Skin telocytes versus fibroblasts: two distinct dermal cell populations

CD34+ stromal cells/telocytes and their role in mouse lung development: Light microscopy, immunofluorescence, ultrastructural and scanning electron microscopy evidence

Telocytes (TCs), a novel type of mesenchymal or interstitial cell with specific, very long and thin cellular prolongations, have been found in various mammalian organs and have potential biological functions. However, their existence during lung development is poorly understood. This study aimed to investigate the existence, morphological features, and role of CD34+ SCs/TCs in mouse lungs from foetal to postnatal life using primary cell culture, double immunofluorescence, transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The immunofluorescence double staining profiles revealed positive expression of CD34 and PDGFR-α, Sca-1 or VEGFR-3, and the expression of these markers differed among the age groups during lung development. Intriguingly, in the E18.5 stage of development, along with the CD34+ SCs/TCs, haematopoietic stem cells and angiogenic factors were also significantly increased in number compared with those in the E14.5, E16.5, P0 and P7. Subsequently, TEM confirmed that CD34+ SCs/TCs consisted of a small cell body with long telopodes (Tps) that projected from the cytoplasm. Tps consisted of alternating thin and thick segments known as podomers and podoms. TCs contain abundant endoplasmic reticulum, mitochondria and secretory vesicles and establish close connections with neighbouring cells. Furthermore, SEM revealed characteristic features, including triangular, oval, spherical, or fusiform cell bodies with extensive cellular prolongations, depending on the number of Tps. Our findings provide evidence for the existence of CD34+ SCs/TCs, which contribute to vasculogenesis, the formation of the air‒blood barrier, tissue organization during lung development and homoeostasis.

Developmental characteristics of cutaneous telocytes in late embryos of the silky fowl

Telocytes (TCs) have been identified in various animals. However, information on TCs in the embryos is still very limited. In this work, the developing skin of the silky fowl was sampled for TCs identification by histology, immunohistochemistry and transmission electron microscopy. In addition, morphological parameters of cutaneous TCs and their location relationships were measured using a morphometry software - ImageJ (FiJi). At the 12th, 16th and 20th day of incubation, in the embryonic skin, telocyte-like cells (TC-L) were observed in the dermis. TCs were PDGFRα+ at the 12th, 16th and 20th day of incubation, but showed CD34+ only at 20th day of incubation in the embryonic dermis. Ultrastructurally, TCs were observed in the dermis at all late embryonic developmental stages. TCs established the homocellular contacts/plasmalemmal adhesion with each other. TCs established heterocellular contacts with melanocytes at 20th day of incubation in the dermis. In addition, the intracellular microvesicles were present in the cytoplasm of TCs. The extracellular microvesicles/exosomes were in close proximity to the TCs. The results confirmed that the locations, immunophenotypes, structural characteristics and relationships of TCs, and revealed the developmental characteristics of cutaneous TCs in late silky fowl embryos.

Immunohistochemical and Ultrastructural Characterization of Telocytes in Normal and Diabetic Human Kidneys

Background: Telocytes are interstitial stromal cells identified in various human organs, including the kidney. Their presence and role in human diabetic kidney disease remain unknown. Methods: To identify and localize telocytes in glomerular and tubule-interstitial compartments, both normal and diabetic human renal tissues were examined using immunohistochemistry, immunofluorescence, and transmission electron microscopy. Results: Renal telocytes are elongated interstitial cells with long, thin telopodes, showing alternating thin and thick segments. They expressed CD34, Nestin, α-SMA, and Vimentin markers. Occasionally, c-Kit expression was observed in some rounded and spindle cells, while no positivity was detected for PDGFR-β and NG2. Telocytes were identified around Bowman's capsule, tubules, and peritubular capillaries in both normal and diabetic conditions. In diabetic renal samples, there was a significant increase in α-SMA expressing telocytes, leading to periglomerular fibrosis. These telocytes also exhibited a synthetic phenotype with proteoglycan deposition in the extracellular matrix and, in some cases, showed pre-adipocytic differentiation. Conclusions: Telocytes were identified in normal and diabetic human kidneys. These cells form an elastic mechanical scaffold in the interstitium and are present in all renal cortical compartments. In diabetic samples, their increased α-SMA expression and synthetic phenotype suggest their potential role in the pathogenesis of diabetic nephropathy.

Skin Telocytes Could Fundament the Cellular Mechanisms of Wound Healing in Platelet-Rich Plasma Administration

For more than 40 years, autologous platelet concentrates have been used in clinical medicine. Since the first formula used, namely platelet-rich plasma (PRP), other platelet concentrates have been experimented with, including platelet-rich fibrin and concentrated growth factor. Platelet concentrates have three standard characteristics: they act as scaffolds, they serve as a source of growth factors and cytokines, and they contain live cells. PRP has become extensively used in regenerative medicine for the successful treatment of a variety of clinical (non-)dermatological conditions like alopecies, acne scars, skin burns, skin ulcers, muscle, cartilage, and bone repair, and as an adjuvant in post-surgery wound healing, with obvious benefits in terms of functionality and aesthetic recovery of affected tissues/organs. These indications were well documented, and a large amount of evidence has already been published supporting the efficacy of this method. The primordial principle behind minimally invasive PRP treatments is the usage of the patient's own platelets. The benefits of the autologous transplantation of thrombocytes are significant, representing a fast and economic method that requires only basic equipment and training, and it is biocompatible, thus being a low risk for the patient (infection and immunological reactions can be virtually disregarded). Usually, the structural benefits of applying PRP are attributed to fibroblasts only, as they are considered the most numerous cell population within the interstitium. However, this apparent simplistic explanation is still eluding those different types of interstitial cells (distinct from fibroblasts) that are residing within stromal tissue, e.g., telocytes (TCs). Moreover, dermal TCs have an already documented potential in angiogenesis (extra-cutaneous, but also within skin), and their implication in skin recovery in a few dermatological conditions was attested and described ultrastructurally and immunophenotypically. Interestingly, PRP biochemically consists of a series of growth factors, cytokines, and other molecules, to which TCs have also proven to have a positive expression. Thus, it is attractive to hypothesize and to document any tissular collaboration between cutaneous administered PRP and local dermal TCs in skin recovery/repair/regeneration. Therefore, TCs could be perceived as the missing link necessary to provide a solid explanation of the good results achieved by administering PRP in skin-repairing processes.

Immunohistochemical-properties of the dermal embryonic telocytes

The current investigation aims to study the embryonic dermis formed in the early stages of development and identify the initial interstitial components of the dermis that serve as biological and structural scaffolds for the development of the dermal tissue. To investigate the dermal structure, the current study used morphological and immunological techniques. TCs identified by TEM. They had a cell body and unique podomeres and podoms. They formed a 3D network spread throughout the dermis. Homocellular contact established between them, as well as heterocellular contacts with other cells. Immunohistochemical techniques using specific markers for TCss CD34, CD117, and VEGF confirmed TC identification. TCs represent the major interstitial component in the dermal tissue. They established a 3D network, enclosing other cells and structures. Expression of VEGF by TC promotes angiogenesis. TCs establish cellular contact with sprouting endothelial cells. At the site of cell junction with TCs, cytoskeletal filaments identified and observed to form the pseudopodium core that projects from endothelial cells. TCs had proteolytic properties that expressed MMP-9, CD68, and CD21. Proteolytic activity aids in the removal of components of the extracellular matrix and the phagocytosis of degraded remnants to create spaces to facilitate the development of new dermal structures. In conclusion, TCs organized the scaffold for the development of future dermal structures, including fibrous components and skin appendages. Studying dermal TCs would be interested in the possibility of developing therapeutic strategies for treating different skin disorders and diseases.

