Histological and immunohistochemical changes in equine sarcoids

The article presents the results of histopathological and immunohistochemical studies of three types of equine skin neoplasms, classified as type 2 fibroblastic sarcoid based on their morphological features. The tumours were localized in the abdominal area, macroscopically presented as dense, fleshy formations without a stalk, yet with small ulcers on the surface. Infection of horses could have occurred through direct or indirect contact with other infected horses and cattle, as well as through insects. Histological examination of the sarcoid established that the epidermis was in a state of hyperkeratosis and acanthosis, with the formation of numerous projections that penetrated deeply into the dermis. The boundaries between the papillary and reticular layers of the dermis were not visualized; the defining feature was the chaotic spindle-shaped fibroblasts that formed a significant number of dense whirl-like structures, or moire patterns, which encircled vessels of a varying caliber. The cell nuclei were predominantly round-shaped and hyperchromatic. Present were both typical and atypical mitotic figures. Replacement of loose connective tissue with collagen fibers was observed intradermally. Upon studying the morphological structure of the sarcoid on semi-thin sections, significant vascularization was observed. Spindle-shaped fibrocytes with branched processes and elongated nuclei were located around vessels of varying sizes, exhibiting well-differentiated single or double nuclei. Fibroblasts of various shapes and sizes with round nuclei were randomly distributed throughout the entire area of the neoplasm. The bulk of the sarcoid consisted of collagen fibers, which turned pink when exposed to methylene blue and further exposure to acid fuchsin. Immunohistochemical studies revealed intense vimentin-positive expression in the sarcoid cells, confirming the hypothesis of their mesodermal origin. Upon detecting Ki-67 antibodies in the sarcoids from three clinical cases, a significant number of cells in the G1 and S phases of mitosis were discovered; the cell nuclei and karyoplasm were stained brown. These actively proliferating cells of the neoplasm constitute the "growth fraction" of the tumour and point to a high risk of recurrence and malignancy.