Platelet-Rich Plasma in Dermatology: New Insights on the Cellular Mechanism of Skin Repair and Regeneration

The skin's recognised functions may undergo physiological alterations due to ageing, manifesting as varying degrees of facial wrinkles, diminished tautness, density, and volume. Additionally, these functions can be disrupted (patho)physiologically through various physical and chemical injuries, including surgical trauma, accidents, or chronic conditions like ulcers associated with diabetes mellitus, venous insufficiency, or obesity. Advancements in therapeutic interventions that boost the skin's innate regenerative abilities could significantly enhance patient care protocols. The application of Platelet-Rich Plasma (PRP) is widely recognized for its aesthetic and functional benefits to the skin. Yet, the endorsement of PRP's advantages often borders on the dogmatic, with its efficacy commonly ascribed solely to the activation of fibroblasts by the factors contained within platelet granules. PRP therapy is a cornerstone of regenerative medicine which involves the autologous delivery of conditioned plasma enriched by platelets. This is achieved by centrifugation, removing erythrocytes while retaining platelets and their granules. Despite its widespread use, the precise sequences of cellular activation, the specific cellular players, and the molecular machinery that drive PRP-facilitated healing are still enigmatic. There is still a paucity of definitive and robust studies elucidating these mechanisms. In recent years, telocytes (TCs)-a unique dermal cell population-have shown promising potential for tissue regeneration in various organs, including the dermis. TCs' participation in neo-angiogenesis, akin to that attributed to PRP, and their role in tissue remodelling and repair processes within the interstitia of several organs (including the dermis), offer intriguing insights. Their potential to contribute to, or possibly orchestrate, the skin regeneration process following PRP treatment has elicited considerable interest. Therefore, pursuing a comprehensive understanding of the cellular and molecular mechanisms at work, particularly those involving TCs, their temporal involvement in structural recovery following injury, and the interconnected biological events in skin wound healing and regeneration represents a compelling field of study.

Skin telocytes versus fibroblasts: two distinct dermal cell populations

It is already accepted that telocytes (TCs) represent a new type of interstitial cells in human dermis. In normal skin, TCs have particular spatial relations with different dermal structures such as blood vessels, hair follicles, arrector pili muscles or segments of sebaceous and/or eccrine sweat glands. The distribution and the density of TCs is affected in various skin pathological conditions. Previous studies mentioned the particular (ultra)structure of TCs and also their immunophenotype, miR imprint or proteome, genome or secretome features. As fibroblast is the most common intersitital cell (also in human dermis), a dedicated comparison between human skin TCs and fibroblasts (Fbs) was required to be performed. In this study, using different techniques, we document several points of difference between human dermis TCs and Fbs. By transmission electron microscopy (TEM) and scanning electron microscopy (SEM), we demonstrated TCs with their hallmark cellular prolongations - telopodes. Thus, we showed their ultrastructural distinctiveness from Fbs. By RayBio Human Cytokine Antibody Array V analyses performed on the supernatant from separately cultured TCs and Fbs, we detected the cytokine profile of both cell types, individually. Two of 79 detected cytokines - epithelial-derived neutrophil-activating peptide 78 and granulocyte chemotactic protein-2 - were 1.5 times higher in the supernatant of TCs (comparing with Fbs). On the other hand, 37 cytokines were at least 1.5 higher in Fbs supernatant (comparing with TCs), and among them six cytokines - interleukin 5, monocyte chemotactic protein-3 (MCP-3), MCP-4, macrophage inflammatory protein-3, angiogenin, thrombopoietin - being 9.5 times higher (results also confirmed by ELISA testing). In summary, using different techniques, we showed that human dermal TCs and Fbs are different in terms of ultrastructure and cytokine profile.

Telocyte dynamics in psoriasis

The presence of telocytes (TCs) as distinct interstitial cells was previously documented in human dermis. TCs are interstitial cells completely different than dermal fibroblasts. TCs are interconnected in normal dermis in a 3D network and may be involved in skin homeostasis, remodelling, regeneration and repair. The number, distribution and ultrastructure of TCs were recently shown to be affected in systemic scleroderma. Psoriasis is a common inflammatory skin condition (estimated to affect about 0.1-11.8% of population), a keratinization disorder on a genetic background. In psoriasis, the dermis contribution to pathogenesis is frequently eclipsed by remarkable epidermal phenomena. Because of the particular distribution of TCs around blood vessels, we have investigated TCs in the dermis of patients with psoriasis vulgaris using immunohistochemistry (IHC), immunofluorescence (IF), and transmission electron microscopy (TEM). IHC and IF revealed that CD34/PDGFRα-positive TCs are present in human papillary dermis. More TCs were present in the dermis of uninvolved skin and treated skin than in psoriatic dermis. In uninvolved skin, TEM revealed TCs with typical ultrastructural features being involved in a 3D interstitial network in close vicinity to blood vessels in contact with immunoreactive cells in normal and treated skin. In contrast, the number of TCs was significantly decreased in psoriatic plaque. The remaining TCs demonstrated multiple degenerative features: apoptosis, membrane disintegration, cytoplasm fragmentation and nuclear extrusion. We also found changes in the phenotype of vascular smooth muscle cells in small blood vessels that lost the protective envelope formed by TCs. Therefore, impaired TCs could be a 'missed' trigger for the characteristic vascular pathology in psoriasis. Our data explain the mechanism of Auspitz's sign, the most pathognomonic clinical sign of psoriasis vulgaris. This study offers new insights on the cellularity of psoriatic lesions and we suggest that TCs should be considered new cellular targets in forthcoming therapies.