The Morphology and Ultrastructure of Dermal Telocytes Characterized by TEM and AFM

This investigation delves into the structural foundation of human dermal telocytes (TCs) with the aim of elucidating their role in signal transmission. Dermal TCs were isolated from human foreskins via enzymatic digestion and flow cytometric sorting, and identified by immunohistochemical staining with an antibody against CD34. The ultrastructure of TCs was examined using transmission electron microscopy (TEM). The proliferation rates of sorted TCs and CD34-negative fibroblasts were compared using the MTS assay (Cell Proliferation Assay). Images of viable cultured TCs were analyzed using atomic force microscopy (AFM) under normal atmospheric pressure and temperature. Results demonstrated that dermal TCs were positive for CD34 and vimentin, predominantly distributed in the reticular dermis and subcutaneous tissue, forming interwoven networks. Each TC had a small body with a high nuclear-plasma ratio and two or three extremely long and thin telopodes (TPs), exhibiting a typical 'moniliform' appearance. Compared with CD34-negative fibroblasts, dermal TCs exhibited significantly lower proliferation rates. Cultured TCs displayed typical moniliform projections (namely, TPs) in the AFM images. The distal ends of TPs were enlarged, shaped like a broom, and extended multiple pseudopods to contact other cell bodies. Slender filamentary pseudopodia and thick, short cone-like structures were observed on the surfaces of the dilated segments and terminals of TPs. These structures are assumed to be evidence of the secretion and release of endosomes, such as exosomes, and the communication between cells. TCs form interstitial networks in the reticular dermis and subcutaneous tissue, providing a structural basis for contacts between cells and the secretion of signal-carrying substances, involving intercellular connections and communication.

Toward understanding the role of the interstitial tissue architects: Possible functions of telocytes in the male gonad

Telocytes represent a relatively recently discovered population of interstitial cells with a unique morphological structure that distinguishes them from other neighboring cells. Through their long protrusions extending from the cell body, telocytes create microenvironments via tissue compartmentalization and create homo- and hetero-cellular junctions. These establish a three-dimensional network enabling the maintenance of interstitial compartment homeostasis through regulation of extracellular matrix organization and activity, structural support, paracrine and juxtracrine communication, immunomodulation, immune surveillance, cell survival, and apoptosis. The presence of telocytes has also been confirmed in testicular interstitial tissue of many species of animals. The objective of this review is to summarize recent findings on telocytes in the male gonad, on which conclusions have been deduced that indicate the involvement of telocytes in maintaining the cytoarchitecture of the testicular interstitial tissue, in the processes of spermatogenesis and steroidogenesis, and photoperiod-mediated changes in the testes in seasonally reproductive animals.

Platelet-Rich Plasma in Dermatology: New Insights on the Cellular Mechanism of Skin Repair and Regeneration

The skin's recognised functions may undergo physiological alterations due to ageing, manifesting as varying degrees of facial wrinkles, diminished tautness, density, and volume. Additionally, these functions can be disrupted (patho)physiologically through various physical and chemical injuries, including surgical trauma, accidents, or chronic conditions like ulcers associated with diabetes mellitus, venous insufficiency, or obesity. Advancements in therapeutic interventions that boost the skin's innate regenerative abilities could significantly enhance patient care protocols. The application of Platelet-Rich Plasma (PRP) is widely recognized for its aesthetic and functional benefits to the skin. Yet, the endorsement of PRP's advantages often borders on the dogmatic, with its efficacy commonly ascribed solely to the activation of fibroblasts by the factors contained within platelet granules. PRP therapy is a cornerstone of regenerative medicine which involves the autologous delivery of conditioned plasma enriched by platelets. This is achieved by centrifugation, removing erythrocytes while retaining platelets and their granules. Despite its widespread use, the precise sequences of cellular activation, the specific cellular players, and the molecular machinery that drive PRP-facilitated healing are still enigmatic. There is still a paucity of definitive and robust studies elucidating these mechanisms. In recent years, telocytes (TCs)-a unique dermal cell population-have shown promising potential for tissue regeneration in various organs, including the dermis. TCs' participation in neo-angiogenesis, akin to that attributed to PRP, and their role in tissue remodelling and repair processes within the interstitia of several organs (including the dermis), offer intriguing insights. Their potential to contribute to, or possibly orchestrate, the skin regeneration process following PRP treatment has elicited considerable interest. Therefore, pursuing a comprehensive understanding of the cellular and molecular mechanisms at work, particularly those involving TCs, their temporal involvement in structural recovery following injury, and the interconnected biological events in skin wound healing and regeneration represents a compelling field of study.

Unveiling Cellular Diversity in the Buffalo Corneal Stroma: Insights into Telocytes and Keratocytes Using Light Microscope, Transmission Electron Microscope, and Immunofluorescence Analysis

Telocytes and keratocytes are important cells that maintain the structure and function of the cornea. The buffalo cornea, known for its resilience in harsh conditions, has not been extensively studied regarding the presence and role of telocytes and keratocytes. We used light microscopy, transmission electron microscopy (TEM), and immunofluorescence assays with platelet-derived growth factor receptor alpha (PDGFRα), CD34, and Vimentin markers to investigate their expression and localization in the cornea. TEM analysis confirmed the presence of spindle-shaped keratocytes with intercellular connections, while telocytes exhibited small spindle-shaped bodies with long, thin branches connecting to corneal keratocytes. Immunofluorescence findings showed that CD34 was more abundant near the endothelium, Vimentin was prominently expressed near the epithelium, and PDGFRα was uniformly distributed throughout the corneal stroma. Co-expression of CD34 and Vimentin, PDGFRα and Vimentin, as well as CD34 and PDGFRα, was observed in keratocytes and telocytes within the stroma, indicating the potential presence of mesenchymal cells. These results suggest the involvement of telocytes and keratocytes in corneal wound healing, transparency maintenance, and homeostasis. The co-expression of these markers highlights the critical role of telocytes and keratocytes in regulating corneal physiological functions, further enhancing our understanding of corneal biology in the buffalo model.

