Relationships between telocytes and cardiomyocytes during pre‐ and post‐natal life

Evidence has been given that the adult heart contains a specific population of stromal cells lying in close spatial relationship with cardiomyocytes and with cardiac stem cells in sub‐epicardial cardiogenic niches. Recently termed ‘telocytes’ because of their long cytoplasmic processes embracing the parenchymal cells, these cells have been postulated to be involved in heart morphogenesis. In our opinion, investigating the occurrence and morphology of telocytes during heart histogenesis may shed further light on this issue. Our findings show that typical telocytes are present in the mouse heart by early embryonic to adult life. These cells closely embrace the growing cardiomyocytes with their long, slender cytoplasmic processes. Hence, in the developing myocardium, telocytes may play nursing and guiding roles for myocardial precursors to form the correct three‐dimensional tissue architectural pattern, as previously suggested.     

Telocytes accompanying cardiomyocyte in primary culture: two- and three-dimensional culture environment

Recently, the presence of telocytes was demonstrated in human and mammalian tissues and organs (digestive and extra-digestive organs, genitourinary organs, heart, placenta, lungs, pleura, striated muscle). Noteworthy, telocytes seem to play a significant role in the normal function and regeneration of myocardium. By cultures of telocytes in two- and three-dimensional environment we aimed to study the typical morphological features as well as functionality of telocytes, which will provide important support to understand their in vivo roles. Neonatal rat cardiomyocytes were isolated and cultured as seeding cells in vitro in two-dimensional environment. Furthermore, engineered myocardium tissue was constructed from isolated cells in three-dimensional collagen/Matrigel scaffolds. The identification of telocytes was performed by using histological and immunohistochemical methods. The results showed that typical telocytes are distributed among cardiomyocytes, connecting them by long telopodes. Telocytes have a typical fusiform cell body with two or three long moniliform telopodes, as main characteristics. The vital methylene blue staining showed the existence of telocytes in primary culture. Immunohistochemistry demonstrated that some c-kit or CD34 immuno-positive cells in engineered heart tissue had the morphology of telocytes, with a typical fusiform cell body and long moniliform telopodes. Also, a significant number of vimentin+ telocytes were present within engineered heart tissue. We suggest that the model of three-dimensional engineered heart tissue could be useful for the ongoing research on the functional relationships of telocytes with cardiomyocytes. Because the heart has the necessary potential of changing the muscle and non-muscle cells during the lifetime, telocytes might play an active role in the heart regeneration process. Moreover, telocytes might be a useful tool for cardiac tissue engineering.     

TELOCYTES IN ENDOCARDIUM: Electron Microscope Evidence

The term TELOCYTES was very recently introduced, for replacing the name Interstitial Cajal‐Like Cells (ICLC). In fact, telocytes are not really Cajal‐like cells, they being different from all other interstitial cells by the presence of telopodes, which are cell‐body prolongations, very thin (under the resolving power of light microscopy), extremely long (tens up to hundreds of micrometers), with a moniliform aspect (many dilations along), and having caveolae. The presence of telocytes in epicardium and myocardium was previously documented. We present here electron microscope images showing the existence of telocytes, with telopodes, at the level of mouse endocardium. Telocytes are located in the subendothelial layer of endocardium, and their telopodes are interposed in between the endocardial endothelium and the cardiomyocytes bundles. Some telopodes penetrate from the endocardium among the cardiomyocytes and surround them, eventually. Telopodes frequently establish close spatial relationships with myocardial blood capillaries and nerve endings. Because we may consider endocardium as a ‘blood–heart barrier’, or more exactly as a ‘blood–myocardium barrier’, telocytes might have an important role in such a barrier being the dominant cell population in subendothelial layer of endocardium.     

