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Telocytes are a novel defined type of interstitial (stromal) cells, in the field of Stem cells, with very long (tens to hundreds of micrometres) and very thin prolongations (mostly below the resolving power of light microscopy).
Rationale for the term telocyte
Professor Laurențiu M. Popescu's group from Bucharest, Romania described a new type of cell. Popescu coined the terms Telocytes (TC) - for these cells, and Telopodes (Tp)  for their extremely long but thin prolongations [1-7] in order to prevent further confusion with other interstitial (stromal) cells (e.g., fibroblast, fibroblast-like cells, myofibroblast, mesenchymal cells) (see Figs. 1-7). Telopodes present an alternation of thin segments,podomeres (with caliber mostly under 200 nm, below the resolving power of light microscopy) and dilated segments, podoms, which accommodate mitochondria, (rough) endoplasmic reticulum and caveolae - the so-called "Ca2+ uptake/release units". The concept of TC was promptly adopted by other laboratories, as well [8-18].
Telocytes and/or fibroblasts ?
The interstitium (stroma) is in most of the cases seen as a connecting "device" for the specific structures of an organ. Usually, people perceive interstitial cells as being mainly (or even, only) fibroblasts. However, fibroblasts have the function of generating connective tissue matrix, specifically, collagen. The distinction between TC and fibroblasts is obvious since they have different ultrastructure and phenotype. Therefore, their functions should be mostly different: TC - intercellular signaling (connections), but fibroblasts - collagen synthesis. In other words, TC are "more" functionally oriented while fibroblasts are "more" structurally oriented, responsible for fibrosis.
There are some clear ultrastructural features that differentiate telocytes from fibroblasts. For instance, the general aspect of TC is of a small oval (piriform/spindle/triangular/stellate)-shaped cellular body, containing a nucleus surrounded by a small amount of cytoplasm. Anyway, the shape of the cell body depends on the number of Tp. Fibroblast cell body is pleomorphic (phenotype heterogeneity?). TC cellular body average dimensions are, as measured on EM images, 9.3 μm ± 3.2 μm (min. 6.3μm; max. 16.4 μm). Fibroblast nucleus is typically euchromatic, but TC nucleus is mostly heterochromatic. Mitochondria represent only 2% of cell body volume and the Golgi complex is small in TC. Fibroblasts Golgi complex is prominent and the rough endoplasmic reticulum is very well developed (usually 5-12%) of cell volume.
Since telopodes are distinctive for telocytes, here are their main features:
- Number: 1–5 (frequently only 2–3 telopodes are observed on a single section, depending on site and angle of section, since their 3D convolutions prevent them to be observed at their full length in a 2D very thin section);
- Length: tens – up to hundreds of μm, as measured on EM images (e.g. Figs. 2-10). However, under favorable conditions in cell cultures, their entire length can be captured in several successive images (Fig. 1);
- Thickness: uneven caliber, mostly below 0.2 μm (below the resolving power of light microscopy), visible under electron microscopy;
- Moniliform aspect: podoms and podomeres; average caliber of podomeres: 0.1 μm ± 0.05μm, min. = 0.003 μm; max. = 0.24 μm; Podoms accommodate: mitochondria, (rough) endoplasmic reticulum, caveolae, a trio called ‘Ca2+-uptake/release units’.
- Branching, with a dichotomous pattern;
- Organization in a labyrinthine system, forming a 3D network anchored by hetero- and homocellular junctions.
Here is shown visual evidence (electron microscopy, electron tomography, phase contrast microscopy) for the existence of Telocytes (TC) in many organs from human and rodents. TC and Tp, and also podoms and podomeres were found in:
- cavitary organs:
- heart (endo-, myo-, and pericardium);
- stomach and intestine, with mesentery;
- gall bladder;
- uterus and Fallopian tube; [19-20]
- non-cavitary organs:
- lungs and pleura; [7, 28-29]
- pancreas (exocrine gland); 
- mammary gland;
- placenta; 
- kidneys; [25-26]
Recent evidence shows the involvement of TC in pathology . TC are strategically located in between blood vessels (capillaries), nerve endings and the specific resident cell population(s) of a given organ. TC establish via Tp homo- and heterocellular junctions and release shed vesicles and exosomes.