Telocytes in the human kidney cortex

Renal interstitial cells play an important role in the physiology and pathology of the kidneys. As a novel type of interstitial cell, telocytes (TCs) have been described in various tissues and organs, including the heart, lung, skeletal muscle, urinary tract, etc. (www.telocytes.com). However, it is not known if TCs are present in the kidney interstitium. We demonstrated the presence of TCs in human kidney cortex interstitium using primary cell culture, transmission electron microscopy (TEM) and in situ immunohistochemistry (IHC). Renal TCs were positive for CD34, CD117 and vimentin. They were localized in the kidney cortex interstitial compartment, partially covering the tubules and vascular walls. Morphologically, renal TCs resemble TCs described in other organs, with very long telopodes (Tps) composed of thin segments (podomers) and dilated segments (podoms). However, their possible roles (beyond intercellular signalling) as well as their specific phenotype in the kidney remain to be established.

Telocytes in human oesophagus

Telocytes (TCs), a new type of interstitial cells, were identified in many different organs and tissues of mammalians and humans. In this study, we show the presence, in human oesophagus, of cells having the typical features of TCs in lamina propria of the mucosa, as well as in muscular layers. We used transmission electron microscopy (TEM), immunohistochemistry (IHC) and primary cell culture. Human oesophageal TCs present a small cell body with 2–3 very long Telopodes (Tps). Tps consist of an alternation of thin segments (podomers) and thick segments (podoms) and have a labyrinthine spatial arrangement. Tps establish close contacts (‘stromal synapses’) with other neighbouring cells (e.g. lymphocytes, macrophages). The ELISA testing of the supernatant of primary culture of TCs indicated that the concentrations of VEGF and EGF increased progressively. In conclusion, our study shows the existence of typical TCs at the level of oesophagus (mucosa, submucosa and muscular layer) and suggests their possible role in tissue repair.

Experimental acute myocardial infarction: telocytes involvement in neo-angiogenesis

We used rat experimental myocardial infarction to study the ultrastructural recovery, especially neo-angiogenesis in the infarction border zone. We were interested in the possible role(s) of telocytes (TCs), a novel type of interstitial cell very recently discovered in myocardim (see http://www.telocytes.com). Electron microscopy, immunocytochemistry and analysis of several proangiogenic microRNAs provided evidence for TC involvement in neo-angiogenesis after myocardial infarction. Electron microscopy showed the close spatial association of TCs with neoangiogenetic elements. Higher resolution images provided the following information: (a) the intercellular space between the abluminal face of endothelium and its surrounding TCs is frequently less than 50 nm; (b) TCs establish multiple direct nanocontacts with endothelial cells, where the extracellular space seems obliterated; such nanocontacts have a length of 0.4–1.5 μm; (c) the absence of basal membrane on the abluminal face of endothelial cell. Besides the physical contacts (either nanoscopic or microscopic) TCs presumably contribute to neo-angiognesis via paracrine secretion (as shown by immunocytochemistry for VEGF or NOS2). Last but not least, TCs contain measurable quantities of angiogenic microRNAs (e.g. let-7e, 10a, 21, 27b, 100, 126-3p, 130a, 143, 155, 503). Taken together, the direct (physical) contact of TCs with endothelial tubes, as well as the indirect (chemical) positive influence within the ‘angiogenic zones’, suggests an important participation of TCs in neo-angiogenesis during the late stage of myocardial infarction.

Telocytes in trachea and lungs

We show the existence of a novel type of interstitial cell-telocytes (TC) in mouse trachea and lungs. We used cell cultures, vital stainings, as well as scanning electron microscopy (SEM), transmission electron microscopy (TEM) and immunohistochemistry (IHC). Phase contrast microscopy on cultured cells showed cells with unequivocally characteristic morphology of typical TC (cells with telopodes-Tp). SEM revealed typical TC with two to three Tp-very long and branched cell prolongations. Tp consist of an alternation of thin segments (podomers) and thick segments (podoms). The latter accommodate mitochondria (as shown by Janus Green and MitoTracker), rough endoplasmic reticulum and caveolae. TEM showed characteristic podomers and podoms as well as close relationships with nerve endings and blood capillaries. IHC revealed positive expression of TC for c-kit, vimentin and CD34. In conclusion, this study shows the presence in trachea and lungs of a peculiar type of cells, which fulfils the criteria for TC.

