Assessment of Connective Tissue in the Equine Uterus and Cervix: Review of Clinical Impact and Staining Options.

Simple Summary: Uterine diseases are the leading cause of infertility in mares, causing increasing costs and losses in horses' breeding. Their current diagnosis is often supported by obtaining endometrial biopsies and hematoxylin–eosin staining, which is the basic staining used in histopathology. This review aims to show the variety of uterine changes affecting fertility and highlights the usefulness of special stains for connective tissue visualization—Masson trichrome, picrosirius red, elastica van Gieson, or periodic acid–Schiff—for a more comprehensive diagnosis. The fibrosis evaluation includes connective tissue changes in the cervix, the endometrium, and around/in the wall of blood vessels. Cervical connective tissue undergoes cyclic changes impacting fertility, whereas vascular changes, especially in multiparous mares, are crucial for adapting to physiological shifts, affecting early pregnancy and hindering placental development. Special stains are valuable for the identification of structural changes in the cervix and fibrosis in uterine blood vessels. Moreover, equine endometrosis, linked to fibrotic processes in the endometrium, emphasizes the need for wider use of special stains in diagnosis. Therefore, we advocate for special staining in reproductive tract evaluation due to its simplicity, accessibility, and effectiveness. We encourage scientists and diagnosticians to adopt additional tools for clearer pathology visualization, enabling reliable fertility predictions. Uterine diseases stand as the primary cause of infertility in mares; however, the diagnostic process often relies on obtaining endometrial biopsies and their hematoxylin–eosin staining. This review seeks to present the variability of uterine changes and their impact on fertility and underscore the utility of special stains, such as Masson trichrome, picrosirius red, elastica van Gieson, or periodic acid–Schiff, in enhancing diagnostic breadth. Connective tissue evaluation in the cervix is discussed, as it is subjected to cyclic changes and the impact on overall fertility. Vascular changes, particularly prevalent in multiparous mares, play a crucial role in adapting to physiological and pathological alterations, affecting early gestation and impeding placental development. Given that uterine vascular pathologies often involve fibrotic changes, connective tissue stains emerge as a valuable tool in this context. Moreover, equine endometriosis, predominantly associated with endometrial fibrosis, further highlights the relevance of special stains, suggesting their underutilization in the diagnostic process. Recognizing the subjective nature of diagnosing uterine pathologies and the need for additional diagnostic tools, we advocate for using dedicated stains in the histopathological evaluation of uterine samples. In conclusion, we encourage scientists and diagnosticians to embrace additional tools that enhance pathology visualization, enabling more reliable diagnoses concerning expected fertility. [ABSTRACT FROM AUTHOR]