Telocytes/CD34+ Stromal Cells in the Normal, Hyperplastic, and Adenomatous Human Parathyroid Glands

Telocytes/CD34+ stromal cells (TCs/CD34+ SCs) have been studied in numerous organs and tissues, but their presence and characteristics in the parathyroid glands have not been explored. Using immunological and ultrastructural procedures, we assess the location, arrangement, and behavior of TCs/CD34+ SCs in normal human parathyroids, during their development and in their most frequent pathologic conditions. In normal parathyroids, TCs/CD34+ SCs show a small somatic body and long thin processes with a moniliform aspect, form labyrinthine systems, connect other neighboring TCs/CD34+ SCs, vessels, adipocytes, and parenchymal cells directly or by extracellular vesicles, and associate with collagen I. TCs/CD34+ SCs and collagen I are absent around vessels and adipocytes within parenchymal clusters. In developing parathyroids, TCs/CD34+ SC surround small parenchymal nests and adipocytes. In hyperplastic parathyroids, TCs/CD34+ SCs are prominent in some thickened internodular septa and surround small extraglandular parenchymal cell nests. TCs/CD34+ SCs are present in delimiting regions with compressed parathyroids and their capsule in adenomas but absent in most adenomatous tissue. In conclusion, TCs/CD34+ SCs are an important cellular component in the human parathyroid stroma, except around vessels within parenchymal nests. They show typical characteristics, including those of connecting cells, are present in developing parathyroids, and participate in the most frequent parathyroid pathology, including hyperplastic and adenomatous parathyroids.

The Cellular Mechanism of Acupuncture for Ulcerative Colitis based on the Communication of Telocytes

Acupuncture can ameliorate or treat diseases according to the meridian theory in traditional Chinese medicine (TCM); however, its mechanism has not been scientifically clarified. On the other hand, telocytes (TCs) are morphologically in accordance with the meridian system, which needs further cytological investigations and acupuncture confirmation. The present study showed that acupuncture could activate TCs in several ways, alleviating rabbit ulcerative colitis. TCs could cytologically communicate the acupoints, the acupuncture sites in skin with their corresponding large intestine by TC homo-cellular junctions, exosomes around TCs, and TC-mediated nerves or blood vessels. TCs expressed transient receptor potential vanilloid type 4, the mechanosensitive channel protein that can transduce the mechanical stimulation of acupuncture into biochemical signals transferring along the extremely thin and long TCs. Collectively, a cellular mechanism diagram of acupuncture was concluded based on TC characteristics. Those results also confirmed the viewpoint that TCs were the key cells of meridian essence in TCM.

Identification and protective role of CD34+ stromal cells/telocytes in experimental autoimmune encephalomyelitis (EAE) mouse spleen

Experimental autoimmune encephalomyelitis (EAE) is a classical animal model of human multiple sclerosis (MS) that is most commonly used to study the neuropathology and therapeutic effects of the disease. Telocytes (TCs) are a specialized type of interstitial or mesenchymal cell first identified by Popescu in various tissues and organs. However, the existence, distribution and role of CD34⁺ stromal cells (SCs)/TCs in the EAE-induced mouse spleen remain to be elucidated. We conducted immunohistochemistry, immunofluorescence (double staining for CD34 and c-kit, vimentin, F4/80, CD163, Nanog, Sca-1, CD31 or tryptase) and transmission electron microscopy experiments to investigate the existence, distribution and role of CD34⁺ SCs/TCs in the EAE-induced mouse spleen. Interestingly, immunohistochemistry, double-immunofluorescence, and transmission electron microscopy results revealed that CD34⁺ SCs/TCs were significantly upregulated in the EAE mouse spleen. Immunohistochemical or double-immunofluorescence staining of CD34⁺ SCs/TCs showed positive expression for CD34, c-kit, vimentin, CD34/vimentin, c-kit/vimentin and CD34/c-kit, and negative expression for CD31 and tryptase. Transmission electron microscopy (TEM) results demonstrated that CD34⁺ SCs/TCs established close connections with lymphocytes, reticular cells, macrophages, endothelial cells and erythrocytes. Furthermore, we also found that M1 (F4/80) or M2 (CD163) macrophages, and haematopoietic, pluripotent stem cells were markedly increased in EAE mice. Our results suggest that CD34⁺ SCs/TCs are abundant and may play a contributing role in modulating the immune response, recruiting macrophages and proliferation of haematopoietic and pluripotent stem cells following injury to promote tissue repair and regeneration in EAE mouse spleens. This suggests that their transplantation combined with stem cells might represent a promising therapeutic target for the treatment and prevention of multiple autoimmune and chronic inflammatory disorders.

Morphologies and potential roles of telocytes in nervous tissue

Structurally similar cells have been found and termed telocytes (TCs) since the first characterisation of interstitial Cajal‐like cells in 1911. TCs are a novel and peculiar interstitial cell type with a small cellular body, markedly long cell processes named telopodes and a wide distribution in numerous tissues throughout the body. Besides specific morphological characteristics and immunohistochemical profiles, TCs build three‐dimensional mixed networks through homocellular (connection to each other) and/or heterocellular contacts (connection with other cell types), interaction with extracellular matrix and their vicinity to nerve endings, and thus might play, as part of an integrated system, roles in maintaining organ/tissue function. In this mini‐review, we summarise physical properties, general characteristics and distribution of TCs in diverse organs and tissues, focusing on their potential functions in nervous tissue and current challenges in investigating TCs as a distinct cell type.

Synthesis and Physiological Remodeling of CD34 Cells in the Skin following the Reversal of Fibrosis through Intensive Treatment for Lower Limb Lymphedema: A Case Report

A novel type of cell underwent identification between 2005 and 2008 and was denominated the “telocyte” in 2010. In 2012, transmission electron microscopy revealed the presence of telocytes in the dermis. The aim of the present study was to report important changes in immunostained CD34 cells following the treatment of lower limb lymphedema using a specific lymphatic therapy technique. A clinical trial involving the evaluation of changes in immunostained CD34 cells in the epidermis and dermis (10 randomly selected histological fields) of a patient before and after intensive treatment for clinical stage II lymphedema was conducted using the Godoy Method, which was adapted to the treatment of skin fibrosis. The evaluation involved the use of the Weibel multi-point morphometric method. Comparisons were performed using the t-test with a 95% significance level. An important increase in CD34 cells was found with redistribution occurring following treatment. The treatment of primary lymphedema of the lower limbs resulted in the clinical reversal of fibrosis and an increase in the number of immunomarked CD34 cells.

Telocytes in Cutaneous Biology: A Reappraisal

The telocytes (TCs) are novel interstitial cells that have been overlooked for a long time due to their histologic similarity to other stromal cells. TCs can be separated from the stromal cells based on their distinct immunohistochemical, ultrastructural, and molecular features. Functionally, TCs are involved in the tissue renewal, mechanical support, and immune modulation. These cells are also involved in the signal transduction either through their direct interactions with the neighboring cells or through the paracrine signaling via extracellular vesicles. TCs are damaged in several inflammatory and fibrotic conditions such as ulcerative colitis, Crohn's disease, hepatic fibrosis, psoriasis, and systemic sclerosis. The transplantation of TCs in the damaged tissue can promote tissue regeneration. Therefore, enhancing tissue TCs either by their transplantation or by promoting their survival and growth using novel medications represents novel therapeutic strategy in the future. In this review, we addressed several aspects of TCs including their origin, distribution, morphologic features, and functions. We also discussed their involvement of the cutaneous TCs in the development various pathologic conditions.

