Mesothelial-to-mesenchymal transition (MMT) is an autoregulated physiological process of tissue repair that in uncontrolled conditions, such as peritoneal dialysis (PD), can lead to peritoneal fibrosis. The maximum expression of sclerotic peritoneal syndromes (SPS) is the encapsulating peritoneal sclerosis (EPS) for which no specific treatment exists. The SPS includes a wide range of peritoneal fibrosis that appears progressively and is considered as a reversible process, while EPS does not. EPS is a serious complication of PD characterized by a progressive intra-abdominal inflammatory process that results in bridles and severe fibrous tissue formation which cover and constrict the viscera. Recent studies show that transdifferentiated mesothelial cells isolated from the PD effluent correlate very well with the clinical events such as the number of hemoperitoneum and peritonitis, as well as with PD function (lower ultrafiltration and high Cr-MTC). In addition, in peritoneal biopsies from PD patients, the MMT correlates very well with anatomical changes (fibrosis and angiogenesis). However, the pathway to reach EPS from SPS has not been fully and completely established. Herein, we present important evidence pointing to the MMT that is present in the initial peritoneal fibrosis stages and it is perpetual over time, with at least theoretical possibility that MMT initiated the fibrosing process to reach EPS. 1. Introduction Peritoneal dialysis (PD) is a form of renal replacement therapy that uses the peritoneal membrane (PM) as semipermeable barrier for the exchange of toxic substances and water. This technique has increased during the last years, in parallel to its complications. Currently, prolonged survival on PD has been reached due to technological advances, prevention, and early diagnosis of uremic complications. The basic objective of DP is the long-term preservation of the PM function. The PM is lined by a monolayer of MCs that have characteristics of epithelial cells and act as a permeability barrier across which ultrafiltration and diffusion take place. The long-term exposure to hyperosmotic, hyperglycaemic, and low pH of dialysis solutions and repeated episodes of peritonitis or hemoperitoneum cause injury of the peritoneum, which progressively becomes denuded of MCs and undergoes fibrosis and neovascularization . Such structural alterations are considered the major cause of ultrafiltration failure [1, 2]. In this context, it has been proposed that local production of vascular endothelial growth factor (VEGF), a potent proangiogenic cytokine,
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