthesis. We hypothesised that there are reductions in cellular expression of the gap junction proteins connexin (Cx) 43 and 32 during maturation and ageing of the superficial digital flexor tendon that do not occur in the non-injury-prone common digital extensor tendon. Cryosections from the superficial digital flexor tendon and common digital extensor tendon of 5 fetuses, 5 foals (1-6 months), 5 young adults (2-7 years) and 5 old horses (18-33 years) were immunofluorescently labelled and quantitative confocal laser microscopy was performed. Expression of Cx43 and Cx32 protein per tenocyte was significantly higher in the fetal group compared with all other age groups in both tendons. The density of tenocytes was found to be highest in immature tissue. Higher levels of cellularity and connexin protein expression in immature tendons are likely to relate to requirements for tissue remodelling and growth. However, if further studies demonstrate that this correlates with greater gap junctional communication efficiency and synthetic responsiveness to mechanical strain in immature compared with adult tendons, it could support the concept of early introduction of controlled exercise as a means of increasing resistance to later injury.

Differentiation of Organotypic Epidermis in the Presence of Skin Disease-Linked Dominant-Negative Cx26 Mutants and Knockdown Cx26

In this study, we chose a differentiation-competent rat epidermal keratinocyte (REK) cell line to examine the role of Cx26 and disease-linked Cx26 mutants in organotypic epidermal differentiation. First, we generated stable REK cell lines expressing three skin disease-linked mutants (G59A, D66H and R75W). Second, we used an RNAi approach to knock down the expression of Cx26 in REKs. Interestingly, the three-dimensional (3D) architecture of the organotypic epidermis altered the intracellular spatial distribution of the mutants in comparison to 2D cultured REKs, highlighting the importance of using organotypic cultures. Unexpectedly, the presence of disease-linked mutants or the overexpression of wild-type Cx26 had little effect on the differentiation of the organotypic epidermis as determined by the architecture of the epidermis, expression of molecular markers indicative of epidermis differentiation (keratin 10, keratin 14, involucrin, loricrin) and stratification/cornification of the epidermis. Likewise, organotypic epidermis continued to differentiate normally upon Cx26 knockdown. While Cx26 has been reported to be upregulated during wound healing, no reduction in wound closure was observed in 2D REK cultures that expressed loss-of-function, dominant Cx26 mutants. In conclusion, we demonstrate that gain or loss of Cx26 function does not disrupt organotypic epidermal differentiation and offer insights into why patients harboring Cx26 mutations do not frequently present with more severe disease that encompasses thin skin.

Rat epidermal keratinocytes as an organotypic model for examining the role of Cx43 and Cx26 in skin differentiation

In order to characterize connexin expression and regulation in the epidermis, we have characterized a rat epidermal keratinocyte (REK) cell line that is phenotypically similar to basal keratinocytes in that they have the ability to differentiate into organotypic epidermis consisting of a basal cell layer, 2-3 suprabasal cell layers, and a cornified layer. RT-PCR revealed that REK cells express mRNA for Cx26, Cx31, Cx31.1, Cx37, and Cx43, which mimics the reported connexin profile for rat tissue. In addition, we report the expression of Cx30, Cx30.3, Cx40, and Cx45 in rat keratinocytes, highlighting the complexity of the connexin complement in rat epidermis. Furthermore, 3-dimensional analysis of organotypic skin revealed that Cx26 and Cx43 are exquisitely regulated during the differentiation process. The life-cycle of these connexins including their expression, transport, assembly into gap junctions, internalization, and degradation are elegantly depicted in organotypic epidermis as keratinocytes proceed from differentiation to programmed cell death.

Species specificity of mammalian connexin-26 to form open voltage-gated hemichannels

Mutations of connexin-26 (Cx26) cause nonsyndromic hearing loss and other syndromes affecting ectoderm-derived tissues. While the exact mechanisms underlying these diseases remain elusive, Cx's are generally considered to mediate cell-to-cell communication by forming gap junction channels. We show here that unlike rat Cx26, human and sheep Cx26 form voltage-gated hemichannels when expressed in oocytes and Neuro2A cells. A single evolutionary amino acidic change at position 159 of the rodent protein, the replacement of aspartic acid with asparagine in the human and sheep proteins, accounts for this species specificity. At the resting potential and in normal millimolar extracellular calcium, open human Cx26 hemichannels can be detected both electrophysiologically and by dye uptake, although they did not affect cell viability. These hemichannels opened at approximately -50 mV and their activation increased by depolarization until they inactivate at positive membrane potentials. Single-channel analysis revealed that activation and inactivation involved two distinct voltage gating mechanisms and that the fully open hemichannel displays a conductance twice that of the intercellular channel. The existence of a hemichannel that opens under physiological control of the membrane potential may have important implications for the normal and pathological activity of Cx26 in humans, particularly with respect to hearing and the epidermis.