Telocytes in Cutaneous Biology: A Reappraisal

The telocytes (TCs) are novel interstitial cells that have been overlooked for a long time due to their histologic similarity to other stromal cells. TCs can be separated from the stromal cells based on their distinct immunohistochemical, ultrastructural, and molecular features. Functionally, TCs are involved in the tissue renewal, mechanical support, and immune modulation. These cells are also involved in the signal transduction either through their direct interactions with the neighboring cells or through the paracrine signaling via extracellular vesicles. TCs are damaged in several inflammatory and fibrotic conditions such as ulcerative colitis, Crohn's disease, hepatic fibrosis, psoriasis, and systemic sclerosis. The transplantation of TCs in the damaged tissue can promote tissue regeneration. Therefore, enhancing tissue TCs either by their transplantation or by promoting their survival and growth using novel medications represents novel therapeutic strategy in the future. In this review, we addressed several aspects of TCs including their origin, distribution, morphologic features, and functions. We also discussed their involvement of the cutaneous TCs in the development various pathologic conditions.

Delimiting CD34+ Stromal Cells/Telocytes Are Resident Mesenchymal Cells That Participate in Neovessel Formation in Skin Kaposi Sarcoma

Kaposi sarcoma (KS) is an angioproliferative lesion in which two main KS cell sources are currently sustained: endothelial cells (ECs) and mesenchymal/stromal cells. Our objective is to establish the tissue location, characteristics and transdifferentiation steps to the KS cells of the latter. For this purpose, we studied specimens of 49 cases of cutaneous KS using immunochemistry and confocal and electron microscopy. The results showed that delimiting CD34+ stromal cells/Telocytes (CD34+SCs/TCs) in the external layer of the pre-existing blood vessels and around skin appendages form small convergent lumens, express markers for ECs of blood and lymphatic vessels, share ultrastructural characteristics with ECs and participate in the origin of two main types of neovessels, the evolution of which gives rise to lymphangiomatous or spindle-cell patterns-the substrate of the main KS histopathological variants. Intraluminal folds and pillars (papillae) are formed in the neovessels, which suggests they increase by vessel splitting (intussusceptive angiogenesis and intussusceptive lymphangiogenesis). In conclusion, delimiting CD34+SCs/TCs are mesenchymal/stromal cells that can transdifferentiate into KS ECs, participating in the formation of two types of neovessels. The subsequent growth of the latter involves intussusceptive mechanisms, originating several KS variants. These findings are of histogenic, clinical and therapeutic interest.

The Osteopath's Imprint: Osteopathic Medicine Under the Nanoscopic Lens

Scientific literature demonstrates how osteopathic manipulative treatments (OMT) are able to improve various somatic functional parameters, change somato-visceral and viscero-somatic reflexes toward a more physiological mechano-metabolic environment and, consequently, bring benefits to patients. These benefits can be long-lasting or short-lived. Multiple reasons can be found to explain the positive responses to OMT, ranging from neurological, vascular, lymphatic, and endocrine explanations. Not only the techniques, but the touch of the clinician prove to be important factors for a favorable adaptation by the patient. Another science capable of explaining the change in cellular status and from which reflections that pave the way for observing the human body in a different light can be extrapolated is quantum physics. The latter is rarely taken into consideration to obtain possible explanations of the physical events that occur between the clinician and the patient. The article tries to put the effects of OMT under the light of a new lens: the nanoscopic.

Dermal Telocytes: A Different Viewpoint of Skin Repairing and Regeneration

Fifteen years after their discovery, telocytes (TCs) are yet perceived as a new stromal cell type. Their presence was initially documented peri-digestively, and gradually throughout the interstitia of many (non-)cavitary mammalian, human, and avian organs, including skin. Each time, TCs proved to be involved in diverse spatial relations with elements of interstitial (ultra)structure (blood vessels, nerves, immune cells, etc.). To date, transmission electron microscopy (TEM) remained the single main microscopic technique able to correctly and certainly attest TCs by their well-acknowledged (ultra)structure. In skin, dermal TCs reiterate almost all (ultra)structural features ascribed to TCs in other locations, with apparent direct implications in skin physiology and/or pathology. TCs' uneven distribution within skin, mainly located in stem cell niches, suggests involvement in either skin homeostasis or dermatological pathologies. On the other hand, different skin diseases involve different patterns of disruption of TCs' structure and ultrastructure. TCs' cellular cooperation with other interstitial elements, their immunological profile, and their changes during remission of diseases suggest their role(s) in tissue regeneration/repair processes. Thus, expanding the knowledge on dermal TCs could offer new insights into the natural skin capacity of self-repairing. Moreover, it would become attractive to consider that augmenting dermal TCs' presence/density could become an attractive therapeutic alternative for treating various skin defects.

Scleroderma-like Impairment in the Network of Telocytes/CD34+ Stromal Cells in the Experimental Mouse Model of Bleomycin-Induced Dermal Fibrosis

Considerable evidence accumulated over the past decade supports that telocytes (TCs)/CD34⁺ stromal cells represent an exclusive type of interstitial cells identifiable by transmission electron microscopy (TEM) or immunohistochemistry in various organs of the human body, including the skin. By means of their characteristic cellular extensions (telopodes), dermal TCs are arranged in networks intermingled with a multitude of neighboring cells and, hence, they are thought to contribute to skin homeostasis through both intercellular contacts and releasing extracellular vesicles. In this context, fibrotic skin lesions from patients with systemic sclerosis (SSc, scleroderma) appear to be characterized by a disruption of the dermal network of TCs, which has been ascribed to either cell degenerative processes or possible transformation into profibrotic myofibroblasts. In the present study, we utilized the well-established mouse model of bleomycin-induced scleroderma to gain further insights into the TC alterations found in cutaneous fibrosis. CD34 immunofluorescence revealed a severe impairment in the dermal network of TCs/CD34⁺ stromal cells in bleomycin-treated mice. CD31/CD34 double immunofluorescence confirmed that CD31^-/CD34⁺ TC counts were greatly reduced in the skin of bleomycin-treated mice compared with control mice. Ultrastructural signs of TC injury were detected in the skin of bleomycin-treated mice by TEM. The analyses of skin samples from mice treated with bleomycin for different times by either TEM or double immunostaining and immunoblotting for the CD34/α-SMA antigens collectively suggested that, although a few TCs may transition to α-SMA⁺ myofibroblasts in the early disease stage, most of these cells rather undergo degeneration, and then are lost. Taken together, our data demonstrate that TC changes in the skin of bleomycin-treated mice mimic very closely those observed in human SSc skin, which makes this experimental model a suitable tool to (i) unravel the pathological mechanisms underlying TC damage and (ii) clarify the possible contribution of the TC loss to the development/progression of dermal fibrosis. In perspective, these findings may have important implications in the field of skin regenerative medicine.

