Adjacent Keratinocytes Research Articles

Breaching the Barrier: Investigating Initial Herpes Simplex Viral Infection and Spread in Human Skin and Mucosa.

Herpes simplex virus (HSV) is sexually transmitted via the anogenital mucosa where it initially infects epidermal keratinocytes and mononuclear phagocytes (MNPs). It then spreads to the dorsal root ganglion via sensory nerve endings, to remain latent for life with periodic reactivation. Currently, there is no cure or vaccine. Initial or recurrent HSV infection can produce serious complications and mediate acquisition of HIV. This review outlines the initial events after the HSV infection of human anogenital mucosa to determine the optimal window to target the virus before it becomes latent. After infection, HSV spreads rapidly within the mid-layers of epidermal keratinocytes in the explanted human inner foreskin. Infected cells produce chemokines, which modulate nectin-1 distribution on the surface of adjacent keratinocytes, facilitating viral spread. Epidermal Langerhans cells and dendritic cells become infected with HSV followed by a "viral relay" to dermal MNPs, which then present viral antigen to T cells in the dermis or lymph nodes. These data indicate the need for interruption of spread within 24 h by diffusible vaccine-induced mediators such as antiviral cytokines from resident immune cells or antibodies. Intradermal/mucosal vaccines would need to target the relevant dermal MNPs to induce HSV-specific CD4+ and CD8+ T cells.

Calcium dynamics of skin-resident macrophages during homeostasis and tissue injury.

Immune cells depend on rapid changes in intracellular calcium activity to modulate cell function. Skin contains diverse immune cell types and is critically dependent on calcium signaling for homeostasis and repair, yet the dynamics and functions of calcium in skin immune cells remain poorly understood. Here, we characterize calcium activity in Langerhans cells, skin-resident macrophages responsible for surveillance and clearance of cellular debris after tissue damage. Langerhans cells reside in the epidermis and extend dynamic dendrites in close proximity to adjacent keratinocytes and somatosensory peripheral axons. We find that homeostatic Langerhans cells exhibit spontaneous and transient changes in calcium activity, with calcium flux occurring primarily in the cell body and rarely in the dendrites. Triggering somatosensory axon degeneration increases the frequency of calcium activity in Langerhans cell dendrites. By contrast, we show that Langerhans cells exhibit a sustained increase in intracellular calcium following engulfment of damaged keratinocytes. Altering intracellular calcium activity leads to a decrease in engulfment efficiency of keratinocyte debris. Our findings demonstrate that Langerhans cells exhibit context-specific changes in calcium activity and highlight the utility of skin as an accessible model for imaging calcium dynamics in tissue-resident macrophages. [Media: see text] [Media: see text] [Media: see text] [Media: see text].

Connexins in epidermal health and diseases: insights into their mutations, implications, and therapeutic solutions.

The epidermis, the outermost layer of the skin, serves as a protective barrier against external factors. Epidermal differentiation, a tightly regulated process essential for epidermal homeostasis, epidermal barrier formation and skin integrity maintenance, is orchestrated by several players, including signaling molecules, calcium gradient and junctional complexes such as gap junctions (GJs). GJ proteins, known as connexins facilitate cell-to-cell communication between adjacent keratinocytes. Connexins can function as either hemichannels or GJs, depending on their interaction with other connexons from neighboring keratinocytes. These channels enable the transport of metabolites, cAMP, microRNAs, and ions, including Ca2+, across cell membranes. At least ten distinct connexins are expressed within the epidermis and mutations in at least five of them has been linked to various skin disorders. Connexin mutations may cause aberrant channel activity by altering their synthesis, their gating properties, their intracellular trafficking, and the assembly of hemichannels and GJ channels. In addition to mutations, connexin expression is dysregulated in other skin conditions including psoriasis, chronic wound and skin cancers, indicating the crucial role of connexins in skin homeostasis. Current treatment options for conditions with mutant or altered connexins are limited and primarily focus on symptom management. Several therapeutics, including non-peptide chemicals, antibodies, mimetic peptides and allele-specific small interfering RNAs are promising in treating connexin-related skin disorders. Since connexins play crucial roles in maintaining epidermal homeostasis as shown with linkage to a range of skin disorders and cancer, further investigations are warranted to decipher the molecular and cellular alterations within cells due to mutations or altered expression, leading to abnormal proliferation and differentiation. This would also help characterize the roles of each isoform in skin homeostasis, in addition to the development of innovative therapeutic interventions. This review highlights the critical functions of connexins in the epidermis and the association between connexins and skin disorders, and discusses potential therapeutic options.

