Keywords: horses; ultrastructure; fibroblasts; fibrocytes; collagen fibers; capillaries; endotheliocytes; myofilaments

Abstract The article presents the results of ultrastructural research on fibroblastic sarcoid, which is one of the most aggressive types of sarcoid in horses. A retrospective study on the prevalence of sarcoids in horses in Ukraine’s western regions was conducted in 2019–2023. It was found that during the period of 2022–2023, the number of horses with sarcoids increased sharply by 15.6% compared to the period 2020–2021. This was associated with the mass movement of animals from eastern regions due to the onset of the war in Ukraine. The fibroblastic type of sarcoid was predominantly localized in the abdominal wall and groin area. To study the ultrastructure of sarcoids, they were surgically removed by excising undamaged skin portions under general anesthesia, adhering to all requirements of the European Convention for the Protection of Animals used for Experimental and Other Scientific Purposes. Electronograms revealed that the tumour formation mainly consisted of fibroblasts of varying differentiation degrees: significant number of vesicles, markedly expanded endoplasmic reticulum cisterns, and irregularly shaped nuclei with numerous invaginations. Open nuclear pores were observed in most nuclei. Active formation of a large number of capillaries was noted between the collagen matrix, fibroblasts, and fibrocytes, indicated by the increased number of endotheliocytes with pseudopodia on the cytoplasm’s marginal part and the basal surface. This suggests their embryonic type. Endotheliocytes contained large round nuclei and a significant number of mitochondria in the cytoplasm. There were both bright and dark endotheliocytes in the blood capillaries’ venous section. The cytoplasm of bright endotheliocytes contained myofilaments, individual mitochondria, free ribosomes, peroxisomes, and vesicles of various sizes. The plasmalemma formed small pseudopodia. The capillary lumen showed moderate electron density. Additionally, high platelet activity was observed, manifested by cell adhesion to the marginal part of the endotheliocyte cytoplasm. It should be noted that most capillaries were of the venous type, as indicated by the endotheliocytes’ height, the presence of an increased number of mitochondria and vesicles. The entire cellular pool was localized among the massive framework consisting of collagen fibers. Elastic fibers could not be detected in the fibrillar component.

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