Critical review: Cardiac telocytes vs cardiac lymphatic endothelial cells

Cardiac Telocytes 16 Years on-What Have We Learned So Far, and How Close Are We to Routine Application of the Knowledge in Cardiovascular Regenerative Medicine?

The regeneration of a diseased heart is one of the principal challenges of modern cardiovascular medicine. There has been ongoing research on stem-cell-based therapeutic approaches. A cell population called telocytes (TCs) described only 16 years ago largely contributed to the research area of cardiovascular regeneration. TCs are cells with small bodies and extremely long cytoplasmic projections called telopodes, described in all layers of the heart wall. Their functions include cell-to-cell signaling, stem-cell nursing, mechanical support, and immunoregulation, to name but a few. The functional derangement or quantitative loss of TCs has been implicated in the pathogenesis of myocardial infarction, heart failure, arrhythmias, and many other conditions. The exact pathomechanisms are still unknown, but the loss of regulative, integrative, and nursing functions of TCs may provide important clues. Therefore, a viable avenue in the future modern management of these conditions is TC-based cell therapy. TCs have been previously transplanted into a mouse model of myocardial infarction with promising results. Tandem transplantation with stem cells may provide additional benefit; however, many underresearched areas need to be addressed in future research before routine application of TC-based cell therapy in human subjects. These include the standardization of protocols for isolation, cultivation, and transplantation, quantitative optimization of TC transplants, cost-effectivity analysis, and many others.

Telocytes and Lymphatics of the Human Colon

Background: Telocytes (TCs) are a peculiar morphological type of stromal cells. They project long and moniliform telopodes, visible on various bidimensional sections. Originally regarded as “interstitial Cajal-like cells”, gastrointestinal TCs were CD34+. Further double-labelling studies found that colon TCs are negative for the expressions of the PDGFR-α and α-SMA. However, the TCs in colon were not distinguished specifically from endothelial cells (ECs), vascular or lymphatic. A combinational approach is important for accurate TC identification. Hence, we designed an immunohistochemical study of human colon to check whether ECs and CD34+ TCs express different markers. Methods: Immunohistochemistry was performed on archived paraffin-embedded samples of human colon (nine cases) for the following markers: CD31, CD34, CD117/c-kit and D2-40 (podoplanin). Results: A distinctive population of CD34+ TCs was found coating the myenteric ganglia. However, also perivascular cells and vascular ECs were CD34+. c-kit expression was equally found in interstitial Cajal cells (ICCs) and perivascular cells. The CD34 TCs did not express c-kit. As they were equally CD31- and D2-40- they were assessed as different from ECs. Conclusions: Testing specific markers of ECs, vascular and lymphatic, in the same tissues in which CD34+ TCs are found, is much more relevant than to identify TCs by transmission electron microscopy alone.

Role of Podoplanin-Positive Cells in Cardiac Fibrosis and Angiogenesis After Ischemia

New insights into the cellular and extra-cellular composition of scar tissue after myocardial infarction (MI) have been identified. Recently, a heterogeneous podoplanin-expressing cell population has been associated with fibrogenic and inflammatory responses and lymphatic vessel growth during scar formation. Podoplanin is a mucin-like transmembrane glycoprotein that plays an important role in heart development, cell motility, tumorigenesis, and metastasis. In the adult mouse heart, podoplanin is expressed only by cardiac lymphatic endothelial cells; after MI, it is acquired with an unexpected heterogeneity by PDGFRα-, PDGFRβ-, and CD34-positive cells. Podoplanin may therefore represent a sign of activation of a cohort of progenitor cells during different phases of post-ischemic myocardial wound repair. Podoplanin binds to C-type lectin-like receptor 2 (CLEC-2) which is exclusively expressed by platelets and a variety of immune cells. CLEC-2 is upregulated in CD11b^high cells, including monocytes and macrophages, following inflammatory stimuli. We recently published that inhibition of the interaction between podoplanin-expressing cells and podoplanin-binding cells using podoplanin-neutralizing antibodies reduces but does not fully suppress inflammation post-MI while improving heart function and scar composition after ischemic injury. These data support an emerging and alternative mechanism of interactome in the heart that, when neutralized, leads to altered inflammatory response and preservation of cardiac function and structure. The overarching objective of this review is to assimilate and discuss the available evidence on the functional role of podoplanin-positive cells on cardiac fibrosis and remodeling. A detailed characterization of cell-to-cell interactions and paracrine signals between podoplanin-expressing cells and the other type of cells that compose the heart tissue is needed to open a new line of investigation extending beyond the known function of these cells. This review attempts to discuss the role and biology of podoplanin-positive cells in the context of cardiac injury, repair, and remodeling.