Telocytes and their structural relationships with surrounding cell types in the skin of silky fowl by immunohistochemistrical, transmission electron microscopical and morphometric analysis

Telocytes (TCs), a novel type of interstitial cells, were identified in various animals. Since TCs have not observed in avian skin, hence, we carried out immunohistochemistrical and transmission electron microscopical studies in the skin of the silky fowl to investigate the TCs. TCs appear as CD34, c-Kit, and PDGFRα immunopositive. The elongated TCs with 2 long and thin telopodes (Tps) are located in the dermis. Generally, a TC possesses a fusiform, ovoid and polygonal cell body with 2 Tps (lengths = 5.27-21.85 μm), which are uneven in thickness including thick sections - podoms (diameters = 0.40-0.47 μm) and thin sections - podomers (diameters = 0.03-0.04 μm). TCs/Tps are observed frequently in close proximity to neighboring cell types/structures, such as adipocytes, collagen fibers, and capillaries. Under a magnified field, homocellular TCs/Tps contacts are observed through gap junctions (distances = 0.01-0.05 μm), whereas some of TCs/Tps have heterocellular close contacts by point contacts with surrounding cells, including stem cells and melanocytes. The multivisicular bodies, especially exosomes (diameters = 0.09-0.23 μm) releasing from TCs/Tps are observed in close proximity to TCs/Tps. Our results illustrated that the novel type of interstitial cells - TCs are present in the dermis of the silky fowl, and they have special structural relationships with surrounding cell types. The study provides histological evidence for TCs involvement in intercellular communication, skin regeneration, and pigmentogenesis in avian skin.

Morphological Evidence of Telocytes in Skeletal Muscle Interstitium of Exercised and Sedentary Rodents

Skeletal muscle atrophy, resulting from states of hypokinesis or immobilization, leads to morphological, metabolic, and functional changes within the muscle tissue, a large variety of which are supported by the stromal cells populating the interstitium. Telocytes represent a recently discovered population of stromal cells, which has been increasingly identified in several human organs and appears to participate in sustaining cross-talk, promoting regenerative mechanisms and supporting differentiation of local stem cell niche. The aim of this morphologic study was to investigate the presence of Telocytes in the tibialis anterior muscle of healthy rats undergoing an endurance training protocol for either 4 weeks or 16 weeks compared to sedentary rats. Histomorphometric analysis of muscle fibers diameter revealed muscle atrophy in sedentary rats. Telocytes were identified by double-positive immunofluorescence staining for CD34/CD117 and CD34/vimentin. The results showed that Telocytes were significantly reduced in sedentary rats at 16 weeks, while rats subjected to regular exercise maintained a stable Telocytes population after 16 weeks. Understanding of the relationship between Telocytes and exercise offers new chances in the field of regenerative medicine, suggesting possible triggers for Telocytes in sarcopenia and other musculoskeletal disorders, promoting adapted physical activity and rehabilitation programmes in clinical practice.

Cd34+ Stromal Cells/Telocytes in Normal and Pathological Skin

We studied CD34+ stromal cells/telocytes (CD34+SCs/TCs) in pathologic skin, after briefly examining them in normal conditions. We confirm previous studies by other authors in the normal dermis regarding CD34+SC/TC characteristics and distribution around vessels, nerves and cutaneous annexes, highlighting their practical absence in the papillary dermis and presence in the bulge region of perifollicular groups of very small CD34+ stromal cells. In non-tumoral skin pathology, we studied examples of the principal histologic patterns in which CD34+SCs/TCs have (1) a fundamental pathophysiological role, including (a) fibrosing/sclerosing diseases, such as systemic sclerosis, with loss of CD34+SCs/TCs and presence of stromal cells co-expressing CD34 and αSMA, and (b) metabolic degenerative processes, including basophilic degeneration of collagen, with stromal cells/telocytes in close association with degenerative fibrils, and cutaneous myxoid cysts with spindle-shaped, stellate and bulky vacuolated CD34+ stromal cells, and (2) a secondary reactive role, encompassing dermatitis—e.g., interface (erythema multiforme), acantholytic (pemphigus, Hailey–Hailey disease), lichenoid (lichen planus), subepidermal vesicular (bullous pemphigoid), psoriasiform (psoriasis), granulomatous (granuloma annulare)—vasculitis (leukocytoclastic and lymphocytic vasculitis), folliculitis, perifolliculitis and inflammation of the sweat and sebaceous glands (perifolliculitis and rosacea) and infectious dermatitis (verruca vulgaris). In skin tumor and tumor-like conditions, we studied examples of those in which CD34+ stromal cells are (1) the neoplastic component (dermatofibrosarcoma protuberans, sclerotic fibroma and solitary fibrous tumor), (2) a neoplastic component with varying presentation (fibroepithelial polyp and superficial myxofibrosarcoma) and (3) a reactive component in other tumor/tumor-like cell lines, such as those deriving from vessel periendothelial cells (myopericytoma), epithelial cells (trichoepithelioma, nevus sebaceous of Jadassohn and seborrheic keratosis), Merkel cells (Merkel cell carcinoma), melanocytes (dermal melanocytic nevi) and Schwann cells (neurofibroma and granular cell tumor).

Telocytes in the atherosclerotic carotid artery: Immunofluorescence and TEM evidence

Telocytes, which possess distinct body shapes and long telopodes, are allocated in the vascular wall. As a fundamental cell type, telocytes construct a three-dimensional network to form a support structure for the artery. This study aims to characterize the morphology and ultrastructure of telocytes in atherosclerotic arteries. ApoE gene-deficient mice were selected as the atherosclerosis animal model and fed a high-fat diet for at least 12 weeks, and immunofluorescence assays and transmission electron microscopy techniques were used to observe changes in telocytes in atherosclerotic arteries. By immunofluorescence staining, CD34, CD117 and PDGFR-α were positive compared with negative CD28/vimentin in telocytes in the atherosclerotic carotid artery, and they were distributed in the tunica intima and tunica adventitia. Under transmission electron microscopy, the bodies of telocytes became larger, while telopodes became shorter compared with their normal condition, and a mass of lipidosomes was present during the progression of atherosclerosis. These results demonstrate that immunofluorescence with TEM is the critical method for identifying TCs and that steatosis of TCs is a reason for atherosclerotic artery dysfunction.

Telocytes are associated with tissue remodeling and angiogenesis during the postlactational involution of the mammary gland in gerbils

The postlactational involution of the mammary gland is a complex process. It involves the collapse of the alveoli and the remodeling of the extracellular matrix, which in turn implies a complex set of interrelations between the epithelial, stromal, and extracellular matrix elements. The telocytes, a new type of CD34-positive stromal cell that differs from fibroblasts in morphological terms and gene expression, were detected in the stroma of several tissues, including the mammary gland; however, their function remains elusive. The present study employed three-dimensional reconstructions and immunohistochemical, ultrastructural, and immunofluorescence techniques in histological sections of the mammary gland of the Mongolian gerbil during lactation and postlactational involution to evaluate the presence of telocytes and to investigate a possible function for these cells. By means of immunofluorescence assays for CD34 and c-kit, major markers of telocytes, and also through morphological and ultrastructural evidences, telocytes were observed to surround the mammary ducts and collapsing alveoli. It was also found that these cells are associated with matrix metalloproteinase 9, which indicates that telocytes can play a role in extracellular matrix digestion, as well as vascular endothelial growth factor, a factor that promotes angiogenesis. Together, these data indicate that telocytes are a distinct cell type in the mammary gland and, for the first time, show that these cells possibly play a role in tissue remodeling and angiogenesis during the postlactional involution of the mammary gland.