A Standardized and Reproducible Workflow for Membrane Glass Slides in Routine Histology and Spatial Proteomics

Defining the molecular phenotype of single cells in situ is key for understanding tissue architecture in health and disease. Advanced imaging platforms have recently been joined by spatial omics technologies, promising unparalleled insights into the molecular landscape of biological samples. Furthermore, high-precision laser microdissection (LMD) of tissue on membrane glass slides is a powerful method for spatial omics technologies and single-cell type spatial proteomics in particular. However, current histology protocols have not been compatible with glass membrane slides and LMD for automated staining platforms and routine histology procedures. This has prevented the combination of advanced staining procedures with LMD. In this study, we describe a novel method for handling glass membrane slides that enables automated eight-color multiplexed immunofluorescence staining and high-quality imaging followed by precise laser-guided extraction of single cells. The key advance is the glycerol-based modification of heat-induced epitope retrieval protocols, termed “G-HIER.” We find that this altered antigen-retrieval solution prevents membrane distortion. Importantly, G-HIER is fully compatible with current antigen retrieval workflows and mass spectrometry–based proteomics and does not affect proteome depth or quality. To demonstrate the versatility of G-HIER for spatial proteomics, we apply the recently introduced deep visual proteomics technology to perform single-cell type analysis of adjacent suprabasal and basal keratinocytes of human skin. G-HIER overcomes previous incompatibility of standard and advanced staining protocols with membrane glass slides and enables robust integration with routine histology procedures, high-throughput multiplexed imaging, and sophisticated downstream spatial omics technologies.

Silk Fibroin-Based Hydrogel Microneedles Deliver α-MSH to Promote Melanosome Delivery for Vitiligo Treatment.

Microneedles have shown great advantages in subcutaneous drug delivery and skin disease treatment. Vitiligo is a difficult-to-cure skin disease characterized by the depigmentation of the epidermis. Melanosomes produced in melanocytes are transported through dendrites to adjacent keratinocytes, where they accumulate, resulting in skin pigmentation. However, melanocytes in vitiligo patients are functionally disrupted. Silk fibroin (SF) methacrylate hydrogel microneedle can deliver α-MSH to the epidermis directly, where α-MSH helps the protection of melanocytes, extension of melanocytic dendrites, and transfer of melanosomes. In addition, the expression of melanogenesis-related melanocyte-inducing transcription factor and tyrosinase-related protein 1 (TRP1) was up-regulated, and the number of hair follicle stem cells increased with good proliferative activity. This slow release α-MSH SF-based hydrogel microneedles provides a new idea for the treatment of vitiligo.

Clinical and Biological Characteristics of a Griscelli Syndrome Type 2: About a Case Report

Introduction: Griscelli Syndrome (GS) is a rare autosomal recessive genetic disorder, characterized by a partial albinism that can be associated with early-onset severe psychomotor retardation, and normal immune status (type 1), or associated with an immune deficiency (type 2). GS type 2 results from mutations in the RAB27A gene, leading to immune deficiency. GS type 2 has a grave prognosis because of a rapidly fatal macrophage activated syndrome. Case Report: A 3-month-old infant having a family history of first-degree consanguinity in the parents and history of silvery gray hair in two uncles, Admitted to our department for management of a prolonged fever and an anemic syndrome. Hair on the scalp, eyebrows, and eyelashes were silvery gray and had a metallic sheen. The lymphocyte sub typing showed a severe combined immune deficiency. Histological examination of the hair showed enlarged, hyper pigmented basal melanocytes with sparse pigmentation of adjacent keratinocytes in favor of a Griscelli syndrome. The evolution was marked by clinical worsening, respiratory distress. On the biological level, a macrophage activation syndrome was retained in front of a hyperferritinemia, a hypertriglyceridemia and hepatic cytolysis. The patient received broad-spectrum antibiotic therapy, antivirals and antifungals, corticosteroid boluses 3 days in a row and an immunoglobulin infusion. Unfortunately, the patient died of septic shock. Conclusion: Type 2 GS is a rare genetic disease with a poor prognosis, which must be diagnosed early to allow better support. The only curative treatment is bone marrow transplantation.