Lymphatic lacunae of the human eye conjunctiva embedded within a stroma containing CD34+ telocytes

An accurate identification of telocytes (TCs) was limited because of the heterogeneity of cell types expressing the markers attributed to TCs. Some endothelial lineage cells also could fit within the pattern of TCs. Such endothelial cells could line conjunctival lacunae previously assessed by laser confocal microscopy. We have been suggested that an accurate distinction of TCs from endothelial cells in the human eye conjunctiva could be achieved by use of CD31, CD34 and D2‐40 (podoplanin); and that the conjunctival lacunae are in fact lymphatic. We aimed as testing the hypothesis by an immunohistochemical study on human eye conjunctiva biopsy samples. Samples of human eye conjunctiva from 30 patients were evaluated immunohistochemically by use of the primary antibodies: CD34, D2‐40 and CD31. D2‐40 was equally expressed within epithelia and laminae propria. Basal epithelial cells were D2‐40 positive. Within the stromal compartment, the lymphatic marker D2‐40 labelled several lymphatic vessels. CD31 labelled both vascular and lymphatic endothelial cells within the lamina propria. When capillary lymphatics were tangentially cut, they gave the false appearance of telocytes. Blood endothelial cells expressed CD34, whereas lymphatic endothelial cells did not. Stromal CD34‐expressing cells/telocytes were found building a consistent pan‐stromal network which was equally CD31‐negative and D2‐40‐negative. The conjunctival lymphatic lacunae seem to represent a peculiar anatomic feature of eye conjunctiva. They are embedded within a CD34‐expressing stromal network of TCs. The negative expression of CD31 and D2‐40 should be tested when discriminating CD34‐expressing TCs.

Telocytes and lymphatic endothelial cells: Two immunophenotypically distinct and spatially close cell entities

Telocytes (TCs) have recently emerged as a peculiar type of stromal cells located in both perivascular and interstitial compartments of multiple anatomical sites in humans, other mammals and vertebrates. Pioneer electron microscopy studies have ultrastructurally defined TCs as "stromal cells with telopodes" (i.e. very long and thin cell processes with a moniliform morphology conferred by the irregular alternation of slender segments and small, bead-like, dilated portions), whereupon it has become apparent that TCs largely correspond to the CD34+ stromal/interstitial cells detectable by immunohistochemical assays. Besides CD34, TCs are also characterized by the expression of platelet-derived growth factor receptor (PDGFR)α. Interestingly, recent works recommended that lymphatic endothelial cell (LEC) markers should be routinely assessed to discriminate with certainty TCs from LECs, because these two cell types may exhibit similar morphological traits, especially when initial lymphatics are sectioned longitudinally and appear as vascular profiles with no obvious lumen. Furthermore, it has been argued that lymphatic microvessels immunostained for the small mucin-type transmembrane glycoprotein podoplanin (PDPN), which is widely used as lymphatic endothelial marker, can be easily misidentified as TCs. Nevertheless, surprisingly these assumptions were not based on double tissue immunostaining for TC and LEC markers. Therefore, the present morphological study was undertaken to precisely investigate the mutual spatial organization and putative relationships of TCs and lymphatic vessels in tissues from different human organs. For this purpose, we carried out a series of double immunofluorescence analyses simultaneously detecting the CD34 or PDGFRα antigen and a marker of LECs, either PDPN or lymphatic vessel endothelial hyaluronan receptor-1 (LYVE-1). In the connective tissue compartment of different organs, TCs were CD34+/PDGFRα+/PDPN-/LYVE-1- while LECs were CD34-/PDGFRα-/PDPN+/LYVE-1+, thus representing two definitely distinct, though spatially close, cell entities. The arrangement of telopodes to intimately surround the abluminal side of LECs suggests a possible role of TCs in the regulation of lymphatic capillary functionality, which is worth investigating further.

Critical Review: What Cell Types Are the Lung Telocytes?