A Two-Step Immunomagnetic Microbead-Based Method for the Isolation of Human Primary Skin Telocytes/CD34+ Stromal Cells

Telocytes (TCs), commonly referred to as TCs/CD34+ stromal cells, are a peculiar type of interstitial cells with distinctive morphologic traits that are supposed to exert several biological functions, including tissue homeostasis regulation, cell-to-cell signaling, immune surveillance, and reparative/regenerative effects. At present, the majority of studies investigating these cells are mainly descriptive and focus only on their morphology, with a consequent paucity of functional data. To gain relevant insight into the possible functions of TCs, in vitro analyses are clearly required, but currently, the protocols for TC isolation are only at the early stages and not fully standardized. In the present in vitro study, we describe a novel methodology for the purification of human primary skin TCs through a two-step immunomagnetic microbead-based cell separation (i.e., negative selection for CD31 followed by positive selection for CD34) capable of discriminating these cells from other connective tissue-resident cells on the basis of their different immunophenotypic features. Our experiments clearly demonstrated that the proposed method allows a selective purification of cells exhibiting the peculiar TC morphology. Isolated TCs displayed very long cytoplasmic extensions with a moniliform silhouette (telopodes) and presented an immunophenotypic profile (CD31−/CD34+/PDGFRα+/vimentin+) that unequivocally differentiates them from endothelial cells (CD31+/CD34+/PDGFRα−/vimentin+) and fibroblasts (CD31−/CD34−/PDGFRα+/vimentin+). This novel methodology for the isolation of TCs lays the groundwork for further research aimed at elucidating their functional properties and possible translational applications, especially in the field of regenerative medicine.

Comparative Morphological Features of Syrinx in Male Domestic Fowl Gallus gallus domesticus and Male Domestic Pigeon Columba livia domestica: A Histochemical, Ultrastructural, Scanning Electron Microscopic and Morphometrical Study

Many studies have been carried out to investigate the morphological structure of the syrinx in many bird species. However, the cellular organization of the syrinx in the fowls and pigeons is still unclear. The current study revealed that in fowl and pigeon, the syrinx is formed of three main parts including tympanum (cranial) part, intermediate syringeal part, and bronchosyringeal (caudal) part, in addition to pessulus and tympaniform membranes. A great variation in the structural characteristics of syrinx of fowl and pigeon was recorded. In fowl, the tympaniform membranes showed a characteristic distribution of elastic and collagen fibers which increase the elasticity of tympaniform membranes. Moreover, the bony pessulus helps the medial tympaniform membranes to be stiffer, vibrate more strongly so that louder sound will be generated. In pigeon, the lateral tympaniform membrane is of greater thickness so that the oscillation of this membrane is reduced and the amplitude is lower. Moreover, the pessulus is smaller in size and is formed mainly of connective tissue core (devoid of cartilaginous or bony plates), resulting in the failure of stretching and vibrating of the medial tympaniform membranes, that leads to the generation of deeper sound. Electron microscopic examination of the syringes of fowls and pigeons revealed numerous immune cells including dendritic cells, plasma cells, mast cells, and lymphocytes distributed within syringeal mucosa and invading the syringeal epithelium. Telocytes were first recorded in the syrinx of fowls and pigeons in this study. They presented two long telopodes that made up frequent close contacts with other neighboring telocytes, immune cells, and blood capillaries.

Telocytes inhibited inflammatory factor expression and enhanced cell migration in LPS-induced skin wound healing models in vitro and in vivo

Background

Cell proliferation and death are key components of wound healing and tissue repair. Telocytes (TCs) represent a newly discovered cell type that can protect tissue from acute injury via cell–cell communication with adjacent cells. The aim of this study was to use a mouse model of skin wound healing and lipopolysaccharide (LPS)-induced cell injury to evaluate the effects of TCs on skin wound healing in vivo and in vitro.

Material/methods

Immunohistochemical staining was performed to evaluate the alteration of TCs in tissues from normal and chronic wound patients. Then, a male C57BL/6 mouse wound model of the back was established. The mice were divided randomly into three groups, and wound healing was estimated according to the wound healing rate and histology. An LPS-induced co-culture model of a mouse lung telocyte cell line (TCs) with human keratinocyte (HaCaT), human dermal microvascular endothelial cell (HDMEC) or murine fibroblast (L929) cell lines was established to analyse the effects of TCs on constitutive cell types of the skin. Cell proliferation, migration and apoptosis were examined, and reactive oxygen species (ROS) and inflammatory factors in HaCaT cells, HDMECs, and L929 cells were detected to study the mechanisms involved in TC protection in skin wounds.

Results

TCs were significantly increased in tissues from chronic wound patients compared with healthy controls. Wound healing was significantly improved in wound mouse models treated with exogenous TCs compared with LPS-induced models. TCs reversed the LPS-induced inhibition of HaCaT cells and HDMECs and reduced the LPS-induced apoptosis of HaCaT cells and the death ratios of HDMECs and L929 cells. TCs reversed LPS-induced ROS in HDMECs and L929 cells and decreased inflammatory factor mRNA levels in HaCaT cells, HDMECs and L929 cells.

Conclusions

TCs reduce wound healing delay, and inflammatory responses caused by LPS might be mediated by inflammatory inhibition, thus restricting apoptosis and promoting migration of the main component cell types in the skin.

Ultrastructural and immunohistochemical characteristics of telocytes in human scalp tissue

This study was designed to characterize the location, morphology and ultrastructure of telocytes (TCs) in human scalp tissue. After obtaining approval for this study and informed consent from the patient, a scalp specimen was obtained. The distribution and morphology of TCs in human scalp tissue was assessed by immunohistochemical staining of CD34 and CD117/c-KIT, and the ultrastructure of TCs was investigated using transmission electron microscopy (TEM). Immunohistochemical staining of CD34 revealed that TCs were located in the connective tissue of human scalp, and were concentrated around hair follicles (HFs), blood vessels, sweat glands, sebaceous glands and adipose lobules. Immunohistochemical staining of CD117 revealed that TCs were mainly located in the dermis of human scalp, surrounding the HFs and sweat glands. Under TEM, TCs were seen and confirmed by their special morphological features. These cells were spindle-shaped, had small cell bodies and long thin processes, and surrounded stem cell clusters in the bulge region of HFs. These results demonstrate that TCs in human scalp were positive for CD34 and CD117, and their strategic positioning surrounding stem cells suggests their possible involvement in local regeneration, remodeling and homeostasis of the skin.

Emerging diverse roles of telocytes

Since the first description of 'interstitial cells of Cajal' in the mammalian gut in 1911, scientists have found structurally similar cells, now termed telocytes, in numerous tissues throughout the body. These cells have recently sparked renewed interest, facilitated through the development of a molecular handle to genetically manipulate their function in tissue homeostasis and disease. In this Primer, we discuss the discovery of telocytes, their physical properties, distribution and function, focusing on recent developments in the functional analysis of Foxl1-positive telocytes in the intestinal stem cell niche, and, finally, the current challenges of studying telocytes as a distinct cell type.