Melanoma stimulates the proteolytic activity of HaCaT keratinocytes

BackgroundKeratinocytes constitute a major part of the melanoma microenvironment, considering their protective role towards melanocytes in physiological conditions. However, their interactions with tumor cells following melanomagenesis are still unclear.MethodsWe used two in vitro models (melanoma-conditioned media and indirect co-culture of keratinocytes with melanoma cells on Transwell inserts) to activate immortalized keratinocytes towards cancer-associated ones. Western Blotting and qPCR were used to evaluate keratinocyte markers and mediators of cell invasiveness on protein and mRNA expression level respectively. The levels and activity of proteases and cytokines were analysed using gelatin-FITC staining, gelatin zymography, chemiluminescent enzymatic test, as well as protein arrays. Finally, to further study the functional changes influenced by melanoma we assessed the rate of proliferation of keratinocytes and their invasive abilities by employing wound healing assay and the Transwell filter invasion method.ResultsHaCaT keratinocytes activated through incubation with melanoma-conditioned medium or indirect co-culture exhibit properties of less differentiated cells (downregulation of cytokeratin 10), which also prefer to form connections with cancer cells rather than adjacent keratinocytes (decreased level of E-cadherin). While they express only a small number of cytokines, the variety of secreted proteases is quite prominent especially considering that several of them were never reported as a part of secretome of activated keratinocytes’ (e.g., matrix metalloproteinase 3 (MMP3), ADAM metallopeptidase with thrombospondin type 1 motif 1). Activated keratinocytes also seem to exhibit a high level of proteolytic activity mediated by MMP9 and MMP14, reduced expression of TIMPs (tissue inhibitor of metalloproteinases), upregulation of ERK activity and increased levels of MMP expression regulators-RUNX2 and galectin 3. Moreover, cancer-associated keratinocytes show slightly elevated migratory and invasive abilities, however only following co-culture with melanoma cells on Transwell inserts.ConclusionsOur study offers a more in-depth view of keratinocytes residing in the melanoma niche, drawing attention to their unique secretome and mediators of invasive abilities, factors which could be used by cancer cells to support their invasion of surrounding tissues.52obFQmRyJn1nHi_mM3M7bVideo abstract

Identification of a Novel MLPH Missense Mutation in a Chinese Griscelli Syndrome 3 Patient

Melanophilin (MLPH) functions as a linker between RAB27A and myosin Va (MYO5A) in regulating skin pigmentation during the melanosome transport process. The MYO5A-MLPH-RAB27A ternary protein complex is required for anchoring mature melanosomes in the peripheral actin filaments of melanocytes for subsequent transfer to adjacent keratinocytes. Griscelli syndrome type 3 (GS3) is caused by mutations in the MLPH gene. So far, only five variants of MLPH associated with GS3 have been reported. Here, we reported the first patient with GS3 in a Chinese population. The proband carried a novel homozygous missense mutation (c.73G>C; p.D25H), residing in the conserved Slp homology domain of MLPH, and presented with hypopigmentation of the hair, eyebrows, and eyelashes. Light microscopy revealed the presence of abnormal pigment clumping in his hair shaft. In silico tools predicted this MLPH variant to be likely pathogenic. Using immunoblotting and immunofluorescence analysis, we demonstrated that the MLPH (D25H) variant had an inhibitory effect on melanosome transport by exhibiting perinuclear melanosome aggregation in melanocytes, and greatly reduced its binding to RAB27A, although the protein level of MLPH in the patient was not changed. Our findings suggest that MLPH (D25H) is a pathogenic variant that expands the genetic spectrum of the MLPH gene.