Telocytes (TCs) are stromal cells defined by peculiar long, thin, moniliform prolongations known as telopodes. When isolated, their morphology often lacks the specificity for the proper definition of a particular cell type. Recent studies have linked TCs with different functions and different cell lineages. Although some authors have studied pulmonary TCs, their research has important limitations that we will attempt to summarize in this article. We will focus our analysis on the following: the culture methods used to study them, the lack of proper discrimination of TCs from lymphatic endothelial cells (LECs), whose ultrastructures are very similar, and the immune phenotype of TCs, which may appear in other cell types such as those related to the endothelial lineage or stem/progenitor cells. In conclusion, the cellular diagnosis of lung TCs should be considered with caution until properly designed studies can positively identify these cells and differentiate them from other cell types such as LECs and stem/progenitor cells. Anat Rec, 303:1280-1292, 2020. © 2019 American Association for Anatomy.

Terminologia Histologica 10 years on: some disputable terms in need of discussion and recent developments

At first sight, the issue of morphological terminology may seem to be a "closed and unchanging chapter", as many of the structures within the human body have been known for decades or even centuries. However, the exact opposite is true. The initial knowledge of the microscopic structure of the human body has been continuously broadening thanks to the development of new specialized staining techniques, discovery of the electron microscope, or later application of histochemical and immunohistochemical methods into routine tissue examination. Contrary to popular belief, histology has a status of constantly developing scientific discipline, with continuous influx of new knowledge, resulting in an unavoidable necessity to revise the histological nomenclature at regular intervals. The team of experts of the Federative International Programme on Anatomical Terminology, a working group of the International Federation of Associations of Anatomists, published in 2008 the First Edition of Terminologia Histologica. Terminologia Histologica (TH) is the best and most extensive of all the histological nomenclatures ever issued. However, here we suggest that several terms of important histological structures are still missing while several other terms are disputable. First, we present some clinically important terms of cells and tissue structures for inclusion in the next TH and, in a second part, we refer to some new terms in the current edition of the TH which are not yet mentioned in current histology textbooks (e.g., fusocellular connective tissue, bundle bone as the third type of bone tissue, spongy layer of vagina or arteria vaginata from the splenic white pulp). With this article we hope to start a wide scientific discussion which will lead to an inambiguous definition and demonstration of typical examples of all terms in the TH, with the result that the new edition of the Terminologia Histologica will become an internationally accepted communication tool for all practitioners and teachers of histology alike.

An Immunohistochemical Study of Gastric Mucosa and Critical Review Indicate that the Subepithelial Telocytes are Prelymphatic Endothelial Cells

Background and Objectives: There are only a few studies regarding gut subepithelial telocytes (TCs). The telopodes, namely peculiar TCs’ prolongations described on two-dimensional cuts, are not enough to differentiate this specific cell type. Subepithelial TCs were associated with the intestinal stem niche but a proper differential diagnosis with lymphatic endothelial cells (LECs) was not performed. In this study, we will also critically review studies suggesting that distinctive TCs could be positioned within the lamina propria. Materials and Methods: We performed an immunohistochemical study of human gastric mucosa to test the expression of D2-40, the lymphatic marker, as well as that of CD31, CD34, CD44, CD117/c-kit, α-smooth muscle actin (α-SMA) and vimentin in the gastric subepithelial niche. Results: The results support the poorly investigated anatomy of intramural gastric lymphatics, with circumferential collectors located on both sides of the muscularis mucosae (mucosal and then submucosal) and myenteric collectors in the muscularis propria. We also found superficial epithelial prelymphatic channels bordered by D2-40+ but CD31–TC-like cells. Deep epithelial lymphatic collectors drain in collectors within the lamina propria. Blood endothelial cells expressed CD31, CD34, CD44, and vimentin. Conclusions: Therefore, the positive diagnosis of TC for subepithelial CD34+ cells should be regarded with caution, as they could also be artefacts, resulting from the two-dimensional examination of three dimensional structures, or as LECs. Lymphatic markers should be routinely used to discriminate TCs from LECs.

Ultrastructure of telocytes, a new type of interstitial cells in the myocardium of the Chinese giant salamander (Andrias davidianus)

Telocytes (TCs) are new interstitial cells, and they are involved in tissue regeneration, particularly in heart. Therefore, TCs are suggested to be promising cells in regenerative medicine. However, the information of location structural characteristics and functions of TCs is still limited. In this study, cardiac TCs of the Chinese giant salamanders (Andrias davidianus) were identified by transmission electron microscopy. TCs were located in the interstitium between cardiomyocytes (CM). TCs possessed distinctive ultrastructural characteristics, including one to two very long and thin moniliform telopodes (Tps), emerging points from the cell body, caveolae, dichotomous branchings, labyrinthic systems, neighbouring exosomes and homocellular contacts between Tps. TCs/Tps were frequently observed in close proximity to cardiomyocytes. Moreover, Tps established hetero-cellular contacts with cardiomyocytes. Our results confirm the presence of TCs in the myocardium of the A. davidianus. This will help us to better understand roles of TCs in amphibian hearts.