Tracheal telocytes

Recently, a novel type of stromal cell - the telocytes (TC) - was identified in mouse trachea. These cells are known to possess the ultrastructural characteristics, which support their role in intercellular signaling. We found TC in all stromal compartments of the tracheal wall. TC with long prolongations (telopodes, Tp) were lining longitudinally the collagen bundles, and were serially arranged (end-to-end connections of Tp were found). Noteworthy, Tp frequently establish stromal synapses with mast cells (MC). Primary cilia were also identified in TC. In conclusion, tracheal TC could be involved in the tracheal regulation (e.g. secretion, contractility). The tandem TC-MC deserves further investigations.     

Telocytes in human epicardium

The existence of the epicardial telocytes was previously documented by immunohistochemistry (IHC) or immunofluorescence. We have also demonstrated recently that telocytes are present in mice epicardium, within the cardiac stem‐cell niches, and, possibly, they are acting as nurse cells for the cardiomyocyte progenitors. The rationale of this study was to show that telocytes do exist in human (sub)epicardium, too. Human autopsy hearts from 10 adults and 15 foetuses were used for conventional IHC for c‐kit/CD117, CD34, vimentin, S‐100, τ, Neurokinin 1, as well as using laser confocal microscopy. Tissue samples obtained by surgical biopsies from 10 adults were studied by digital transmission electron microscopy (TEM). Double immunolabelling for c‐kit/CD34 and, for c‐kit/vimentin suggests that in human beings, epicardial telocytes share similar immunophenotype features with myocardial telocytes. The presence of the telocytes in human epicardium is shown by TEM. Epicardial telocytes, like any of the telocytes are defined by telopodes, their cell prolongations, which are very long (several tens of μm), very thin (0.1–0.2 μm, below the resolving power of light microscopy) and with moniliform configuration. The interconnected epicardial telocytes create a 3D cellular network, connected with the 3D network of myocardial telocytes. TEM documented that telocytes release shed microvesicles or exocytotic multivesicular bodies in the intercellular space. The human epicardial telocytes have similar phenotype (TEM and IHC) with telocytes located among human working cardiomyocyte. It remains to be established the role(s) of telocytes in cardiac renewing/repair/regeneration processes, and also the pathological aspects induced by their ‘functional inhibition’, or by their variation in number. We consider telocytes as a real candidate for future developments of autologous cell‐based therapy in heart diseases.     

Cardiac telocytes exist in the adult Xenopus tropicalis heart

Recent research has revealed that cardiac telocytes (CTs) play an important role in cardiac physiopathology and the regeneration of injured myocardium. Recently, we reported that the adult Xenopus tropicalis heart can regenerate perfectly in a nearly scar‐free manner after injury via apical resection. However, whether telocytes exist in the X tropicalis heart and are affected in the regeneration of injured X tropicalis myocardium is still unknown. The present ultrastructural and immunofluorescent double staining results clearly showed that CTs exist in the X tropicalis myocardium. CTs in the X tropicalis myocardium were mainly twined around the surface of cardiomyocyte trabeculae and linked via nanocontacts between the ends of the telopodes, forming a three‐dimensional network. CTs might play a role in the regeneration of injured myocardium.     

Electron microscopy of human fascia lata: focus on telocytes

From the histological point of view, fascia lata is a dense connective tissue. Although extracellular matrix is certainly the most predominant fascia's feature, there are also several cell populations encountered within this structure. The aim of this study was to describe the existence and characteristics of fascia lata cell populations viewed through a transmission electron microscope. Special emphasis was placed on telocytes as a particular interstitial cell type, recently discovered in a wide variety of tissues and organs such as the heart, skeletal muscles, skin, gastrointestinal tract, uterus and urinary system. The conducted study confirmed the existence of a telocyte population in fascia lata samples. Those cells fulfil main morphological criteria of telocytes, namely, the presence of very long, thin cell processes (telopodes) extending from a relatively small cell body. Aside from telocytes, we have found fibroblasts, mast cells and cells with features of myofibroblastic differentiation. This is the first time it has been shown that telocytes exist in human fascia. Currently, the exact role of those cells within the fascia is unknown and definitely deserves further attention. One can speculate that fascia lata telocytes likewise telocytes in other organs may be involved in regeneration, homeostasis and intracellular signalling.     

