Telocytes in pleura: two- and three-dimensional imaging by transmission electron microscopy

Information about the ultrastructure of connective (interstitial) cells supporting the pleural mesothelium is scarce. Our aim has been to examine whether telocytes (TCs) are present in pleura, as in epicardium and mesentery. TCs are a distinct type of cell, characterized by specific prolongations named telopodes (Tp). We have used transmission electron microscopy (TEM) and electron tomography (ET) to determine whether ultrastructural diagnostic criteria accepted for TCs are fulfilled by any of the cell subpopulations existing in the sub-mesothelial layer in mouse and human pleura. TCs have been identified with TEM by their characteristic prolongations. Tp appear long and moniliform, because of the alternation of podomeres (thin segments of less than 0.2 μm) and podoms (small dilations accommodating caveolae, mitochondria, and endoplasmic reticulum). Tp ramifications follow a dichotomic pattern and establish specialized cell-to-cell junctional complexes. TCs, via their Tp, seem to form an interstitial network beneath the mesothelium, covering about two-thirds of the abluminal mesothelial layer. ET has revealed complex junctional structures and tight junctions connecting pleural TCs, and small vesicles at this level in Tp. Thus, pleural TCs share significant similarities with TCs described in other serosae. Whether TCs are a (major) player in mesothelial-cell-induced tissue repair remains to be established. Nevertheless, the extremely long thin Tp and complex junctional structures that they form and the release of vesicles (or exosomes) indicate the participation of TCs in long-distance homo- or heterocellular communication

Telocytes and putative stem cells in the lungs: electron microscopy, electron tomography and laser scanning microscopy

This study describes a novel type of interstitial (stromal) cell — telocytes (TCs) — in the human and mouse respiratory tree (terminal and respiratory bronchioles, as well as alveolar ducts). TCs have recently been described in pleura, epicardium, myocardium, endocardium, intestine, uterus, pancreas, mammary gland, etc. (see www.telocytes.com). TCs are cells with specific prolongations called telopodes (Tp), frequently two to three per cell. Tp are very long prolongations (tens up to hundreds of μm) built of alternating thin segments known as podomers (≤ 200 nm, below the resolving power of light microscope) and dilated segments called podoms, which accommodate mitochondria, rough endoplasmic reticulum and caveolae. Tp ramify dichotomously, making a 3-dimensional network with complex homo- and heterocellular junctions. Confocal microscopy reveals that TCs are c-kit- and CD34-positive. Tp release shed vesicles or exosomes, sending macromolecular signals to neighboring cells and eventually modifying their transcriptional activity. At bronchoalveolar junctions, TCs have been observed in close association with putative stem cells (SCs) in the subepithelial stroma. SCs are recognized by their ultrastructure and Sca-1 positivity. Tp surround SCs, forming complex TC-SC niches (TC-SCNs). Electron tomography allows the identification of bridging nanostructures, which connect Tp with SCs. In conclusion, this study shows the presence of TCs in lungs and identifies a TC-SC tandem in subepithelial niches of the bronchiolar tree. In TC-SCNs, the synergy of TCs and SCs may be based on nanocontacts and shed vesicles