Human papillomavirus type 16 E6 induces cell competition.

High-risk human papillomavirus (HPV) infections induce squamous epithelial tumors in which the virus replicates. Initially, the virus-infected cells are untransformed, but expand in both number and area at the expense of uninfected squamous epithelial cells. We have developed an in vitro assay in which colonies of post-confluent HPV16 expressing cells outcompete and displace confluent surrounding uninfected keratinocytes. The enhanced colony competition induced by the complete HPV16 genome is conferred by E6 expression alone, not by individual expression of E5 or E7, and requires E6 interaction with p53. E6-expressing keratinocytes undermine and displace adjacent normal keratinocytes from contact with the attachment substrate, thereby expanding the area of the E6-expressing colony at the expense of normal keratinocytes. These new results separate classic oncogenicity that is primarily conferred by HPV16 E7 from cell competition that we show is primarily conferred by E6 and provides a new biological role for E6 oncoproteins from high-risk human papillomaviruses.

WNT5A drives interleukin-6-dependent epithelial-mesenchymal transition via the JAK/STAT pathway in keloid pathogenesis.

BackgroundKeloid scarring is a fibroproliferative disease caused by aberrant genetic activation with an unclear underlying mechanism. Genetic predisposition, aberrant cellular responses to environmental factors, increased inflammatory cytokines and epithelial–mesenchymal transition (EMT) phenomena are known as major contributors. In this study, we aimed to identify the molecular drivers that initiate keloid pathogenesis.MethodsBulk tissue RNA sequencing analyses of keloid and normal tissues along with ex vivo and in vitro tests were performed to identify the contributing genes to keloid pathogenesis. An animal model of inflammatory keloid scarring was reproduced by replication of a skin fibrosis model with intradermal bleomycin injection in C57BL/6 mice.ResultsGene set enrichment analysis revealed upregulation of Wnt family member 5A (WNT5A) expression and genes associated with EMT in keloid tissues. Consistently, human keloid tissues and the bleomycin-induced skin fibrosis animal model showed significantly increased expression of WNT5A and EMT markers. Increased activation of the interleukin (IL)-6/Janus kinase (JAK)/signal transducer and activator of transcription (STAT) pathway and subsequent elevation of EMT markers was also observed in keratinocytes co-cultured with WNT5A-activated fibroblasts or keloid fibroblasts. Furthermore, WNT5A silencing and the blockage of IL-6 secretion via neutralizing IL-6 antibody reversed hyperactivation of the STAT pathway and EMT markers in keratinocytes. Lastly, STAT3 silencing significantly reduced the EMT-like phenotypes in both keratinocytes and IL-6-stimulated keratinocytes.ConclusionsIntercellular communication via the WNT5A and STAT pathways possibly underlies a partial mechanism of EMT-like phenomena in keloid pathogenesis. IL-6 secreted from WNT5A-activated fibroblasts or keloid fibroblasts activates the JAK/STAT signaling pathway in adjacent keratinocytes which in turn express EMT markers. A better understanding of keloid development and the role of WNT5A in EMT will promote the development of next-generation targeted treatments for keloid scars.

Expression and mechanism of long non-coding RNA HCG22 in oral squamous cell carcinoma.

