Histology, immunohistochemistry and ultrastructure of the equine tubal tonsil.

This research study focuses on the structure and characteristics of the tubal tonsil in horses, which is a part of the immune system found near the eustachian tube’s pharyngeal opening. The findings reveal unique cellular features and differences when compared to the nasopharyngeal tonsil, providing a deeper understanding of the horse’s immune response and overall physiology.

Study methodology and findings

The researchers examined the tubal tonsil using various microscopy techniques and studied its structure, cells, and associated lymphatic tissue. Key findings included:

• The tubal tonsil is lined by pseudostratified columnar ciliated epithelium, with areas densely filled with lymphocytes but lacking goblet and ciliated cells.
• Features characteristic of M cells, such as reduced microvilli or a bare surface, more mitochondria, small vesicles, and lysosomes were observed.
• M cells were also found in areas of respiratory epithelium that weren’t associated with lymphoid follicles, suggesting they could be the nasal mucosal counterparts of recently described intestinal villous M cells in mice.

Lymphoid tissue characteristics

In their study of the underlying lymphoid tissue, the researchers found it organized in a specific manner:

• Lymphoid follicles were generally solitary and lacked germinal centers, a contrast to the large and diffuse lymphoid follicles with germinal centers found in the nasopharyngeal tonsil.
• There were much fewer CD8+ T and B-lymphocytes when compared to the nasopharyngeal tonsil.

Other observations

In terms of vascular characteristics and additional unique features of the tubal tonsil, the authors found:

• High endothelial venules were mainly oriented towards the parafollicular area and contained fewer endothelial pores and vesiculo-vacuolar organelles.
• Furthermore, they identified small clusters of mucus acini and striated muscles, emphasizing differing characteristics between the tubal and nasopharyngeal tonsils.

Overall, these findings provide new insights into the immune system physiology of horses, specifically the cellular makeup, lymphatic tissue arrangement, and the diversity between the tubal and nasopharyngeal tonsils.