Telocytes in the Spleen

Histological study of telocytes in mice intrauterine adhesion model and their positive effect on mesenchymal stem cells in vitro

Intrauterine adhesions (IUA), the main cause of secondary infertility in women, result from irreversible fibrotic repair of the endometrium due to inflammation or human factors, accompanied by disruptions in the repair function of endometrial stem cells. This significantly impacts the physical and mental health of women in their childbearing years. Telocytes (TCs), a distinctive type of interstitial cells found in various tissues and organs, play diverse repair functions due to their unique spatial structure. In this study, we conduct the inaugural exploration of the changes in TCs in IUA disease and their potential impact on the function of stem cells. Our results show that in vivo, through double immunofluorescence staining (CD34+/Vimentin+; CD34+/CD31-), as endometrial fibrosis deepens, the number of TCs gradually decreases, telopodes shorten, and the three-dimensional structure becomes disrupted in the mouse IUA mode. In vitro, TCs can promote the proliferation and cycle of bone mesenchymal stem cells (BMSCs) by promoting the Wnt/β-catenin signaling pathway, which were inhibited using XAV939. TCs can promote the migrated ability of BMSCs and contribute to the repair of stem cells during endometrial injury. In addition, TCs can inhibit the apoptosis of BMSCs through the Bcl-2/Bax pathway. In conclusion, our study demonstrates, for the first time, the resistance role of TCs in IUA disease, shedding light on their potential involvement in endometrial repair through the modulation of stem cell function.

Ultrastructural evidence of telocytes in the embryonic chick heart

The cardiac telocyte (TC) is a novel interstitial cell type with a unique ultrastructure and great potential in therapy. The present study examined its presence in the heart of chicken embryos ageing 7-15 days old (Hamburger-Hamilton [HH] stages 31-41) using transmission electron microscopy. TCs were identified across all stages in the atrial and ventricular myocardium, close to maturing cardiomyocytes, blood vessels and lymphatics. Early-stage TCs have immature features resembling mesenchymal cells. Late-stage TCs were distinct, possessing the cytoplasmic prolongations termed telopodes (Tps), which are very long and thin, usually 1-3 in number, and display a moniliform appearance and have an average thickness below 0.2 μm. TCs residing in the epicardium and endocardium were also detected. In the subepicardium near developing coronary vessels, they were localized in the cardiac stem cell niches, coexisting with cardiac stem cells and cardiomyocyte progenitors. Electron-dense structures and the release of extracellular vesicles were observed between embryonic TCs and surrounding structures, suggesting roles in intercellular communication, cardiomyocyte differentiation and maturation, angiogenesis, and stem cell nursing and guidance.

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.

A cellular regulator of the niche: telocyte

Interstitial cells are present in the environment of stem cells in order to increase stem cell proliferation and differentiation and they are important to increase the efficiency of their transplantation. Telocytes (TCs) play an important role both in the preservation of tissue organ integrity and in the pathophysiology of many diseases, especially cancer. They make homo- or heterocellular contacts to form the structure of 3D network through their telopodes and deliver signaling molecules via a juxtacrine and/or paracrine association by budding shed vesicles into the vascular, nervous and endocrine systems. During this interaction, along with organelles, mRNA, microRNA, long non-coding RNA, and genomic DNA are transferred. This review article not only specifies the properties of TCs and their roles in the tissue organ microenvironment but also gives information about the factors that play a role in the transport of epigenetic information by TCs.

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.

