Dysfunctional endothelial cells in patients with chronic thromboembolic pulmonary hypertension. Rationale: The material obtained from pulmonary endarterectomy (PEA) offers the unique opportunity to study the pathophysiological mechanisms of chronic thromboembolic pulmonary hypertension (CTEPH) at disease site. Mitochondrial disarrangements in endothelial cells might explain a hyperproliferative resistant phenotype that could explain vascular changes occurring in CTEPH. We aimed to develop an in vitro model of CTEPH using patient-derived cell lines and assess potential mitochondrial disturbances. Methods: Isolated cells from specimens obtained at PEA, were confirmed as being endothelial cells based on cobblestone morphology, endothelial phenotype (flow cytometry, RT-PCR, immunofluorescence) and functional analysis (tubule formation, proliferation and migration). We also measured: i) mitochondrial membrane potential (MMP), mitochondrial content and apoptosis/necrosis by flow cytometry and ii) mitochondrial dynamics (MD) by confocal microscopy. Results: Isolated cells maintained cobblestone morphology and stained positive for endothelial markers. They showed a hyperproliferative phenotype when compared with control human pulmonary artery endothelial cell lines (HPAE): number of Ki67+cells (50.33±13.4 vs 32.5±9.5; p<0.05), and fold expansion (1.56±0.08 vs 0.8±0.05; p<0.002). Functionally, they showed reduced capacity to form tubule structures (150±44 vs 96±21; p<0.03). CEPTH cells tended to show lower rates of depolarized MMP (49.91±14.70 vs. 59.87±8.41, p=NS), a decrease of mitochondrial content (148.94±69.96 vs. 295.57±178.60, PNS) and lower levels of necrosis/apoptosis (23.57±8.03 vs. 29.33±5.94, p=NS). Mitochondria from CTEPH patients tended to be smaller and to show higher circularity (0.45±0.009 vs. 0.43±0.012, p=NS), with less branching (2.77±0.14 vs. 2.93±0.06, p=NS) with respect to controls, both considered as pathologic markers. Conclusions: Endothelial cells obtained from PEA in CTEPH show a hyperproliferative phenotype, impaired function and mitochondrial material derangement, that may play a role in the pathogenesis of pulmonary hypertension after pulmonary embolism.