Temporal regulation of connexin phosphorylation in embryonic and adult tissues

Gap junctions, composed of proteins from the connexin family, allow for intercellular communication between cells in tissues and are important in development, tissue/cellular homeostasis, and carcinogenesis. Genome databases indicate that there are at least 20 connexins in the mouse and human. Connexin phosphorylation has been implicated in connexin assembly into gap junctions, gap junction turnover, and cell signaling events that occur in response to tumor promoters and oncogenes. Connexin43 (Cx43), the most widely expressed and abundant gap junction protein, can be phosphorylated at several different serine and tyrosine residues. Here, we focus on the dynamic regulation of Cx43 phosphorylation in tissue and how these regulatory events are affected during development, wound healing, and carcinogenesis. The activation of several kinases, including protein kinase A, protein kinase C, p34cdc2/cyclin B kinase, casein kinase 1, mitogen-activated protein kinase, and pp60src kinase, can lead to the phosphorylation of different residues in the C-terminal region of Cx43. The use of antibodies specific for phosphorylation at defined residues has allowed the examination of specific phosphorylation events both in tissue culture and in vivo. These new antibody tools and those under development will allow us to correlate specific phosphorylation events with changes in connexin function.

Immunohistochemical demonstration of connexin-related epithelization defects in a lamb caused by (erythro)keratodermia variabilis

The distribution of four important connexins (Cx 26, 30, 31, 43) in the skin of a neonatal lamb with (erythro)keratodermia (EKV) was demonstrated using immunohistochemistry, including a very sensitive visualization system. The reaction staining for Cx 26, 30 and 31 was negative to weak in all skin structures, and only Cx 43 displayed weak to moderate positive reactions, although not uniformly distributed throughout the important systems (e.g. epidermis, hair follicle sheaths). Thus, the study proves for the first time that connexin defects are associated with the development of EKV in domesticated mammals, as they are in humans.

Directing stem cells into the keratinocyte lineage in vitro

A major area of research in regenerative medicine is the potential application of stem cells in skin grafting and tissue engineering. This would require well defined and efficient protocols for directing the commitment and differentiation of stem cells into the keratinocyte lineage, together with their selective purification and proliferation in vitro. The development of such protocols would reduce the likelihood of spontaneous differentiation of stem cells into divergent lineages upon transplantation, as well as reduce the risk of teratoma formation in the case of embryonic stem cells. Additionally, such protocols could provide useful in vitro models for studying skin tissue biology, as well as facilitate the genetic manipulation of stem cells for therapeutic applications. The development of pharmacokinetic and cytotoxicity/genotoxicity screening tests for skin-related biomaterials and drugs could also utilize protocols developed for the commitment and differentiation of stem cells into the keratinocyte lineage. Hence, this review critically examines the various strategies that could be employed to direct the commitment and differentiation of stem cells into the keratinocyte lineage in vitro.

Gap junction development in the human fetal hair follicle and bulge region

BACKGROUND

Gap junctions, composed of connexin (Cx) subunits, are channels that allow intercellular communication between adjacent cells and are thought to play a key role in the regulation of cell proliferation and differentiation. The Cx expression pattern and formation of gap junctions in human fetal hair follicles has yet to be clarified, including the prominent follicular bulge region that is believed to be a site rich in stem cells.

OBJECTIVES

To study the expression of two major Cxs, Cx26 and Cx43, in developing hair follicles in skin samples from a series of human fetuses of estimated gestational age (EGA) 88-163 days, and to determine quantitatively the presence of gap junctions.

METHODS

We used immunofluorescence labelling to investigate the sequential expression pattern of Cx26 and Cx43 in developing human hair follicles. Gap junction formation was observed by electron microscopy and the numbers of gap junctions were analysed quantitatively. Results Both Cx26 and Cx43 expression were observed at 88 days' EGA in the inner part of the hair peg. At 135 days' EGA, Cx26 was expressed in the outer root sheath (ORS) and the inner root sheath (IRS), while Cx43 was expressed chiefly in the IRS, hair matrix and sebaceous glands. At 163 days' EGA, Cx26 expression was most intense in the outermost layer of the ORS, in contrast to Cx43 expression which was in the inner part of the ORS. In the bulge region, only Cx43 was expressed in a subset of cells in the bulge. Ultrastructurally, gap junctions were observed at 102 days' EGA in the hair peg, and the number of gap junctions increased as the hair follicle matured. Gap junctions were also observed between the bulge cells in considerable numbers.

CONCLUSIONS

The changing expression patterns of Cx26 and Cx43 and the increasing gap junction numbers suggest a close association of Cx expression and gap junction formation with hair follicle morphogenesis. In addition, the present ultrastructural observations demonstrate that considerable numbers of the bulge cells, a putative site rich in hair follicle stem cells, form gap junctions during human hair follicle development.