Evaluation of the Plasticity of Novel Regulatory Cells-Telocytes-in the Gonad of the Male Chinese Soft-Shelled Turtle (Pelodiscus sinensis) Associated with Seasonal Reproductive Activity

Telocyte (TC)—a new type of interstitial cell with long telopodes, can form cellular junctions with various tissues or cells to participate in the regulation of multitudes of physiological activities and diseases. This study aimed to characterize the morphology, molecular features, and potential functions of hormone regulation in Chinese soft-shelled turtle (Pelodiscus sinensis) testis TCs at different reproductive stages by histological evaluation, immunohistochemistry (IHC), immunofluorescence (IF), and transmission electron microscopy. During hibernation, TCs were widely distributed in the interstitial tissue. In contrast, during reproductive activity, TCs were noted to be in close proximity with peritubular myoid cells surrounding the seminiferous tubule. Moreover, formed cell–cell junctions were observed between TCs and PTMs. The results of IHC and IF showed that the immunophenotype of testicular TCs in hibernating Chinese soft-shelled turtles is CD34+Vimentin−, while the reproductive telopodes (Tps) show low expression of vimentin. The androgen receptor is expressed in Tps of TCs of testis during hibernation. Our results showed also that TCs in seasonal breeding animals regulate the activity of neighboring cells by releasing extracellular microvesicles (EXMVs), thus influencing the activity of spermatogenesis and steroidogenesis. Consideration of our novel and interesting results indicate that the whole area warrants further research.

Telocytes: localization, structure, functions and significance in pathology

The review article presents the material analyzing and summarizing the information about the localization, ultramicroscopic structure, functions and significance in pathological processes of newly discovered cells - telocytes. The information about molecular markers of telocytes are considered. The questions about the tissue belonging of these cells are discussed, the relevance of studying their role in the development of pathological processes in different organs is emphasized. The analysis of literature data was carried out taking into account the fundamental provisions of the doctrine of the cell-differon organization of tissues.

Telocytes in Fibrosis Diseases: From Current Findings to Future Clinical Perspectives

Telocytes (TCs), a distinct type of interstitial (stromal) cells, have been discovered in many organs of human and mammal animals. TCs, which have unique morphological characteristics and abundant paracrine substance, construct a three-dimensional (3D) interstitial network within the stromal compartment by homocellular and heterocellular communications which are important for tissue homeostasis and normal development. Fibrosis-related diseases remain a common but challenging problem in the field of medicine with unclear pathogenesis and limited therapeutic options. Recently, increasing evidences suggest that where TCs are morphologically or numerically destructed, many diseases continuously develop, finally lead to irreversible interstitial fibrosis. It is not difficult to find that TCs are associated with chronic inflammation and fibrosis. This review mainly discusses relationship between TCs and the occurrence of fibrosis in various diseases. We analyzed in detail the potential roles and speculated mechanisms of TCs in onset and progression of systemic fibrosis diseases, as well as providing the most up-to-date research on the current therapeutic roles of TCs and involved related pathways. Only through continuous research and exploration in the future can we uncover its magic veil and provide strategies for treatment of fibrosis-related disease.

Telocytes Enhances M1 Differentiation and Phagocytosis While Inhibits Mitochondria-Mediated Apoptosis Via Activation of NF-κB in Macrophages

Telocytes (TCs), which are a recently discovered interstitial cell type present in various organs and tissues, perform multiple biological functions and participate in extensive crosstalk with neighboring cells. Endometriosis (EMs) is a gynecological disease characterized by the presence of viable endometrial debris and impaired macrophage phagocytosis in the peritoneal environment. Here, CD34/vimentin-positive TCs were co-cultured with RAW264.7 cells in vitro. M1/M2 differentiation-related markers were detected; phagocytosis, energy metabolism, proliferation, apoptosis, and pathway mechanisms were studied; and the mitochondrial membrane potential (ΔΨm) was measured. Furthermore, in an EMs mouse model, the differentiation of macrophages in response to treatment with TC-conditioned medium (TCM) in vivo was studied. The results showed that upon in vitro co-culture with TCM, RAW264.7 cells differentiated more toward the M1 phenotype with enhancement of phagocytosis, increase in energy metabolism and proliferation owing to reduced the loss of ΔΨm, and suppression of dexamethasone-induced apoptosis. Further, along with the activation of NF-κB, Bcl-2 and Bcl-xl, the expression of Bax, cleaved-caspase9, and cleaved-caspase3 reduced in RAW264.7 cells. In addition, the M1 subtype was found to be the dominant phenotype among tissue and peritoneal macrophages in the EMs model subjected to in vivo TCM treatment. In conclusion, TCs enhanced M1 differentiation and phagocytosis while inhibiting apoptosis via the activation of NF-κB in macrophages, which potentially inhibited the onset of EMs. Our findings provide a potential research target and the scope for developing a promising therapeutic strategy for EMs.

