Telocytes

Abstract

Here, we review the history, morphology, immunohistochemical phenotype, and presumptive roles of a new type of interstitial tissue cells, formerly called interstitial Cajal-like cells (ICLC) and by 2010 named 'telocytes' (TC). Many different techniques have been used to characterize TC and provide their unequivocal identification: (i) in vitro, cultures and isolated cells; (ii) in situ, fixed specimens examined by light and fluorescence microscopy, transmission (TEM) and scanning electron microscopy, and electron tomography. TEM allowed sure identification and characterization of the most peculiar feature of TC: the long, thin, and convoluted prolongations named 'telopodes'. An enormous variety of antibodies have been tested, but presently none are reliable to specifically label TC. TC have a mesenchymal origin and are resident connective tissue (stromal) cells. Possible identification with 'already identified' stromal cell types (fibroblasts, fibrocytes, fibroblast-like cells, and mesenchymal stromal cells) is discussed. We conclude that in adulthood, most of the TC have the morphology of fibrocytes. Apparently, immunocytochemistry suggests that a variety of TC populations showing different, likely organ-specific, immunophenotypes might exist. Several roles have been hypothesized for TC: mechanical roles, intercellular signaling, guiding and nursing of immature cells during organogenesis, and being themselves a pool of precursors for many of the mesenchyme-derived cells in adulthood; however, none of these roles have been proven yet. On the basis of the available data, we propose TC may be key players in organ regeneration and repair.

Telocytes of the mammary gland stroma

“…They were not searched then for the CD34 expression. It was documented that, in human chorial villi, some TCs are CD34-positive, some are c-kit-positive [15,41]. Here a CD34-positive but c-kit-negative phenotype was found.…”

“…In 2010 the Romanian and Italian researchers renamed these cells from ICLCs to telocytes (TCs), their processes, long of "tens-up to hundreds of [micro]m", being named telopodes [31]. It was unequivocally stated that only transmission electron microscopy (TEM) "allows sure identification of telocytes, evaluation of the cell-tocell interrelationships, and a detailed description of the telopodes" [15]. This attitude was kept until 2014 when it was reinforced that "the features of telocytes are obvious only by transmission electron microscopy" [42].…”

“…The only proofs of mammary telocyte-like cells resulted after an in vitro experiment of breast cancer reconstitution performed recently, in which TCs were identified in primary culture cells by phase contrast microscope [25], no specific marker being used then. Indeed, at this time a specific immune phenotype of TCs was not clearly established [34]; although the most used markers which point to TCs, but not exclusively, are CD34 and c-kit [15].…”

“…It was also repeatedly stated that TCs "nurse" stem cells (SCs) in the stem cell niches [15,31,32,47]. However, CD34, as well as CD117/c-kit, are well known markers of cells with different levels of stemness [44].…”

Morphological Evidence of Telocytes in Canine Inferior Vena Cava

“…TC, a particular identified type of stromal cell, has been identified morphologically by a small cell body and specific long prolongations called Telopodes (Tp) alternating thin segments (podomers) with dilations (podoms) [3]. TCs have been described in various organs and tissues, such as spleen [4], skin [5], esophagus [6], parotid gland [7], salivary glands [8], endocardium [9], bone marrow [10], pleura [11], vasculature [12], lung [13], duodenum [14], liver [15], eye [16], cardiac valves [17], uterus [18], et al Although TCs were also identified in large arteries [19], whether TCs exist in large veins was not yet reported. Here, the ultrastructural features of TCs in inferior vena cava were, for the first time, clarified under transmission electron microscope (TEM).…”