Histology and ultrastructure of the equine lingual tonsil. II. Lymphoid tissue and associated high endothelial venules.

This study investigates the structure and cellular composition of the lingual tonsil in horses, particularly its lymphoid tissue and associated high endothelial venules, and identifies a structural base for enhanced permeability and movement of cells and large molecules.

Study Overview

The research focused on the microscopic structure and constituents of the lingual tonsil on the tongue of young horses. This tonsil is an essential component of the immune system, rich with various types of immune cells. The authors screened for the presence of different immune cells, identified a notable organ structure, and proposed a mechanism for molecule transport within the tonsil.

Observations and Findings

• The researchers found that the surface layer of the tonsil was occupied by CD4 and CD8 positive cells, macrophages, and IgGb and IgA positive cells. CD4 and CD8 are markers for T helper cells and cytotoxic T cells respectively, while IgG and IgA are antibodies indicating B-cell activity. Macrophages are involved in ingesting harmful pathogens.
• The lymphoid follicles of the tonsil had 3 distinct zones – the parafollicular area, corona, and the germinal center. Each zone had a unique composition of immune cells.
• In the parafollicular zone, there were many CD4 and CD8 positive lymphocytes, macrophages, interdigitating cells, and a small number of B-lymphocytes. Interdigitating cells are antigen-presenting cells that play a role in initiating immune response.
• The germinal centre contained mostly IgGb and IgG(T) positive cells, plasma cells (which secrete antibodies), and fewer numbers of follicular dendritic cells (important in B-cell maturation), macrophages, CD4 and CD8 positive, and IgA positive cells.

High Endothelial Venules and a New Organelle

• Some blood vessels (venules) in the parafollicular and inter-follicular areas had characteristics of high endothelial venules. These are specialized blood vessels that facilitate the entry of lymphocytes from the bloodstream into lymph nodes.
• The researchers identified an unprecedented organelle, referred to as a vesiculo-vacuolar organelle, situated towards the lateral, luminal, and abluminal surfaces of these high endothelial venules.
• These vesiculo-vacuolar organelles, with their openings (diaphragms and stomata) communicated with each other and with spaces between the endothelial cells (inter-endothelial clefts). This was suggested to provide a route for increased permeability and transmigration of macromolecules and lymphocytes across the venules.