Architecture and Cellular Composition of the Spleen in the Japanese Quail (Coturnix japonica)

The spleen is considered a key player in birds' immunity. The stroma and the parenchyma of the spleen of the adult quail were demonstrated histologically, histochemically, and ultrastructurally. A thin capsule and the absence of trabeculae were the most characteristics of spleen stroma. The demarcation between white pulp and red pulp was not observed in the quail. White pulp formed from the periarterial lymphatic sheath and the periellipsoidal lymphatic sheath, both of which were surrounded by arteriole and ellipsoid, respectively. Ellipsoids appeared more numerous and were characterized by cuboidal lining of the epithelium and supporting cells. Red pulp consisted of sinuses and cords. White pulp and red pulp of the quail spleen contained various cells, such as red blood cells, macrophages, heterophils with characteristic granules, lymphocytes of different sizes, dendritic cells, plasma cells, and telocytes. In addition, closed circulation and open circulation established the blood flow on the spleen.

A Systematic Review on Cornea Epithelial-Stromal Homeostasis

INTRODUCTION

This review aims to summarise the role of different cells, genes, proteins and lipid in regulating cornea epithelial-stromal homeostasis.

METHODS

We performed an Entrez PubMed literature search using keywords "human," "cornea," "epithelial," "stromal," "homeostasis," "fibrosis response," and "pathogenesis" on 24th of September 2019, resulting in 35 papers, of which 18 were chosen after filtering for "English language" and "published within 10 years" as well as curation for relevance by the authors.

RESULTS

The 18 selected papers showed that corneal epithelial cells, fibroblasts and telocytes, together with genes such as Klf4, Pax6 and Id found in the cells, play important roles in achieving homeostasis to maintain corneal integrity and transparency. Proteins classified as pro-fibrotic ligands and anti-fibrotic ligands are responsible for regulating cornea stromal fibrosis and extracellular matrix deposition, thus regulators of scar formation during wound healing. Anti-inflammatory ligands and wound repairing ligands are critical in eliciting protective inflammation and promoting epithelial healing, respectively. Protein receptors located on cellular membrane play a role in maintaining intercellular connections as well as corneal hydration.

DISCUSSION/CONCLUSION

These studies prompt development of novel therapeutic strategies such as tear drops or ointments that target certain proteins to maintain corneal homeostasis. However, more in vitro and in vivo studies are required to prove the effectiveness of exogenous administration of molecules in improving healing outcome. Hence, future investigations of the molecular pathways highlighted in this review will reveal novel therapeutic tools such as gene or cell therapy to treat corneal diseases.

Immunohistochemical biomarkers and distribution of telocytes in ApoE-/- mice

Telocytes had been identified as a peculiar stromal cell type implicated in tissue homeostasis and the development and pathophysiology of diseases. Telocyte existed in most organs and tissues in humans and animals. However, few studies have examined telocytes in ApoE gene deficient mice. In our studies, we verified the existence, the morphology and immunohistochemical characteristics of telocytes in critical organs of the ApoE-/- mice. Male adult ApoE-/- mice were selected as an experimental model. Immunohistochemical bio-markers, such as CD34, CD117, CD28, Vimentin and PDGFR-α were utilized to determine the distribution and morphology of telocytes in the heart, liver and kidney. Telocyte expressed positively for CD34 and CD117, and partial telocyte and telopode expressed positively for PDGFR-α in heart and liver, but negatively in kidney. Double immunofluorescence assays for CD28/Vimentin, CD34/CD117 and CD34/PDGFR-α were used to demonstrate the biochemistry speciality of telocytes, respectively. The evidence of telocytes in the ApoE-/- mice is the first step of our sturdy, which aims to demonstrate changes in telocytes in atherosclerosis in this animal model.