Perspectives: regenerative medicine
TC and SC make a tandem (due to specific intercellular junctions) within the so-called SC niches, at least in heart  and lungs. Hence, TC could be key-players in regenerating and repair of some organs. The tandem TC-SC could be a better option for therapy rather than SC alone. Published studies suggest that cardiac TCs could be regarded as a potential cell source for therapeutic use to improve cardiac repair and function after a myocardial infarction, either alone or in tandem with SC . Recent data show that TCs are completely different from FBs, using a quantitative proteomics approach, suggesting that TCs might play specific roles in mechanical sensing and mechanochemical conversion task, tissue homoeostasis and remodelling/renewal .
Figure 9. Human mammary gland stroma: TEM; original magnification 9,100x. A: Lymphocyte establishing a multicontact synapse (MS) with a TC. The blue rectangle shows the synaptic ‘kiss and run’ region. The synaptic membranes appear traced in B (violet - TC, orange - lymphocyte). The distances between membranes are shown in C. Note (asterisk) a peculiar conformation of ER connecting mitochondria with the cell surface, suggestive for a possible role in synaptic Ca2+ homeostasis. Reproduced with permission from 
Figure 11. Digitally coloured electron micrograph of mouse ventricular endocardium (burgundy). TC (blue) make an interstitial network in the heart. Subendocardial telocytes (TC1) sends Tp between cardiomyocytes (CM) and communicate with TC2. Cap, blood capillary. Scale bar 5μm. Reproduced with permission from 
Figure 12. This electron tomography (thick section of about 300 nm) shows nanostructures connecting the TC and cardiomyocytes in adult mouse heart. The bridging structures (encircled) have 10-15 nm and suggest a molecular interaction between the Tp of one TC and the two adjacent cardiomyocytes. The dilated segment of Tp involved in the heterocellular connection (podom) - contains a mitochondrion (m).
Figure 15. Mice lung. Terminal bronchiole. At least 4 TC with their extensive Tp are visible between the epithelium and an arteriole (SMC - smooth muscle cells). Note, the striking labyrinthine network formed by Tp. In the upper part a mitosis (prophase) is obvious (orange circle). In addition, a putative stem cell (SC, green oval) is in close contacts with telocytes prolongations, establishing a heterocellular junctions, visible at higher magnification only). The tandem TC-SC forms, presumably, a TC-SC niche. In the lower part, a macrophage (MF) makes a stromal synapse with Tp.
Figure 16. Rat striated skeletal muscle (diaphargm). A typical TC (blue) with two convoluted Tp is shown, by transmission electron microscopy. Note, two shed vesicles (sv, violet). m-mitochondria, Ly-lymphocyte. The asterisks indicate, presumably, two empty exosomes, which probably released their vesicle content. BV-small blood vessel.
Figure 17. Rat jejunum. Toluidine blue stained Semithin Epon sections of jejunum mucosa showing the bottom of Lieberkuhn glands in transverse section and a telocyte (red star) surrounding one of the gland. Note the spindle-shape body sending off two telopodes, one of which measure at least 50 µm in the section plane.
Figure 19. Rat jejunum mucosa. A. This electron microscope image disclose a telopode (blue) in the profund region of lamina propria, close to the muscularis mucosa (brown) and in the proximity of a nerve ending (green). Note the alternating podom and podomere. B. Inset disclosing the organelle details of the podomere – intermediate filaments and free ribosomes, and of the podom – mitochondrion and endoplasmic reticulum cisternae. C. High resolution image illustrating in detail multiple mitochondria, endoplamic reticulum cisterne and caveolae (arrow).
Figure 20. Rat jejunum. A. Photomicrograph of an interstitial cell of Cajal (violet) in muscularis externa. Note the large cell body which extend slender and relatively short connection towards the nerve endings (green). B. Digitally coloured TEM image showing a fibroblast (garnet) and a telocyte (blue) in the lamina propria. C. Coloured transmission electron micrograph (TEM) of a tangential section through a fibroblast cell. The internal structure can be seen, including the dilated rough endoplasmic reticulum (blue). responsible for synthesising collagen. In blue a telopode underlying the intestinal epithelium.
Figure 22. Rat jejunum. A-E. 3-D image reconstruction from 5 serial sections of telocytes (blue) in lamina propria: telopodes branching in a 3-D pattern. Telocyte’s nucleus is colored in violet. F-J. Computer-aided volume rendering and different-angle stereoscopic views of a telocyte (blue) surrounding a nerve fiber (green) in muscularis mucosa (dark red).
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