Uptake and intracytoplasmic storage of pigmented particles by human CD34+ stromal cells/telocytes: endocytic property of telocytes

We studied the phagocytic‐like capacity of human CD34+ stromal cells/telocytes (TCs). For this, we examined segments of the colon after injection of India ink to help surgeons localize lesions identified at endoscopy. Our results demonstrate that CD34+ TCs have endocytic properties (phagocytic‐like TCs: phTCs), with the capacity to uptake and store India ink particles. phTCs conserve the characteristics of TCs (long, thin, bipolar or multipolar, moniliform cytoplasmic processes/telopodes, with linear distribution of the pigment) and maintain their typical distribution. Likewise, they are easily distinguished from pigment‐loaded macrophages (CD68+ macrophages, with oval morphology and coarse granules of pigment clustered in their cytoplasm). A few c‐kit/CD117+ interstitial cells of Cajal also incorporate pigment and may conserve the phagocytic‐like property of their probable TC precursors. CD34+ stromal cells in other locations (skin and periodontal tissues) also have the phagocytic‐like capacity to uptake and store pigments (hemosiderin, some components of dental amalgam and melanin). This suggests a function of TCs in general, which may be related to the transfer of macromolecules in these cells. Our ultrastructural observation of melanin‐storing stromal cells with characteristics of TCs (telopodes with dichotomous branching pattern) favours this possibility. In conclusion, intestinal TCs have a phagocytic‐like property, a function that may be generalized to TCs in other locations. This function (the ability to internalize small particles), together with the capacity of these cells to release extracellular vesicles with macromolecules, could close the cellular bidirectional cooperative circle of informative exchange and intercellular interactions.     

Tbx18+心外膜祖细胞参与成年小鼠部分心脏组织形成

转录因子Tbx18在胚胎心脏发育过程中起重要调控作用,是心外膜祖细胞标记之一|故以Tbx18为标记的阳性祖细胞群被称为：Tbx18+心外膜祖细胞(epicardial progenitor cells, EPCs)。小鼠胚胎、新生和成年期心脏组织细胞的特性区别较大,成年小鼠的心脏属于终末分化组织。但是,Tbx18+EPCs对成年小鼠心脏组织的贡献大小尚存争议。本研究拟定量分析Tbx18+EPCs对成年小鼠心脏组织的贡献大小。采用整体和组织切片X-gal染色检测成年心脏组织LacZ的表达|荧光激活细胞分选法(fluorescence activated cell sorting,FACS)分离成年Tbx18Cre/R26EYFP小鼠心脏组织EYFP+细胞。结果显示,在Tbx18+EPCs遗传谱系示踪小鼠,报告基因LacZ和EYFP在成年小鼠心脏的心室、心房、冠状动脉、室间隔等处表达|成年Tbx18Cre/R26EYFP小鼠心脏组织细胞用FACS分离,分选的EYFP+细胞比例平均约为33.94%。由此可见,成年小鼠心脏的心室、心房、冠状动脉、室间隔等心脏组织均可来源于Tbx18+EPCs|约1/3成年小鼠心脏组织细胞来源于Tbx18+EPCs。故Tbx18+EPCs参与成年小鼠心脏组织的部分形成。     