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.

Recently Discovered Interstitial Cell Population of Telocytes: Distinguishing Facts from Fiction Regarding Their Role in the Pathogenesis of Diverse Diseases Called "Telocytopathies"

In recent years, the interstitial cells telocytes, formerly known as interstitial Cajal-like cells, have been described in almost all organs of the human body. Although telocytes were previously thought to be localized predominantly in the organs of the digestive system, as of 2018 they have also been described in the lymphoid tissue, skin, respiratory system, urinary system, meninges and the organs of the male and female genital tracts. Since the time of eminent German pathologist Rudolf Virchow, we have known that many pathological processes originate directly from cellular changes. Even though telocytes are not widely accepted by all scientists as an individual and morphologically and functionally distinct cell population, several articles regarding telocytes have already been published in such prestigious journals as Nature and Annals of the New York Academy of Sciences. The telocyte diversity extends beyond their morphology and functions, as they have a potential role in the etiopathogenesis of different diseases. The most commonly described telocyte-associated diseases (which may be best termed "telocytopathies" in the future) are summarized in this critical review. It is difficult to imagine that a single cell population could be involved in the pathogenesis of such a wide spectrum of pathological conditions as extragastrointestinal stromal tumors ("telocytomas"), liver fibrosis, preeclampsia during pregnancy, tubal infertility, heart failure and psoriasis. In any case, future functional studies of telocytes in vivo will help to understand the mechanism by which telocytes contribute to tissue homeostasis in health and disease.

Localization of telocytes in rabbits testis: Histological and immunohistochemical approach

Telocyte (TC) is an interesting unique interstitial cell demonstrated in many human and animal tissues and organs. This study verified, for the first time, the pattern of TC distribution in the testicular tissue of New Zealand White rabbits using histological, immunohistochemical, and electron microscopic tools. Rabbit testicular tissue samples were obtained from three pairs of adult healthy New Zealand white rabbit by surgical procedures. The testicular tissues were stained with hematoxyline-eosin, Crossmon's trichrome and Periodic acid Schiff. The immunohistochemistry was performed using three different antibodies CD34, CD117, and vimentin. The testes were examined by scanning and transmission electron microscopy. Histologically, TCs formed a sheath surrounding the seminiferous tubules. Other TCs were located in the interstitial tissue of the rabbit testis. Immunohistochemically, TCs reacted strongly with CD34, CD117, and vimentin. Scanning electron microscopic findings clearly elucidated the spreading pattern of TCs and their cytoplasmic processes with the interstitial tissue including blood vessels. Both homocellular and heterocellular junctions were demonstrated by transmission electron microscope. On the basis of TCs distribution and connections, the before mentioned data suggested that, TCs may play a potential role in maintaining the testicular construction and regulation. A future work is needed to clarify the actual role played by TCs in monitoring testicular fertility. RESEARCH HIGHLIGHTS: Telocyte (TC) is a unique cell demonstrated in human and animal tissues. TCs formed a sheath surrounding the seminiferous tubules in rabbits and may be located in interstitial tissue. Immunohistochemically, TCs reacted strongly with CD34 and CD117.

A Tale of Two Cells: Telocyte and Stem Cell Unique Relationship

Telocytes have been identified as a distinctive type of interstitial cells and have been recognized in most tissues and organs. Telocytes are characterized by having extraordinary long cytoplasmic processes, telopodes, that extend to form three-dimensional networks and commonly constitute specialized forms of cell-to-cell junctions with other neighboring cells. Telocytes have been localized in the stem cell niche of different organs such as the heart, lung, skeletal muscle, and skin. Electron microscopy and electron tomography revealed a specialized link between telocytes and stem cells that postulates a potential role for telocytes during tissue regeneration and repair. In this review, the distribution of telocytes in different stem cell niches will be explored, highlighting the intimate relationship between the two types of cells and their possible functional relationship.