Identification of telocytes in skeletal muscle interstitium: implication for muscle regeneration

Skeletal muscle interstitium is crucial for regulation of blood flow, passage of substances from capillaries to myocytes and muscle regeneration. We show here, probably, for the first time, the presence of telocytes (TCs), a peculiar type of interstitial (stromal) cells, in rat, mouse and human skeletal muscle. TC features include (as already described in other tissues) a small cell body and very long and thin cell prolongations-telopodes (Tps) with moniliform appearance, dichotomous branching and 3D-network distribution. Transmission electron microscopy (TEM) revealed close vicinity of Tps with nerve endings, capillaries, satellite cells and myocytes, suggesting a TC role in intercellular signalling (via shed vesicles or exosomes). In situ immunolabelling showed that skeletal muscle TCs express c-kit, caveolin-1 and secrete VEGF. The same phenotypic profile was demonstrated in cell cultures. These markers and TEM data differentiate TCs from both satellite cells (e.g. TCs are Pax7 negative) and fibroblasts (which are c-kit negative). We also described non-satellite (resident) progenitor cell niche. In culture, TCs (but not satellite cells) emerge from muscle explants and form networks suggesting a key role in muscle regeneration and repair, at least after trauma.

Neuroregeneration in neurodegenerative disorders

Background

Neuroregeneration is a relatively recent concept that includes neurogenesis, neuroplasticity, and neurorestoration - implantation of viable cells as a therapeutical approach.

Discussion

Neurogenesis and neuroplasticity are impaired in brains of patients suffering from Alzheimer's Disease or Parkinson's Disease and correlate with low endogenous protection, as a result of a diminished growth factors expression. However, we hypothesize that the brain possesses, at least in early and medium stages of disease, a "neuroregenerative reserve", that could be exploited by growth factors or stem cells-neurorestoration therapies.

Summary

In this paper we review the current data regarding all three aspects of neuroregeneration in Alzheimer's Disease and Parkinson's Disease.

Telocytes in endocardium: electron microscope evidence

The term TELOCYTES was very recently introduced, for replacing the name Interstitial Cajal-Like Cells (ICLC). In fact, telocytes are not really Cajal-like cells, they being different from all other interstitial cells by the presence of telopodes, which are cell-body prolongations, very thin (under the resolving power of light microscopy), extremely long (tens up to hundreds of micrometers), with a moniliform aspect (many dilations along), and having caveolae. The presence of telocytes in epicardium and myocardium was previously documented. We present here electron microscope images showing the existence of telocytes, with telopodes, at the level of mouse endocardium. Telocytes are located in the subendothelial layer of endocardium, and their telopodes are interposed in between the endocardial endothelium and the cardiomyocytes bundles. Some telopodes penetrate from the endocardium among the cardiomyocytes and surround them, eventually. Telopodes frequently establish close spatial relationships with myocardial blood capillaries and nerve endings. Because we may consider endocardium as a 'blood-heart barrier', or more exactly as a 'blood-myocardium barrier', telocytes might have an important role in such a barrier being the dominant cell population in subendothelial layer of endocardium.

Telocytes in human epicardium

The existence of the epicardial telocytes was previously documented by immunohistochemistry (IHC) or immunofluorescence. We have also demonstrated recently that telocytes are present in mice epicardium, within the cardiac stem-cell niches, and, possibly, they are acting as nurse cells for the cardiomyocyte progenitors. The rationale of this study was to show that telocytes do exist in human (sub)epicardium, too. Human autopsy hearts from 10 adults and 15 foetuses were used for conventional IHC for c-kit/CD117, CD34, vimentin, S-100, τ, Neurokinin 1, as well as using laser confocal microscopy. Tissue samples obtained by surgical biopsies from 10 adults were studied by digital transmission electron microscopy (TEM). Double immunolabelling for c-kit/CD34 and, for c-kit/vimentin suggests that in human beings, epicardial telocytes share similar immunophenotype features with myocardial telocytes. The presence of the telocytes in human epicardium is shown by TEM. Epicardial telocytes, like any of the telocytes are defined by telopodes, their cell prolongations, which are very long (several tens of μm), very thin (0.1-0.2 μm, below the resolving power of light microscopy) and with moniliform configuration. The interconnected epicardial telocytes create a 3D cellular network, connected with the 3D network of myocardial telocytes. TEM documented that telocytes release shed microvesicles or exocytotic multivesicular bodies in the intercellular space. The human epicardial telocytes have similar phenotype (TEM and IHC) with telocytes located among human working cardiomyocyte. It remains to be established the role(s) of telocytes in cardiac renewing/repair/regeneration processes, and also the pathological aspects induced by their 'functional inhibition', or by their variation in number. We consider telocytes as a real candidate for future developments of autologous cell-based therapy in heart diseases.

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.