Morphologic evidence of telocytes in human thyroid stromal tissue

Despite the evidence accumulated over the past decade that telocytes (TCs) are a distinctive, though long neglected, cell entity of the stromal microenvironment of several organs of the human body, to date their localization in the endocrine glands remains almost unexplored. This study was therefore undertaken to examine the presence and characteristics of TCs in normal human thyroid stromal tissue through an integrated morphologic approach featuring light microscopy and ultrastructural analysis. TCs were first identified by immunohistochemistry that revealed the existence of an intricate network of CD34⁺ stromal cells spread throughout the thyroid interfollicular connective tissue. Double immunofluorescence allowed to clearly differentiate CD34⁺ stromal cells lacking CD31 immunoreactivity from neighbour CD31⁺ microvascular structures, and the evidence that these stromal cells coexpressed CD34 and platelet‐derived growth factor receptor α further strengthened their identification as TCs. Transmission electron microscopy confirmed the presence of stromal cells ultrastructurally identifiable as TCs projecting their characteristic cytoplasmic processes (i.e., telopodes) into the narrow interstitium between thyroid follicles and blood microvessels, where telopodes intimately surrounded the basement membrane of thyrocytes. Collectively, these morphologic findings provide the first comprehensive demonstration that TCs are main constituents of the human thyroid stroma and lay the necessary groundwork for further in‐depth studies aimed at clarifying their putative implications in glandular homeostasis and pathophysiology.     

Telocytes in the human kidney cortex

Renal interstitial cells play an important role in the physiology and pathology of the kidneys. As a novel type of interstitial cell, telocytes (TCs) have been described in various tissues and organs, including the heart, lung, skeletal muscle, urinary tract, etc. ( www.telocytes.com ). However, it is not known if TCs are present in the kidney interstitium. We demonstrated the presence of TCs in human kidney cortex interstitium using primary cell culture, transmission electron microscopy (TEM) and in situ immunohistochemistry (IHC). Renal TCs were positive for CD34, CD117 and vimentin. They were localized in the kidney cortex interstitial compartment, partially covering the tubules and vascular walls. Morphologically, renal TCs resemble TCs described in other organs, with very long telopodes (Tps) composed of thin segments (podomers) and dilated segments (podoms). However, their possible roles (beyond intercellular signalling) as well as their specific phenotype in the kidney remain to be established.     

Heterocellular communication in the heart: electron tomography of telocyte-myocyte junctions

Myocardium is composed of two main cell populations: cardiomyocytes (CMs) and interstitial cells (e.g. fibroblasts, immunoreactive cells, capillaries). However, very recently we have showed that a novel type of interstitial cell called telocytes (TCs) does exist in epi-, myo- and endocardium. They have very long and thin telopodes (Tp) formed by alternating podomeres and podoms. Heterocellular communication between TCs and CMs it is supposed to occur by shed vesicles and close apposition. If TCs have to play a role in cardiac physiology it is expected to develop direct and unambiguous contacts with CMs. Because a clear membrane-to-membrane junction has not been reported by electron microscopy we have investigated the heterocellular communication in the mouse heart by electron tomography. This advanced technique showed that small dense structures (10-15 nm nanocontacts) directly connect TCs with CMs. More complex and atypical junctions could be observed between TCs and CMs at the level of intercalated discs. This study proves that TCs and CMs are directly connected and might represent a 'functional unit'.     

Persistent expression of the ATP-binding cassette transporter, Abcg2, identifies cardiac SP cells in the developing and adult heart

Stem cells are important in the maintenance and repair of adult tissues. A population of cells, termed side population (SP) cells, has stem cell characteristics as they have been shown to contribute to diverse lineages. In this study, we confirm that Abcg2 is a determinant of the SP cell phenotype. Therefore, we examined Abcg2 expression during murine embryogenesis and observed robust expression in the blood islands of the E8.5 yolk sac and in developing tissues including the heart. During the latter stages of embryogenesis, Abcg2 identifies a rare cell population in the developing organs. We further establish that the adult heart contains an Abcg2 expressing SP cell population and these progenitor cells are capable of proliferation and differentiation. We define the molecular signature of cardiac SP cells and compare it to embryonic stem cells and adult cardiomyocytes using emerging technologies. We propose that the cardiac SP cell population functions as a progenitor cell population for the development, maintenance, and repair of the heart.     

