Interstitial Cajal-like cells in rat mesentery: an ultrastructural and immunohistochemical approach

Myometrial interstitial cells and the coordination of myometrial contractility

A strict regulation of contractility in the uterus and fallopian tube is essential for various reproductive functions. The uterus contributes, through either increased contractility or periods of relative quiescence, to: (i) expulsion of menstrual debris, (ii) sperm transport, (iii) adequate embryo placement during implantation, (iv) enlarging its capacity during pregnancy and (v) parturition. The dominant cell population of the uterine wall consists of smooth muscle cells that contain the contractile apparatus responsible for the generation of contractile force. Recent interest has focused on a new population of cells located throughout the myometrium on the borders of smooth muscle bundles. These cells are similar to interstitial cells of Cajal (ICC) in the gut that are responsible for the generation of electrical slow waves that control peristalsis. A precise role for myometrial Cajal-like interstitial cells (m-ICLC) has not been identified. m-ICLC express the c-kit receptor, involved in creating and maintaining the ICC phenotype in the gastrointestinal tract. However, both acute and prolonged inhibition of this receptor with the c-kit antagonist imatinib mesylate does not appear to affect the spontaneous contractility of myometrium. Calcium imaging of live tissue slices suggests that contractile signalling starts on the borders of smooth muscle bundles where m-ICLC are located and recently the possible role of extracellular ATP signalling from m-ICLC has been studied. This manuscript reviews the evidence regarding tissue-level signalling in the myometrium with a particular emphasis on the anatomical and possible functional aspects of m-ICLC as new elements of the contractile mechanisms in the uterus.

Cardiac renewing: interstitial Cajal-like cells nurse cardiomyocyte progenitors in epicardial stem cell niches

Recent studies suggested that various cell lineages exist within the subepicardium and we supposed that this area could host cardiac stem cell niches (CSCNs). Using transmission electron microscopy, we have found at least 10 types of cells coexisting in the subepicardium of normal adult mice: adipocytes, fibroblasts, Schwann cells and nerve fibres, isolated smooth muscle cells, mast cells, macrophages, lymphocytes, interstitial Cajal-like cells (ICLCs) and cardiomyocytes progenitors (CMPs). The latter cells, sited in the area of origin of coronary arteries and aorta, showed typical features of either very immature or developing cardiomyocytes. Some of these cells were connected to each other to form columns surrounded by a basal lamina and embedded in a cellular network made by ICLCs. Complex intercellular communication occurs between the ICLCs and CMPs through electron-dense nanostructures or through shed vesicles. We provide here for the first time the ultrastructural description of CSCN in the adult mice myocardium, mainly containing ICLCs and CMPs. The existence of resident CMPs in different developmental stages proves that cardiac renewing is a continuous process. We suggest that ICLCs might act as supporting nurse cells of the cardiac niches and may be responsible for activation, commitment and migration of the stem cells out of the niches. Briefly, not only resident cardiac stem cells but also ICLCs regulate myocyte turnover and contribute to both cardiac cellular homeostasis and endogenous repair/remodelling after injuries.

Epicardium: interstitial Cajal-like cells (ICLC) highlighted by immunofluorescence

During the last few years, there is an increasing interest in the role of the epicardium in cardiac development, myocardial remodelling or repair and regeneration. Several types of cells were described in the subepicardial loose connective tissue, beneath the epicardial mesothe-lium. We showed previously (repeatedly) the existence of interstitial Cajal-like cells (ICLCs) in human and mammalian myocardium, either in atria or in ventricles. Here, we describe ICLCs in adult mice epicardium and primary culture as well as in situ using frozen sections. The identification of ICLCs was based on phase contrast microscopy and immunophenotyping. We found cells with characteristic morphologic aspects: spindle-shaped, triangular or polygonal cell body and typical very long (tens to hundreds micrometres) and very thin cyto-plasmic processes, with a distinctive ‘beads-on-a-string’ appearance. The dilations contain mitochondria, as demonstrated by MitoTracker Green FM labelling of living cells. Epicardial ICLCs were found positive for c-kit/CD117 and/or CD34. However, we also observed ICLCs positive for c-kit and vimentin. In conclusion, ICLCs represent a distinct cell type in the subendocardium, presumably comprising at least two subpopulations: (i) c-kit/CD34-positive and (ii) only c-kit-positive. ICLCs might be essential as progenitor (or promoter) cells for developing cardiomyocyte lineages in normal and/or injured heart.

