Expression and Distribution of Osteopontin, Osteocalcin and Osteonectin in the Bull Reproductive Tract

Osteopontin (OPN), osteocalcin (OC) and osteonectin (SPARC/ON) are multifunctional proteins present in both hard and soft tissues, where they contribute to various physiological processes, including reproductive function. However, their concurrent expression and distribution in the bull testis and epididymis have not been previously characterised. This study aimed to immunohistochemically evaluate the expression patterns and localisation of OPN, OC and SPARC/ON across different segments of the testicular and epididymal structures in bulls. The findings revealed that OPN exhibited variable expression across seminiferous tubules, with localisation in germ cells—particularly spermatogonia and spermatocytes—as well as in Sertoli and Leydig cells within the interstitial compartment. In the epididymis and associated ducts, OPN expression was predominantly observed in epithelial cells. OC demonstrated strong expression in the epithelial cells of the ductus epididymis and ductus deferens, as well as in select connective tissue and smooth muscle cells of the ductus deferens. SPARC/ON expression ranged from moderate to strong across spermatogenic cell lineages, Sertoli and Leydig cells and blood vessels within seminiferous tubules. These findings indicate that OPN, OC and SPARC/ON are differentially expressed throughout the bull testis, rete testis, ductuli efferentes, epididymis and ductus deferens, highlighting their potential roles in maintaining reproductive function under physiological conditions.