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Abstract

The molecular and ultrastructural characteristics of the epithelial cells in the seminiferous tubules as well as the peritubular wall cells of mammalian testes have been subject to controversial debates for several decades that have not been clarified yet. Using biochemical as well as light and electron microscopical methods, in particular immunolocalization techniques, I have studied sexually mature testes from several mammalian species (man, bull, boar, guinea pig, rat and mouse). The analyses included direct interspecies comparison of the tubular and peritubular structures with epithelial tissues of the adjacent excurrent duct system, including the epididymis, and in addition other comparative epithelial and muscular tissue controls. The present study of cytoplasmic filaments of Sertoli cells in seminiferous tubules confirmed the presence of vimentin intermediate-sized filaments (IFs) and the absence of cytokeratin IFs. Furthermore, my analyses of the corresponding cell-cell adhering junctions (AJs) of Sertoli cells and germ cells validated the absence of desmosomes or “desmosome-like” junctions. In addition, epithelial molecules such as E-cadherin or EpCAM were absent. In contrast, AJs present in the seminiferous tubules are based on N-cadherin clusters anchored in cytoplasmic plaques. These plaques contain α- and β-catenin, plakoglobin, proteins p120 and p0071 as well as a protein of the striatin family. My findings revealed that the intratubular Sertoli cells are interconnected with adjacent Sertoli cells as well as with germ cells by a novel type of AJs: Specific N-cadherin-based AJs, i.e., variously-sized, often very large cell-cell contacts (“areae adhaerentes”). In addition, in certain regions of bovine Sertoli cells, I have found small clusters of sieve-like cell-cell contacts perforated by cytoplasm-to-cytoplasm channels 5–7 nm in luminal diameter (“cribelliform junctions”). They are generally associated and laterally connected by tight junction-like membrane-membrane contacts. The cells of the seminiferous tubules are surrounded by a well-developed basal lamina. However, I could show that the basal lamina is not attached to the Sertoli cells by any hemidesmosomal structures and lacks hemidesmosomal marker molecules such as the integrin α6/β4 complex, protein HD230/233 (bullous pemphigoid antigen, BPA 230) and tetraspanin CD151. The basal lamina of the seminiferous tubules is enclosed by a lamellar encasement structure, the peritubular walls. These are composed of a bandage system of monolayers of peritubular smooth muscle cells (SMCs) interspersed with layers of extracellular matrix (ECM). The peritubular SMCs are for the most part polyhedral and very flat (down to ca. 30–100 nm cytoplasmic thickness). The number of these lamellar monolayers can vary from one to six per bandage system in different species, developmental stages and regions. They contain smooth muscle α-actin, the corresponding myosin light and heavy chains, tropomyosin, α-actinin, smoothelin, desmin, vimentin, talin, filamin A, dystrophin, drebrin, caveolin, caldesmon, calponin and protein SM22α, i.e. a typical molecular complement of SMCs. In addition, the peritubular SMC cytoplasm is enriched with myofilament bundles and other typical SMC structures such as "dense bodies", plasma membrane-associated “focal adhesions” and caveolae. Hence, these peritubular cells represent a novel kind of smooth muscle cells and tissue. In this study, I refer to these cells as “lamellar smooth muscle cells” (LSMCs) and not as they have been described in the literature as “myoid cells”, “myofibroblasts” or “myoepithelial cells”. Furthermore, within a monolayer, the LSMCs are connected by end-to-end junctional contacts and laterally by numerous, vertical AJs located in variously-sized, closely overlapping cell processes (“processus alter supra alterum”). The major transmembrane glycoprotein present in the AJs of LSMCs is cadherin-11, often in addition with P-cadherin in some species. These AJ cadherins are anchored in cytoplasmic junction plaques formed by β-catenin, plakoglobin, occasionally protein p0071 and also the 54 kDa plaque protein myozap as well as protein LUMA. Using endothelial cell type marker molecules such as VE-cadherin, claudin-5, protein PE-CAM (CD31), protein LYVE-1 and podoplanin, I could show that the peritubular wall structure of all species examined is not surrounded by a continuous lymphatic endothelium, as it has often been claimed in the literature for rodent testes. The novel types of AJ structures are compared with the AJs hitherto known. The special cell type characters of the seminiferous tubule cells and the LSMCs of the peritubular wall are discussed with respect to their architectonic and physiological functions as well as to structural changes during development, aging and in pathogeneses. Finally, my data contributes to the understanding of molecular characteristics of the male genital tract and can be used for the diagnostic pathology of genital tumors.