Histopathological Analysis of Thrombi in Acute Ischemic Stroke: An Exploratory Study of Thrombus Composition and CD34-Positive Endothelial Cells

Background/Objectives: Mechanical thrombectomy (MT) enables direct examination of the retrieved thrombus in acute ischemic stroke. Thrombus composition may influence treatment outcomes and reflect underlying stroke mechanisms. This study aimed to analyze thrombus histological composition and CD34-positive endothelial cells and evaluate their association with clinical and radiological characteristics. Methods: Fifty-six patients with acute ischemic stroke who underwent MT were included. Thrombi were classified as fibrin-dominant or red blood cell (RBC)-dominant using hematoxylin–eosin staining (H&E). Endothelial cells were identified via CD34 immunostaining. Associations between thrombus composition, procedural variables, imaging findings, and clinical outcomes were analyzed. Results: Forty-one (73.2%) thrombi were fibrin-dominant, and 15 (26.8%) were RBC-dominant. Fibrin-dominant thrombi were significantly associated with more distal occlusions (p = 0.027) and with the use of stent-retrievers (p = 0.045). RBC-dominant thrombi were more frequently associated with the hyperdense artery sign (HAS) (p = 0.015). CD34-positive staining correlated with shorter symptom-to-door (p = 0.017) and symptom-to-puncture times (p < 0.001). Endothelial ingrowth was more common in thrombi from proximal occlusions (p = 0.017). No significant associations were observed between thrombus composition and recanalization success, number of passes, or functional outcomes. The association between RBC-dominant thrombi and HAS supports the potential role of imaging markers in predicting thrombus composition prior to intervention. In addition, the presence and distribution of CD34-positive endothelial cells in relation to time intervals and occlusion location may reflect dynamic processes such as thrombus organization and vessel wall interaction. Conclusions: These findings highlight the heterogeneous nature of thrombus in acute ischemic stroke. Further studies are needed to clarify the biological and clinical implications of these observations.