To investigate the expression and mechanism of the long non-coding RNA (lncRNA) HCG22 in oral squamous cell carcinoma (OSCC). HCG22 levels were detected in the OSCC and adjacent tissues, OSCC cells, and normal oral keratinocytes. HCG22 expression in SCC-25 and HSC-3 cells was upregulated by transfection of the overexpressing plasmi dvector. Methyl thiazolyl tetrazolium (MTT) assay, flow cytometry, and Transwell assay were employed to detect changes in cell proliferation, apoptosis, migration, and invasion ability, while Western blotting was used to detect the expression of epithelial-mesenchymal transformation-related proteins. The expression level of miR-650 in the cells was detected by real-time quantitative polymerase chain reaction (RT-qPCR), and dual-luciferase reporter gene assay was applied to assess the targeting relationship between HCG22 and miR-650. Compared with that in adjacent tissues, the expression of HCG22 significantly decreased in OSCC tissues (P<0.05). Moreover, the prognostic survival of patients in the low-HCG22 expression group was significantly lower than that in the high-expression group (P<0.05). Compared with that in HOK cells, the expression of HCG22 was significantly lower in SCC-25, HN13, HSC-3, and CAL-27 cells (P<0.05). Upregulation of HCG22 expression could inhibit the proliferation, migration, invasion, and apoptosis of SCC-25 and HSC-3 cells, upregulatethe expression of E-cadherin, and downregulate the expression of N-cadherin and vimentin (P<0.05). miR-650 mimics could reduce the luciferase activity of HCG22 wild-type plasmid cells (P<0.05), and the expression of miR-650 in SCC-25 and HSC-3 cells decreased after upregulation of HCG22 expression (P<0.05). HCG22 is expressed at low levels in OSCC. Upregulation of the expression of this lncRNA can inhibit the proliferation, migration, invasion, and epithelial-mesenchymal transition of OSCC cells. The mechanism of action of HCG22 may be related to its targeted regulation of miR-650.

Comparative study of doxycycline, sancycline, and 4-dedimethylamino sancycline (CMT-3) on epidermal melanogenesis.

Melanogenesis is regulated by melanocytes, which synthesize the pigment melanin inside melanosomes; these melanosomes are exported through dendritic extensions to adjacent keratinocytes and result in skin coloration. Chemically modified tetracyclines (CMTs) are nonantimicrobial tetracyclines that retain the capacity to inhibit matrix metalloproteinases (MMPs) and have shown several biological benefits; in particular, CMT-3 [(4-dedimethylamino sancycline (SAN)] has emerged as a candidate for therapeutic benefits in our previous studies. However, to date, studies of the effects of CMT-3 or SAN on melanogenesis are lacking. We have previously reported the anti-melanogenic activity of CMT-308 (the 9-amino derivative of CMT-3). Herein, we have compared the three tetracycline analogs, doxycycline (DOX), SAN, and CMT-3, for their effects on melanogenesis using B16F10 mouse melanoma cells and have validated results in primary human melanocytes (HEMn-DP). DOX did not show any significant effects on intracellular melanin or melanosome export in DP cells while SAN was cytotoxic at high doses but without effects on melanogenesis at lower doses. However, CMT-3 showed a robust suppression of dendricity parameters (dendrite number, dendrite length, and proportion of dendritic cells) in DP cells which was associated, at least in part, with a significant reduction of intracellular tyrosinase activity. In spite of its inhibition of tyrosinase activity, CMT-3 had no significant effects on intracellular melanin levels, suggesting that it selectively targets melanosome export. Our results demonstrate a unique structure-activity relationship (SAR) for the effects of these compounds on melanogenesis and support the conclusion that removal of the 4-dimethylamino moiety confers the selective capacity to suppress melanosome export. Collectively, these results indicate that CMT-3 might be a candidate for diminishing hyperpigmentation skin disorders.

146 Betaine, a natural osmolyte, regulates tight junction protein amount and localization in normal human epidermal keratinocytes

Tight junctions (TJs) connecting adjacent keratinocytes to each other are part of skin’s permeability barrier. Betaine is a natural intracellular osmolyte, that is obtained in diet but it is also used as a cosmetic ingredient. We utilized liquid chromatography to determine the betaine amount in the human skin samples. In addition, freshly isolated human epidermal keratinocytes (NHEKs) were used to investigate how betaine affects TJ protein and differentiation markers expression, as well as their localization with indirect immunofluorescence.

