The testes, which are specialized organs of the male reproductive system, ensure the continuity of species through the production and release of spermatozoa and male sex hormones [1,2]. The testes are composed of seminiferous tubules, where spermatogenesis takes place, and interstitial sections surrounding the seminiferous tubules, where steroidogenesis takes place. The interstitial space consists of loose connective tissue and contains testosterone-producing Leydig cells as well as fibroblasts, immune cells, and blood and lymph vessels [3,4]. The seminiferous tubules, which host the spermatogenesis process and constitute 60–80% of the testicular volume, contain germinal, peritubular myoid and Sertoli cells [1,5]. Sperm cells produced by spermatogenesis are morphologically mature, but have not yet acquired advanced motility and fertilizing capacity. Sperm cells complete their maturation by undergoing molecular and physiological changes in the highly convoluted epididymal duct [6]. Mature sperm cells, together with the secretion of the accessory genital glands, are ejaculated via the epididymis and ductus deferens [7]. Male reproductive physiology involves the differentiation and development of testicular germinal cells, which commence with puberty. These developmental processes are associated with the level and type of motility of germinal cells, both of which are related to the activity of cytoskeletal proteins [2,8]. In general, proteins found in the cytoskeleton include microfilaments (actins), intermediary filaments (vimentin, desmin, cytokeratins, neurofilament proteins and glial fibril acidic protein (GFAP)) and microtubuli (tubulins). These cytoskeletal filaments are engaged in various structural and functional tasks, including among others the maintenance of the shape and polarity of cells, the positioning of cell organelles, the support of cytoplasmic extensions, and the fixation of organelles to the plasma membrane [9].

Actin filaments are the main component of the inner cytoskeleton in both prokaryotes and eukaryotes, as well as the primary determinant of cell shape. Actin filaments have critical roles in multiple physical and cellular processes, including the regulation of cell shape changes in the first place, as well as cell adhesion, cell migration, cell division and the production of mechanical forces [10]. The distribution of these filaments concentrates at certain points in the basal region of cells, in between the adhesion components [11]. Actin filaments exhibit ATP-dependent polymerization. This polymerization structure enables the building of junctions between Sertoli and germinal cells, which prevent the free passage of molecules from the adluminal region of the seminiferous tubules, and thereby, contribute to the blood-testis barrier by establishing an immunologically protected microenvironment [12].

Different from other intermediary filaments, vimentin is formed by the polymerization of a single protein. In vertebrates, vimentin expression has been detected in the connective and fat tissues, as well as in mesenchymal cells during early development, and in malignant epithelial diseases. Overall, vimentin is an important protein in that it mediates intercellular signaling, acts as a cell surface cofactors for the sequestration or activation of proteins and nucleic acids, and provides mechanical support to cells [13,14]. Desmin is the main component of the intermediary filaments of striated and smooth muscle cells and undertakes the primary role of supplying structural and mechanical support to cells. Recent reports indicate that desmin affects various biological processes, including among others myogenesis, muscle contraction and mitochondrial functions [15]. Cytokeratins (CKs), which are another type of intermediary filament, are classified under two groups, namely, low molecular weight-CKs (CK10-20) and high molecular weight-CKs (CK1-9) [16]. CKs are the main cytoskeletal component of epithelial cells and provide mechanical support to cells. Furthermore, CKs are expressed by organs at specific levels and influence the differentiation of various types of epithelial cells. CK-18 is the primary protein of simple epithelial cells, including various parenchymatous epithelia, and is found in the duct epithelium, mesothelium and single-layered epithelium [17].

Although cats have been a long-time pet animal, companion and entertainer of humans, very little is known about the structure and biology of their reproductive system. To date, research on feline reproduction has mainly focused on gamete biology, endocrinology and cryobiology. Thus, molecular factors localized to the feline reproductive organs, feline reproductive endocrinology and pre- and post-pubertal molecular mechanisms of the male domestic cat are topics that receive considerable academic attention [[18], [19], [20]]. The availability of scientific data on the functions of the reproductive system of the male cat is important because it serves as a model for the investigation of the structure and physiology of the reproductive system of other felids. Given that certain wild felids and domestic cat breeds are endangered, research on the common cat provides an understanding of the expression and localization of molecular factors in the reproductive system of all felids [21]. Research has shown the involvement of intermediary filaments in cell migration, cell division and the organization of cell organelles during the different phases of spermatogenesis [22], as well as in the maintenance of the morphology of spermatogonia and the fixation of differentiated Sertoli and germinal cells to the epithelium lining the seminiferous tubules [[23], [24], [25]]. Furthermore, it has been reported that actin filaments are also effective on some spermatogenic cells of the seminiferous epithelium, Sertoli cells and peritubular myoid cells [26]. Based on this data set, the present study was designed to determine periodical differences in the localization and distribution of the proteins actin, vimentin, desmin and CK-18 in the testes, epididymides and ductus deferentes of Persian and Turkish Angora and Van cats during the pre-pubertal and post-pubertal periods by immunohistochemistry, and to ascertain the potential roles of these proteins in the reproductive physiology of male cats.