Morphological Characteristics of Genital Organ-Associated Lymphoid Tissue in the Vaginal Vestibule of Goats and Pigs

SIMPLE SUMMARY: Mucosa-associated lymphoid tissue is a diffuse system of aggregations of lymphoid tissues found in the gut, nasopharynx, bronchus, tonsils, eye, skin, salivary glands, and urinary tract, which are in close contact with the outer environment. We previously reported the presence of lymphoid tissue in the mucosa of the vaginal vestibule in cows, but information regarding other animals is scarce. This study investigated the gross anatomical and histological features of lymphoid tissue in the mucosa of the vaginal vestibule of goats and pigs. The vaginal vestibules of healthy adult goats and pigs were examined using hematoxylin and eosin staining, immunohistochemistry, and immunofluorescence. Furthermore, scanning electron microscopy was used for analyzing the ultrastructure of the mucosal surface. Goat and pig lymphoid tissues comprised B cells, T cells, macrophages, dendritic cells, and plasma cells. Furthermore, the vaginal vestibule epithelium covering lymphoid tissues was disrupted by a large intercellular space in both goats and pigs, and immune cells directly contacted the luminal space through this area lacking epithelium. No obvious differences were found between goats and pigs, suggesting that genital organ-associated lymphoid tissue is a conserved local immunological barrier in these animals. ABSTRACT: Mucosa-associated lymphoid tissue (MALT) is a specialized form of peripheral lymphoid tissue (LT), which is found on mucosal surfaces exposed to the environment. However, morphological data of these tissues in farm animals are scarce. This study investigated the gross anatomical and histological features of genital organ-associated lymphoid tissues (GOALTs) in the vaginal vestibule (VV) of healthy, non-pregnant, adult goats and pigs. Their VVs were composed of stratified squamous, non-keratinized epithelium, and various-sized dark-blue hematoxylin-positive spots were observed in whole-mount specimens, which were diffusely distributed throughout the mucosal surfaces. These spots were histologically identified as LTs and consisted of lymphatic nodules (LNs) or diffuse lymphoid tissue (DLTs). Both LNs and DLTs contained B cells, T cells, macrophages, dendritic cells, plasma cells, and high endothelial venules. Only the numbers of B cells were significantly higher in both the LNs and DLTs of pigs compared to goats. Furthermore, the surface of the VV epithelium covering the LTs was partially disrupted with a large intercellular space containing abundant connective tissue fibers with numerous lymphocytes. In conclusion, GOALTs in the VV appear to be common local immunological barriers in both examined animals. This knowledge is crucial for understanding the structures and disorders of female reproductive organs in farm animals.