The Telocytes in the Subepicardial Niche

A great interest has developed over the last several years in research on interstitial Cajal-like cells (ICLCs), later renamed to telocytes (TCs). Such studies are restricted by diverse limitations. We aimed to critically review (sub)epicardial ICLCs/TCs and to bring forward supplemental immunohistochemical evidence on (sub)epicardial stromal niche inhabitants. We tested the epicardial expressions of CD117/c-kit, CD34, Cytokeratin 7 (CK7), Ki67, Platelet-Derived Growth Factor Receptor (PDGFR)-α and D2-40 in adult human cardiac samples. The mesothelial epicardial cells expressed D2-40, CK7, CD117/c-kit and PDGFR-α. Subepicardial D2-40-positive lymphatic vessels and isolated D2-40-positive and CK7-positive subepicardial cells were also found. Immediate submesothelial spindle-shaped cells expressed Ki-67. Submesothelial stromal cells and endothelial tubes were PDGFR-α-positive and CD34-positive. The expression of CD34 was pan-stromal, so a particular stromal cell type could not be distinguished. The stromal expression of CD117/c-kit was also noted. It seems that epicardial TCs could not be regarded as belonging to a unique cell type until (pre)lymphatic endothelial cells are inadequately excluded. Markers such as CD117/c-kit or CD34 seem to be improper for identifying TCs as a distinctive cell type. Care should be taken when using the immunohistochemical method and histological interpretations, as they may not produce accurate results.

Telocytes constitute a widespread interstitial meshwork in the lamina propria and underlying striated muscle of human tongue

Telocytes have recently emerged as unique interstitial cells defined by their extremely long, thin and moniliform prolongations termed telopodes. Despite growing evidence that these cells consistently reside in the stromal compartment of various organs from human beings, studies dealing with telocytes in structures of the oral cavity are scarce. Hence, the present morphologic study was undertaken to explore for the first time the presence and specific localization of telocytes within tissues of the normal human tongue, a complex muscular organ whose main functions include taste, speech, and food manipulation in the oral cavity. Telocytes were initially identified by CD34 immunostaining and confirmed by CD34/PDGFRα double immunofluorescence and transmission electron microscopy. CD34+/PDGFRα+ telocytes were organized in interstitial meshworks either in the tongue lamina propria or in the underlying striated muscle. Lingual telocytes were immunonegative for CD31, c-kit and α-SMA. Telopodes were finely distributed throughout the stromal space and concentrated beneath the lingual epithelium and around CD31+ vessels, skeletal muscle bundles/fibers, and intramuscular nerves and ganglia. They also enveloped salivary gland units outside the α-SMA+ myoepithelial cells and delimited lymphoid aggregates. These findings establish telocytes as a previously overlooked interstitial cell population worth investigating further in the setting of human tongue pathophysiology.

Myocardial Telocyte-Like Cells: A Review Including New Evidence

Telocytes (TCs) are a controversial cell type characterized by the presence of a particular kind of prolongations, known as telopodes, which are long, thin, and moniliform. A number of attempts has been made to establish the molecular phenotype of cardiac TCs (i.e., expression of c-kit, CD34, vimentin, PDGRFα, PDGRFβ, etc.). We designed an immunohistochemical study involving cardiac tissue samples obtained from 10 cadavers with the aim of determining whether there are TC-like interstitial cells that populate the interstitial space other than the mural microvascular cells. We applied the markers for CD31, CD34, PDGRFα, CD117/c-kit, and α-smooth muscle actin (α-SMA). We found that, in relation to two-dimensional cuts, the endothelial tubes could be misidentified as TC-like cells, the difference being the positive identification of endothelial lumina. Moreover, we found that cardiac pericytes express PDGRFα, CD117/c-kit, and α-SMA, and that they could also be misidentified as TCs when using light microscopy. We reviewed the respective values of the previously identified markers for achieving a clear-cut identification of cardiac TCs, highlighting the critical lack of specificity.

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.