Nicotinamide as a Skin Whitener: Evidence and Controversies

Background: Topical nicotinamide (NAM) can reduce excessive melanin deposition in cell culture, by reversibly blocking the transfer of melanosomes from melanocytes to the adjacent keratinocytes. Thus, it has been increasingly used as a whitening agent.&#x0D; Objective: To assess the efficacy and safety of topical nicotinamide used for the treatment of melasma and hyperpigmentation.&#x0D; Methods: An electronic search for topical nicotinamide was carried out on Pubmed and Medline databases to identify studies that addressed this topic as a whitening agent. And to review the primary and secondary outcomes.&#x0D; Results: A significant decrease in hyperpigmentation and increased skin lightness was found with the use of topical nicotinamide, compared with the vehicle In two small sample size clinical studies. Combined regimens including nicotinamide and other ingredients offer more synergistic effects than monotherapy.&#x0D; Conclusion: Due to the lack of sufficient evidence, the use of nicotinamide for melasma remains controversial. Extended randomized, double-blind, placebo-controlled trials with long-term follow-up periods are needed to assess the efficacy of nicotinamide as a whitening agent.

Expression of Hedgehog signalling molecules inmicrocystic adnexal carcinoma.

Microcystic adnexal carcinoma (MAC) is a rare skin neoplasm that has not been characterized on a molecular basis. To assess expression profiles of Hedgehog (HH) signalling molecules in MAC and control tumours. Immunohistochemistry was performed for Sonic Hedgehog (SHH), Indian Hedgehog (IHH), Patched 1 (PTCH1) and Smoothened (SMO) on patient MAC tissue (n=26) and control tumour tissue, including syringoma (SyG; n=11), trichoepithelioma (TE; n=11) and basal cell carcinoma (BCC; n=12) tissues. Patched 1 and SMO immunoreactivity was significantly higher in BCC than in SyG, TE or MAC (P<0.001 and P<0.03, respectively). The highest IHH expression was observed in BCC and TE compared with SyG and MAC (P<0.04). Notably, the highest SHH protein expression was observed in SyG compared with MAC, TE and even BCC (P<0.001). In patients with MAC, SMO immunoreactivity significantly (r=0.51; P<0.01) correlated with PTCH1 expression. Further correlation studies did not show significant associations between the HH expression markers assessed (P>0.05). Our results indicate that alterations of the HH signalling are unlikely to play a major role in the pathogenesis of MAC, which is in contrast to the morphologically similar BCC and TE. Our observation provides additional information to the limited molecular pathology knowledge on this rare tumour.

Inhibitory effect of CADI on melanin transfer in the B16F10-HaCAT cells co-culture system and anti-melanogenesis of CNCE in zebrafish

Skin pigmentation depends not only on melanosomes produced by melanocytes but also on the migration of melanosomes to adjacent keratinocytes and their distribution on the skin surface. The inhibitory effects of cardanol diene (CADI), a major phenolic compound extracted from cashew nut shell liquid (CNSL), on melanin transfer in B16F10-HaCAT co-culture cells and a crude extracted compound of cashew nut shell oil (CNCE) on melanin production in zebrafish were investigated. β-arbutin was used as a positive control. CADI inhibited the expression levels of the melanin phagocytosis-associated protein PAR-2 and filopodia-associated protein NMDAR1, and the effect was better than that of β-arbutin at the same concentration. To further study the inhibitory mechanism of CADI in melanin transfer, fluorescence microspheres were used to simulate melanin and co-culture with keratinocytes. The results indicated that CADI inhibited melanin transfer through suppressing keratinocyte phagocytosis, dendrite formation and filopodia three channels. We also found that CNCE inhibited melanin production in zebrafish and showed significant inhibitory effects on TYR, DCT and α-MSH. Thus, the natural components extracted from CNSL effectively inhibited skin pigmentation, providing insight into the application of new anti-melanin drugs in cosmetics and medicine.

Here comes the cavalry!