Expression of toll-like receptors and their regulatory roles in murine cardiac telocytes

Telocytes, newly discovered in the last decade, are interstitial cells found in numerous organs, with multiple proposed potential biological functions. Toll-like receptors (TLRs) play an important role in innate and adaptive immunity by recognizing pathogen-associated molecular patterns (PAMPs). However, it is still unknown whether telocytes express these innate receptors. We sought to determine the expression and role of TLRs in telocytes. In our study, we primarily detected TLR1-9 expression in telocytes. The proliferation, apoptosis and immunoregulatory activity of telocytes activated with or without TLR ligands were determined. Our results showed that purified telocytes expressed TLR2, TLR3 and TLR5. In particular, telocytes expressed high levels of TLR2 as observed using flow cytometry. When we stimulated telocytes with TLR2 or TLR3 agonists (Pam3CSK4, PolyI:C), iNOS expression was greatly increased after Pam3CSK4 treatment. Additionally, telocyte proliferation was reduced and cell apoptosis was increased after TLR agonist stimulation. A co-culture experiment showed that supernatant from telocytes pretreated with Pam3CSK4 inhibited T cell activation much more than that from untreated telocytes and this effect was mediated by iNOS. Overall, our results demonstrated TLR expression on telocytes for the first time and provided evidence of an immunoregulatory role of telocytes, indicating their clinical potential.

How hormones may modulate human skin pigmentation in melasma: An in vitro perspective

Melasma is a common acquired hyperpigmentary disorder occurring primarily in photo-exposed areas and mainly affecting women of childbearing age. To decipher the role of sex hormones in melasma, this viewpoint reviews the effects of sex hormones on cutaneous cells cultured in monolayers, in coculture, in 3D models and explants in the presence or the absence of UV. The data show that sex steroid hormones, especially oestrogen, can modulate in vitro pigmentation by stimulating melanocytes and keratinocyte pro-pigmentary factors, but not via fibroblast or mast cell activation. In vitro data suggest that oestrogen acts on endothelial cell count, which may in turn increase endothelin-1 concentrations. However, data on explants revealed that sex steroid even at doses observed during pregnancy cannot induce melanogenesis alone nor melanosome transfer but that it acts in synergy with UVB. In conclusion, we hypothesize that in predisposed persons, sex steroid hormones initiate hyperpigmentation in melasma by amplifying the effects of UV on melanogenesis via direct effects on melanocytes or indirect effects via keratinocytes and on the transfer of melanosomes. They also help to sustain hyperpigmentation by increasing the number of blood vessels and, in turn, the level of endothelin-1.

Telocytes role during the postnatal development of the Mongolian gerbil jejunum

Telocytes are recently categorised CD34-positive interstitial cells that comprise the cells which were previously called interstitial Cajal-like cells (ICLCs). These were detected in the stroma of various organs such as the prostate, lungs, mammary glands, liver, gallbladder, and jejunum, among others. Several functions have been proposed for telocytes, such as a supportive role in smooth muscle contraction and immune function in adult organs, and tissue organisation and paracrine signalling during development, as well as others. In the jejunum, little is known about the function of telocytes in the adult organ, or is there any information about when these cells develop or if they could have an auxiliary role in the development of the jejunum. The present study employed histological, immunohistochemical and immunofluorescence techniques on histological sections of the jejunum of Mongolian gerbil pups on two different days of postnatal development of the jejunum, covering the maturation period of the organ. By immunolabelling for CD34, it was observed that telocytes are already present in the jejunum during the first week of postnatal life and exist in close association with the developing muscularis mucosae, which are therefore TGFβ1-positive. The telocytes are still present at the end of the first month of life, and a portion of them present co-localisation with c-Kit. Fibroblast-like cells, which are exclusively c-Kit-positive, are also observed, which may indicate the presence of interstitial Cajal cells (ICCs). Finally, it can be hypothesised that a portion of the telocytes may give rise to ICCs, which are c-Kit-positive but CD34 negative.

Telocytes in normal and keratoconic human cornea: an immunohistochemical and transmission electron microscopy study

Telocytes (TC) are typically defined as cells with telopodes by their ultrastructural features. Their presence was reported in the interstitium of various organs in vertebrates, including humans. However, no study has yet described the presence of TC in the human eye and in particular, within the stromal compartment of the cornea. To address this issue, samples of normal and pathologic (keratoconic) human corneas were tested by immunohistochemistry for CD34, platelet‐derived growth factor receptor α (PDGFRα) and c‐kit/CD117 or examined by transmission electron microscopy. We found that TC coexpressing CD34 and PDGFRα were distributed throughout the whole normal corneal stroma with different TC subtypes being distinguishable on the basis of the expression of the stemness marker c‐kit (i.e. c‐kit‐positive and c‐kit‐negative TC subpopulations). Transmission electron microscopy examination confirmed the existence of spindle‐shaped and bipolar TC typically displaying two long and thin moniliform telopodes establishing intercellular contacts formed by gap junctions. Keratoconic corneas were characterized by ultrastructural damages and patchy loss of TC with an almost complete depletion of the c‐kit‐positive TC subpopulation. We propose that TC may contribute to the maintenance of corneal stromal homoeostasis and that, in particular, the c‐kit‐positive TC subtype might have stemness capacity participating in corneal regeneration and repair processes. Further studies are needed to clarify the differential roles of corneal TC subtypes as well as the possible therapeutic applications of TC in degenerative corneal disorders such as keratoconus.

The Cutaneous Telocytes

Telocytes (TCs) are interstitial cells found in stroma of many organs, including the skin dermis. Ultrastructurally, normal skin TCs recapitulates all the previously documented features in interstitum of other organs. Their (ultra)structural hallmark is the presence of particular shaped cellular prolongations (termed telopodes), along other features as cellular organelles representation and their distribution within cell body and its prolongations. Transmission electron microscopy (TEM) or high magnification light microscopy indicated that the particular shape of telopodes alternate characteristically thin segments (termed podomeres) and dilated segments (called podoms). A new and powerful technique, focused ion beam scanning electron microscopy (FIB-SEM), indicated that, ultrastructurally, telopodes could be either irregular ribbon-like structures, or uneven tubular-like structures. TEM images shown that podoms consists mitochondria, elements of endoplasmic reticulum and caveolae. Immunohisochemical studies on skin TCs revealed their positive expression for CD34 and PDGFRα, but for vimentin and c-kit, also. In normal dermis, TCs are involved in junctions, either homocellular (TCs-TCs), or heterocellular (TCs - other type of cells). The junctional attribute of TCs underlies their ability of forming a 3D network within dermis. Beyond the physical interactions, the connections between TCs and other cells could be also chemical, by paracrine secretion via shed vesicles as ultrastructural studies demonstrated. In normal dermis, TCs were found distributed in particular spatial relationships with other interstitial structures and/or cells: vascular structures, nerves, skin adnexa, stem cells and immune reactive cells.To date, the study of TCs was approached into two pathologic conditions: systemic sclerosis and psoriasis. In both diseases, the normal ultrastructure of TCs and also their distribution were shown to be altered. Moreover, the pattern of TCs ultrastructural changes differs in systemic sclerosis (cytoplasmic vacuolization, swollen mitochondria, lipofuscin bodies) from those appeared in psoriasis, characterized by important dystrophic changes (telopodes fragmentation, cytoplasmic disintegration, apoptotic nuclei, nuclear extrusions). Furthermore, in psoriasis, the lesional remission is (ultra)structurally displaying a recovery of dermal TCs at values similar to normal.Considering TCs ultrastructural features, their connections and spatiality in normal dermis and also their pathologic changes, TCs are credited with roles in skin homeostasis and/or pathogeny of dermatological disorders. In our opinion, further researches should be focused on identifying a specific marker for TCs and also on comprehending the pattern of their response in different dermatoses.