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: a potential defender in the spleen of Npc1 mutant mice

Niemann–Pick disease, type C1 (Npc1), is an atypical lysosomal storage disorder caused by autosomal recessive inheritance of mutations in Npc1 gene. In the Npc1 mutant mice (Npc1^−/−), the initial manifestation is enlarged spleen, concomitant with free cholesterol accumulation. Telocytes (TCs), a novel type of interstitial cell, exist in a variety of tissues including spleen, presumably thought to be involved in many biological processes such as nursing stem cells and recruiting inflammatory cells. In this study, we found that the spleen is significantly enlarged in Npc1^−/− mice, and the results from transmission electron microscopy examination and immunostaining using three different TCs markers, c‐Kit, CD34 and Vimentin revealed significantly increased splenic TCs in Npc1^−/− mice. Furthermore, hematopoietic stem cells and macrophages were also elevated in Npc1^−/− spleen. Taken together, our data indicate that splenic TCs might alleviate the progress of splenic malfunction via recruiting hematopoietic stem cells and macrophages.

Subsets of telocytes: Myocardial telocytes

Telocytes (TCs) are morphologically defined as small-sized cells with long, thin, moniliform processes called telopodes (Tps). Numerous papers imply that TCs are a distinctive cell type, and that transmission electron microscopy (TEM) is the gold standard tool for their identification. We aimed to reproduce previous studies on myocardial TCs to check their validity. For this purpose we performed an immunohistochemical study on human cardiac samples from six autopsied donor cadavers, using antibodies against CD10, CD31, CD34, CD146, Ki67, alpha-smooth muscle actin (α-SMA), Platelet-Derived Growth Factor Receptor-alpha (PDGFRα) and laminin. Additionally we performed a TEM study on cardiac samples from three human autopsied donor cadavers and five adult Sprague-Dawley rats. We found endothelial cells (ECs), cords, and filopodia-projecting endothelial tip cells (ETCs) that expressed CD10, CD31, CD34, CD146, and PDGFR-α. Often, endothelial cells closely neighbored the sarcolemmal basal laminae. Endothelial progenitor cells, as well as nascent capillaries, were CD31+/CD34+. Proliferative endothelial cells expressed Ki67. In larger vessels we found pericytes that expressed CD146 and α-SMA; scarce α-SMA-expressing spindle-shaped cells lining cardiomyocytes were suggestive of a pericytic role in angiogenic sprout guidance. The TEM study showed that endothelial tubes are almost exclusively found in the narrow myocardial interstitia. ECs that built them up appeared identical to the cells that previous TEM studies have suggested to be myocardial telocytes. A subset of stromal cells with TC-like phenotype and telopodes-like processes actually seem to configure blood vessels, and therefore belong to the endothelial lineage. This study shows that data presented in previous studies on myocardial telocytes is not enough to allow the reproducibility of the results. At least a subset of cells considered to be TCs might belong to the endothelial lineage.

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.

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.

The History of Telocyte Discovery and Understanding

Telocytes (TCs) are identified as a peculiar cell type of interstitial cells in various organs. The typical features of TCs from the other cells are the extending cellular process as telopodes with alternation of podomeres and podoms. Before the year of 2010, TCs were considered as interstitial Cajal-like cells because of the similar morphology and immunohistochemical features with interstitial cells of Cajal which were found more than 100 years ago and considered to be pacemakers for gut motility. Subsequently, it demonstrated that TCs were not Cajal-like cells, and thus the new name "telocyte" was proposed in 2010. With the help of different techniques, e.g., transmission electron microscopy, immunohistochemistry, or omics science, TCs have been detected in various tissues and organs from different species. The pathological role of TCs in different diseases was also studied. According to observation in situ or in vitro, TCs played a vital role in mechanical support, signaling transduction, tissue renewal or repair, immune surveillance, and mechanical sensor via establishing homo- or heterogenous junctions with neighboring cells to form 3D network or release extracellular vesicles to form juxtacrine and paracrine. This review will introduce the origin, distribution, morphology, functions, omics science, methods, and interaction of TCs with other cells and provide a better understanding of the new cell type.

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.