Cell Biology Healing small scratch wounds in the skin is much less well understood than the closing of deeper wounds. Bornes et al. used intravital microscopy to study the healing of scratches on the backs of mice. The authors followed individual keratinocytes as the cells multiplied and migrated to rapidly reestablish the skin's protective barrier. Basal keratinocytes multiplied at sites somewhat distant from the wound and then swarmed toward the wound, moving as a sheet of independently migrating cells. This allowed the replacement cells to bypass any intervening obstacles such as intact immobile hair follicles or sweat glands. The findings challenge a model proposing that the leading edge of migrating cells directly transmits forces to adjacent keratinocytes more distant from the wound bed to physically drag them along. Instead, leading-edge keratinocytes may initiate wound closure followed by individually moving keratinocytes within a cohesive layer. Life Sci. Alliance 4 , e202000765 (2020).

Laminins and interaction partners in the architecture of the basement membrane at the dermal-epidermal junction.

The basement membrane at the dermal-epidermal junction keeps the epidermis attached to the dermis. This anatomical barrier is made up of four categories of extracellular matrix proteins: collagen IV, laminin, nidogen and perlecan. These proteins are precisely arranged in a well-defined architecture through specific interactions between the structural domains of the individual components. Some of the molecular constituents are provided by both fibroblasts and keratinocytes, while others are synthesized exclusively by fibroblasts or keratinocytes. It remains to be determined how the components from the fibroblasts are targeted to the dermal-epidermal junction and correctly organized and integrated with the proteins from the adjacent keratinocytes to form the basement membrane.

Three-dimensional structure analysis of melanocytes and keratinocytes in senile lentigo.

Senile lentigo or age spots are hyperpigmented macules of skin that commonly develop following long-term exposure to ultraviolet radiation. This condition is caused by accumulation of large numbers of melanosomes (melanin granules) produced by melanocytes within neighboring keratinocytes. However, there is still no consensus regarding the melanosome transfer mechanism in senile lentigo. To date, most pathohistological studies of skin have been two-dimensional and do not provide detailed data on the complex interactions of the melanocyte-keratinocyte network involved in melanosome transfer. We performed a three-dimensional reconstruction of the epidermal microstructure in senile lentigo using three different microscopic modalities to visualize the topological melanocyte-keratinocyte relationship and melanosome distribution. Confocal laser microscopy images showed that melanocyte dendritic processes are more frequently branched and elongated in senile lentigo skin than in normal skin. Serial transmission electron micrographs showed that dendritic processes extend into intercellular spaces between keratinocytes. Focused ion beam-scanning electron micrographs showed that dendritic processes in senile lentigo encircle adjacent keratinocytes and accumulate large numbers of melanosomes. Moreover, melanosomes transferred to keratinocytes are present not only in the supranuclear area but throughout the perinuclear area except on the basal side. The use of these different microscopic methods helped to elucidate the three-dimensional morphology and topology of melanocytes and keratinocytes in senile lentigo. We show that the localization of melanosomes in dendritic processes to the region encircling recipient keratinocytes contributes to efficient melanosome transfer in senile lentigo.

Recent advances in understanding the molecular basis of melanogenesis in melanocytes.

Melanin pigments are responsible for human skin and hair color, and they protect the body from harmful ultraviolet light. The black and brown melanin pigments are synthesized in specialized lysosome-related organelles called melanosomes in melanocytes. Mature melanosomes are transported within melanocytes and transferred to adjacent keratinocytes, which constitute the principal part of human skin. The melanosomes are then deposited inside the keratinocytes and darken the skin (a process called tanning). Owing to their dark color, melanosomes can be seen easily with an ordinary light microscope, and melanosome research dates back approximately 150 years; since then, biochemical studies aimed at isolating and purifying melanosomes have been conducted. Moreover, in the last two decades, hundreds of molecules involved in regulating melanosomal functions have been identified by analyses of the genes of coat-color mutant animals and patients with genetic diseases characterized by pigment abnormalities, such as hypopigmentation. In recent years, dynamic analyses by more precise microscopic observations have revealed specific functions of a variety of molecules involved in melanogenesis. This review article focuses on the latest findings with regard to the steps (or mechanisms) involved in melanosome formation and transport of mature melanosomes within epidermal melanocytes. Finally, we will touch on current topics in melanosome research, particularly on the "melanosome transfer" and "post-transfer" steps, and discuss future directions in pigment research.