Telocytes in Chronic Inflammatory and Fibrotic Diseases

Telocytes are a peculiar stromal (interstitial) cell type implicated in tissue homeostasis and development, as well as in the pathophysiology of several disorders. Severe damage and reduction of telocytes have been reported during fibrotic remodeling of multiple organs in various diseases, including scleroderma, Crohn's disease, ulcerative colitis, and liver fibrosis, as well as in chronic inflammatory lesions like those of primary Sjögren's syndrome and psoriasis. Owing to their close relationship with stem cells, telocytes are also supposed to contribute to tissue repair/regeneration. Indeed, telocytes are universally considered as "connecting cells" mostly oriented to intercellular signaling. On the basis of recent promising experimental findings, in the near future, telocyte transplantation might represent a novel therapeutic opportunity to control the evolution of chronic inflammatory and fibrotic diseases. Notably, there is evidence to support that telocytes could help in preventing abnormal activation of immune cells and fibroblasts, as well as in attenuating the altered matrix organization during the fibrotic process. By targeting telocytes alone or in tandem with stem cells, we might be able to promote regeneration and prevent the evolution to irreversible tissue injury. Besides exogenous transplantation, exploring pharmacological or non-pharmacological methods to enhance the growth and/or survival of telocytes could be an additional therapeutic strategy for many disorders.

Telocytes - a Hope for Cardiac Repair after Myocardial Infarction

Cardiovascular diseases, particularly myocardial infarction, remain the leading cause of morbidity and mortality worldwide, even though pharmacological and interventional therapies improved significantly in the last years. Moreover, despite encouraging results of cell - based therapies in experimental myocardial infarction models, clinical trials showed inconsistent and modest efficiency. Therefore the next step should be the revealing of a new cell type, capable of regenerating the damaged myocardium. Telocytes (TCs), a relatively new type of interstitial cells, were described few years ago and are credited with important roles in regenerative therapies. In this paper we review their most important characteristics and functions, showing the evidences of their potential role in cardiac repair and regeneration. Our research leads to the conclusion that TCs might be a novel target for therapeutic strategies in myocardial infarction.

Telocytes heterogeneity: From cellular morphology to functional evidence

Telocytes (TCs), located ubiquitously in the internal organs of vertebrates, are a heterogeneous, recently described, cell population of the stromal space. Characterized by lengthy cytoplasmic extensions that can reach tens of microns and are called telopodes (Tps), TCs are difficult to see using conventional microscopes. It was the electron microscopy which led to their first identification and Popescu's team the first responsible for the reconstructions indicating TCs 'organization' in a three-dimensional (3D) network that is believed to be accountable for the complex roles of TCs. Gradually, it became increasingly evident that TCs are difficult to characterize in terms of immunophenotype and that their phenotype is different depending on the location and needs of the tissue at one time. This review discusses the growing body of evidence accumulated since TCs were discovered and highlights how the complex interplay between TCs and stem cells might be of importance for tissue engineering and regenerative medicine.

Telocytes in exercise-induced cardiac growth

Exercise can induce physiological cardiac growth, which is featured by enlarged cardiomyocyte cell size and formation of new cardiomyocytes. Telocytes (TCs) are a recently identified distinct interstitial cell type, existing in many tissues and organs including heart. TCs have been shown to form a tandem with cardiac stem/progenitor cells in cardiac stem cell niches, participating in cardiac regeneration and repair. Although exercise‐induced cardiac growth has been confirmed as an important way to promote cardiac regeneration and repair, the response of cardiac TCs to exercise is still unclear. In this study, 4 weeks of swimming training was used to induce robust healthy cardiac growth. Exercise can induce an increase in cardiomyocyte cell size and formation of new cardiomyocytes as determined by Wheat Germ Lectin and EdU staining respectively. TCs were identified by three immunofluorescence stainings including double labelling for CD34/vimentin, CD34/platelet‐derived growth factor (PDGF) receptor‐α and CD34/PDGF receptor‐β. We found that cardiac TCs were significantly increased in exercised heart, suggesting that TCs might help control the activity of cardiac stem/progenitor cells, cardiomyocytes or endothelial cells. Adding cardiac TCs might help promote cardiac regeneration and renewal.

Potential roles of telocytes in lung diseases

Telocytes (TCs) are a unique type of interstitial cells with specific, extremely long prolongations named telopodes (Tps), as shown by immune-positive staining against CD34, c-kit and vimentin. They were found in many organs of mammals, with potential biological functions, including the trachea and lung, even though the exact function remains unclear. Here, we give a historical overview of the TCs research field and summarize the latest findings associated with TCs, with a special focus on the recent progress about TCs specific gene and protein profiles that has been made in understanding that TCs may play a potential, but important, role in the pathogenesis of lung diseases.

Telocyte implications in human pathology: An overview

Telocytes are a recently described interstitial cell population widely distributed in the stromal compartment of many organs in vertebrates, including humans. Owing to their close spatial relationship with multiple cell types, telocytes are universally considered as 'connecting cells' mostly committed to intercellular signaling by converting the interstitium into an integrated system that drives organ development and contributes to the maintenance of local tissue homeostasis. Increasing evidence indicates that telocytes may cooperate with tissue-resident stem cells to foster organ repair and regeneration, and that telocyte damage and dysfunction may occur in several disorders. The goal of this review is to provide an overview of the most recent findings concerning the implication of telocytes in a variety of pathologic conditions in humans, including heart disease, chronic inflammation and multiorgan fibrosis. Based on recent promising experimental data, there is realistic hope that by targeting telocytes alone or in tandem with stem cells, we might be able to promote organ regeneration and/or prevent irreversible end-stage organ damage in different pathologies.

Immunohistochemistry of Telocytes in the Uterus and Fallopian Tubes

The seminal work of Popescu and colleagues first demonstrated the existence of a new cell type - the telocytes. We were among the first who reported the presence of such cells in the female genital tract and performed TEM examinations, as well as immunohistochemical staining in the attempt to find a specific marker. Telocytes from rat and from the human uterus and from human fallopian tube were extensively investigated initially by comparison with interstitial cells of Cajal. Progress in telocyte research led to the identification of different subtypes suggestive for a heterogeneous telocyte population which can even coexist in the same location. As a consequence, the functions of TCs are still elusive and can be considered a versatile phenomenon that depends on a variety of conditions, including signal reception and transmission of information via extracellular vesicles or by direct intercellular contact.

Decoding Telocytes

Telocytes (TCs) are a novel type of interstitial cells, with extremely long and thin cellular prolongations termed telopodes (Tps). TCs were first identified by Popescu et al. and described their finding as "cells with telopodes." The presence of TCs has been reported in the majority of tissues and organs (for details please visit www.telocytes.com ). TCs have been ignored or overlooked for a long time due to our inability to observe these cells via a light microscopy. TCs represent a distinct cell population, different from other types of interstitial cells, based on their distinct (ultra)structure, immunophenotype, microRNA profile, gene feature, proteome signature, and secretome features. As TCs have been suggested as new cellular targets for forthcoming therapies, developing specific methods to modulate TC numbers represents an important objective.