Cardiac telocytes were decreased during myocardial infarction and their therapeutic effects for ischaemic heart in rat

Recently, cardiac telocytes were found in the myocardium. However, the functional role of cardiac telocytes and possible changes in the cardiac telocyte population during myocardial infarction in the myocardium are not known. In this study, the role of the recently identified cardiac telocytes in myocardial infarction (MI) was investigated. Cardiac telocytes were distributed longitudinally and within the cross network of the myocardium, which was impaired during MI. Cardiac telocytes in the infarction zone were undetectable from approximately 4 days to 4 weeks after an experimental coronary occlusion was used to induce MI. Although cardiac telocytes in the non‐ischaemic area of the ischaemic heart experienced cell death, the cell density increased approximately 2 weeks after experimental coronary occlusion. The cell density was then maintained at a level similar to that observed 1–4 days after left anterior descending coronary artery (LAD)‐ligation, but was still lower than normal after 2 weeks. We also found that simultaneous transplantation of cardiac telocytes in the infarcted and border zones of the heart decreased the infarction size and improved myocardial function. These data indicate that cardiac telocytes, their secreted factors and microvesicles, and the microenvironment may be structurally and functionally important for maintenance of the physiological integrity of the myocardium. Rebuilding the cardiac telocyte network in the infarcted zone following MI may be beneficial for functional regeneration of the infarcted myocardium.     

Cytoskeletal heart-enriched actin-associated protein (CHAP) is expressed in striated and smooth muscle cells in chick and mouse during embryonic and adult stages

We recently identified a new Z-disc protein, CHAP (Cytoskeletal Heart-enriched Actin-associated Protein), which is expressed in striated muscle and plays an important role during embryonic muscle development in mouse and zebrafish. Here, we confirm and further extend these findings by (i) the identification and characterization of the CHAP orthologue in chick and (ii) providing a detailed analysis of CHAP expression in mouse during embryonic and adult stages. Chick CHAP contains a PDZ domain and a nuclear localization signal, resembling the human and mouse CHAPa. CHAP is expressed in the developing heart and somites, as well as muscle precursors of the limb buds in mouse and chick embryos. CHAP expression in heart and skeletal muscle is maintained in adult mice, both in slow and fast muscle fibers. Moreover, besides expression in striated muscle, we demonstrate that CHAP is expressed in smooth muscle cells of aorta, carotid and coronary arteries in adult mice, but not during embryonic development.     

Extrinsic regulation of cardiomyocyte differentiation of embryonic stem cells

Cardiovascular disease is one of leading causes of death throughout the U.S. and the world. The damage of cardiomyocytes resulting from ischemic injury is irreversible and leads to the development of progressive heart failure, which is characterized by the loss of functional cardiomyocytes. Because cardiomyocytes are unable to regenerate in the adult heart, cell-based therapy of transplantation provides a potential alternative approach to replace damaged myocardial tissue and restore cardiac function. A major roadblock toward this goal is the lack of donor cells; therefore, it is urgent to identify the cardiovascular cells that are necessary for achieving cardiac muscle regeneration. Pluripotent embryonic stem (ES) cells have enormous potential as a source of therapeutic tissues, including cardiovascular cells; however, the regulatory elements mediating ES cell differentiation to cardiomyocytes are largely unknown. In this review, we will focus on extrinsic factors that play a role in regulating different stages of cardiomyocyte differentiation of ES cells.     

动物特络细胞的研究进展

特络细胞是一种新型间质细胞,其最显著的形态特征是具有极其细长且粗细不均而呈念珠形的细胞突起,可以和周围的同/异型细胞、血管、神经末梢等形成细胞连接.特络细胞还可释放细胞外囊泡(EVs)和其他信号分子,从而发挥其潜在的生理功能.先前的研究表明,特络细胞的功能与动物组织再生有关,因此对于低等动物特络细胞的研究有助于进一步理...     

