Abstract：Pharyngeal tonsil (PhaT) belongs to mucosal-associated lymphoid tissue and plays an important role in mucosal immunity of respiratory tract. IgA (immunoglobulin A) and IgG (immunoglobulin G), which are 2 important effector molecules of mucosal immunity, have extensive immunological function. This research aims to study the changes of histological structure characteristics of pharyngeal tonsils during the development of yak (Bos grunniens), and the distributive characteristics and changing regularities of IgA and IgG antibody secreting cells (ASCs) in pharyngeal tonsils. In this study, histology method was used to observe and compare the histological structure characteristics of pharyngeal tonsils between newborn and adult yak. The expression of IgA and IgG in pharyngeal tonsils of newborn and adult yak was detected by immunohistochemistry (IHC). Histological results showed that pharyngeal tonsilar epithelium was roughly divided into 2 types, namely non-reticular epithelium and reticular epithelium. Non-reticular epithelium referred to pseudostratified columnar epithelium with a large number of alcian blue-periodic acid schiff (AB-PAS) positive goblet cells and non-keratinized stratified squamous epithelium. However, the reticular epithelium overlying the pharyngeal tonsil was the lymphoepithelium that was frequently infiltrated by lymphocyte. The lamina propria of yak pharyngeal tonsils was composed of lymphoid follicles and diffuse lymphoid tissue. A large number of high endothelial veins and lymphatic vessels could be found in the diffuse lymphoid tissue. After comparing with the pharyngeal tonsils of newborn and adult yak, only a several primary lymphoid follicles in the newborn yak's pharyngeal tonsils was found. However, not only primary lymphoid follicles, but also secondary lymphoid follicles can be found in adult yak's pharyngeal tonsils. In addition, the statistics of the number of lymphoid follicles showed that in the same unit field of view (100×), the number of lymphoid follicles was approximately (2±0.6019) in newborn yak, while was approximately (13±0.4079) in adult yak, which was much higher than that of the newborn(P＜0.01). The immunohistochemical results showed that IgA and IgG ASCs were distributed in the subepithelial region of non-reticular epithelium, the reticular epithelium region, inter-follicles area, lymphoid follicles and glandular compartments in the 2 groups, and the distribution of IgA and IgG ASCs was the highest in the subepithelial region of non-reticular epithelium, respectively. In addition, the average number of IgG ASCs per unit area(0.023 mm2) were higher than that of IgA ASCs in both 2 groups, and there were significant difference in the adult group(P＜0.01), but no difference in the newborn group (P＞0.05). The results indicated that the primary lymphoid follicles of yak pharyngeal tonsils have been formed in the newborn stage; IgA and IgG ASCs played an important immunoprotective effect on the mucosal surface through the non-reticular epithelial; IgG may be dominant in the immune defense of yak pharyngeal tonsils due to there was a preponderance of IgG ASCs in 2 groups. This study provides morphological and immunological information for the mucosal immunity and the pathogenesis of related diseases.