Histochemical and ultrastructural characteristics of an interstitial cell type different from ICC and resident in the muscle coat of human gut

CD117 (or c-kit) is expressed by the interstitial cells of Cajal (ICC), which are located within the gastrointestinal (GI) muscle coat and directly involved in its motility. CD34 is expressed by several cell types some of which have features and location resembling the ICC; however, a sure identification of these cells is still lacking. In order to establish whether the CD34-positive cells of the human GI tract are to be considered as ICC subpopulation or a novel independent cell type, and to hypothesize their nature and role, we verified CD34 and CD117 receptor expression under light and fluorescence microscope and performed a routine and a CD34-immuno-electron microscopy. CD34-positive cells were seen in the entire human GI tract. In the muscularis propria, shared morphologies similar to the c-kit-positive cells, in the submucosa, resembled fibroblasts. Their ultrastructure resembled that of the fibrocytes/fibroblasts and of the interstitial Cajal-like cells (ICLC). Double labelling and immunoelectro-microscopy demonstrated that they are unequivocally different to the ICC and, due to the similarities with the ICLC, we identified them as ICLC. The novelty of these results is that two types of interstitial cells are present in the GI muscle coat of humans: the ICC and the ICLC. We hypothesize a mechanical role for the septal ICLC, those at the myenteric plexus level and those bordering the muscle layers; a helping role in neurotransmission is proposed for the ICLC intercalated with the intramuscular ICC, possibly in spreading the slow waves generated by the ICC. Furthermore, the possibility that the ICLC represent the adult mesenchymal stromal cells able to guarantee the ICC renewal deserves to be considered.

A distinct type of cell in myocardium: interstitial Cajal-like cells (ICLCs)

The existence of a novel type of interstitial cells in the heart, interstitial Cajal-like cells (ICLCs), had been described for the first time in 2005. Their identification was mainly based on ultrastructural criteria: very long (tens up to hundreds of micrometres) and moniliform prolongations, which are extremely thin (less than 0.2 μm), below the resolving power of light microscopy. Myocardial ICLCs were also identified by methylene-blue vital staining, silver impregnation, and immunoreactivity for CD 34, vimentin, CD117/c-kit, etc. Although a series of studies provided evidence for the existence of ICLCs in human atria and rat ventricles, further investigations in other laboratories, using additional techniques, are required to substantiate the consistency of these findings. Here we provide further evidence for the existence of ICLCs in human and mammalian hearts (by transmission and scanning electron microscopy, as well as confocal laser scanning microscopy). Noteworthy, we confirm that ICLCs communicate with neighbouring cells via shedding (micro)vesicles. Although these so-called ICLCs represent a distinct type of cells, different from classical interstitial cells of Cajal, or fibroblasts, their role(s) in myocardium remain(s) to be established. Several hypotheses are proposed: (i) adult stromal (mesenchymal) stem cells, which might participate in cardiac repair/remodelling; (ii) intercellular signalling (e.g. via shedding microvesicles); (iii) chemo-mechanical transducers and (iv) players in pacemaking and/or arrhytmogenesis, and so on.

Interstitial cells from rat middle cerebral artery belong to smooth muscle cell type

It is now established that non-contractile cells with thin filopodia, also called vascular interstitial cells (VICs), are constitutively present in the media of many, if not all, blood vessels. The aim of this study was to determine the type of cell lineage to which arterial VICs belong using immunocytochemical, and real-time and reverse transcription PCR (RT-PCR). Using RT-PCR, we compared gene expression profiles of single VICs and smooth muscle cells (SMCs) freshly dispersed from rat middle cerebral artery. Both VICs and SMCs expressed the SMC marker, smooth muscle myosin heavy chain (SM-MHC), but did not express fibroblast, pericyte, neuronal, mast cell, endothelial or stem cell markers. Freshly isolated VICs also did not express c-kit, which is the marker for interstitial cells of Cajal in the gastrointestinal tract. Immunocytochemical labelling of contractile proteins showed that VICs and SMCs expressed SM-MHC similarly to the same degree, but VICs in contrast to SMCs had decreased expression of alpha-SM-actin and very low or no expression of calponin. Real-time RT-PCR was consistent with immunocytochemical experiments and showed that VICs had four times lower gene expression of calponin comparing to SMCs, which may explain VICs' inability to contract. VICs had greater expression than SMCs of structural proteins such as non-muscular beta-actin and desmin. The results obtained suggest that VICs represent a subtype of SMCs and may originate from the same precursor as SMCs, but later develop filopodia and a non-contractile cell phenotype.