Cardiomyocyte precursors and telocytes in epicardial stem cell niche: electron microscope images

A highly heterogeneous population of stem and progenitor cells has been described by light immunohistochemistry in the mammalian adult heart, but the ultrastructural identity of cardiac stem cells remains unknown. Using electron microscopy, we demonstrate the presence of cells with stem features in the adult mouse heart. These putative cardiac stem cells are small (6–10 μm), round cells, with an irregular shaped nucleus, large nucleolus, few endoplasmic reticulum cisternae and mitochondria, but numerous ribosomes. Stem cells located in the epicardial stem cell niche undergo mitosis and apoptosis. Cells with intermediate features between stem cells and cardiomyocyte progenitors have also been seen. Moreover, electron microscopy showed that cardiomyocyte progenitors were added to the peripheral working cardiomyocytes. Telocytes make a supportive interstitial network for stem cells and progenitors in the stem cell niche. This study enhances the hypothesis of a unique type of cardiac stem cell and progenitors in different stages of differentiation. In our opinion, stem cells, cardiomyocyte progenitors and telocytes sustain a continuous cardiac renewal process in the adult mammalian heart.     

Dynamic changes in endoglin expression in the developing mouse heart

Endoglin (ENG) is essential for cardiovascular development and is expressed in the heart from its earliest developmental stages. ENG expression has been reported in the cardiac crescent, endocardium, valve mesenchyme and coronary vascular endothelial cells. However, its expression in these cell types is non-uniform and the dynamic changes in ENG expression during heart development have not been systematically studied.Using immunofluorescent staining we tracked ENG protein expression in mouse embryonic hearts aged from 11.5 to 17.5 days, and in postnatal and adult hearts. ENG is expressed in the endocardium and in venous endothelial cells throughout these developmental stages. ENG protein is down-regulated by approximately two-fold as a subset of early coronary veins reprogram to form arteries within the developing myocardium from E13.5. This two-fold higher ratio of ENG protein in veins versus arteries is maintained throughout cardiac development and in the adult heart.ENG is also down-regulated two-fold following mesenchymal transition of endocardial cells to form cardiac valve mesenchyme, whilst expression of the pan-endothelial marker CD31 is completely lost. A subset of epicardial cells (which do not express ENG protein) delaminate and undergo a similar mesenchymal transition to form epicardially derived cells (EPDCs). This transient intra-myocardial mesenchymal cell population expresses low levels of ENG protein, similar to valve mesenchyme.In conclusion, ENG shows dynamic changes of expression in vascular endothelial cells, endocardial cells and mesenchymal cells in the developing heart that vary according to cardiovascular cell type.     

Telocytes and putative stem cells in ageing human heart

Tradition considers that mammalian heart consists of about 70% non‐myocytes (interstitial cells) and 30% cardiomyocytes (CMs). Anyway, the presence of telocytes (TCs) has been overlooked, since they were described in 2010 (visit www.telocytes.com ). Also, the number of cardiac stem cells (CSCs) has not accurately estimated in humans during ageing. We used electron microscopy to identify and estimate the number of cells in human atrial myocardium (appendages). Three age‐related groups were studied: newborns (17 days–1 year), children (6–17 years) and adults (34–60 years). Morphometry was performed on low‐magnification electron microscope images using computer‐assisted technology. We found that interstitial area gradually increases with age from 31.3 ± 4.9% in newborns to 41 ± 5.2% in adults. Also, the number of blood capillaries (per mm²) increased with several hundreds in children and adults versus newborns. CMs are the most numerous cells, representing 76% in newborns, 88% in children and 86% in adults. Images of CMs mitoses were seen in the 17‐day newborns. Interestingly, no lipofuscin granules were found in CMs of human newborns and children. The percentage of cells that occupy interstitium were (depending on age): endothelial cells 52–62%; vascular smooth muscle cells and pericytes 22–28%, Schwann cells with nerve endings 6–7%, fibroblasts 3–10%, macrophages 1–8%, TCs about 1% and stem cells less than 1%. We cannot confirm the popular belief that cardiac fibroblasts are the most prevalent cell type in the heart and account for about 20% of myocardial volume. Numerically, TCs represent a small fraction of human cardiac interstitial cells, but because of their extensive telopodes, they achieve a 3D network that, for instance, supports CSCs. The myocardial (very) low capability to regenerate may be explained by the number of CSCs, which decreases fivefold by age (from 0.5% to 0.1% in newborns versus adults).