The tonsils are

Tonsils

The tonsils are accumulations of lymphoid tissue surrounding the openings of the digestive and respiratory tracts. The tonsils and smaller accumulations of lymphoid tissue, which may be found between them, are also called Waldeyer's ring.

Depending on their localisation we distinguish between

• palatine tonsils (THE tonsils), which are located in the lateral wall of the oropharynx and covered by a stratified squamous epithelium,
   
• lingual tonsils which are situated in the lamina propria at the root of the tongue and also covered by a stratified squamous epithelium, and
   
• pharyngeal tonsils (also called nasopharyngeal tonsils or adenoids) which are located in the upper posterior part of the throat (nasopharynx) and covered by a pseudostratified ciliated epithelium with goblet cells.

The tonsils do not have afferent lymph vessels. Efferent lymph vessels are present. Exposure to antigens relies on the contact of antigens with cells of the immune system across the epithelium which covers the tonsils. The epithelium of the palatine and lingual tonsils forms deep crypts into the lymphoid tissue, and the resulting increase of the surface area is one way to facilitate the contact of antigens with the immune cells. In addition, the epithelium may specialise in places to form an open meshwork of cells with an incomplete basal lamina (a reticulated epithelium) which allows the infiltration of the epithelium by lymphocytes and macrophages.
Tonsillar lymphoid nodules consist mainly of B-lymphocytes. Other areas are occupied by T-lymphocytes, activated B-lymphocytes and other cells of the immune system.

The tonsils share some histological features with lymph nodes:

1. cells in the tonsils are supported by a fine network of reticular fibres and
2. high-endothelial (postcapillary ~) venules function in the "homing" of circulating lymphocytes - this is actually a shared feature of all lymphoid tissues and organs.

The palatine tonsils are surrounded by a thick hemicapsule of connective tissue, which delimits them from the pharyngeal muscle and facilitates their removal in tonsillitis.

Tonsil, human - H&E
Identify the surface epithelium covering the lymphoid tissue of the tonsils, lymph nodules and tonsillar crypts. Have a look at the epithelium both inside and outside the crypts. Outside the crypts, the epithelium will typically look like ordinary stratified squamous epithelium. Inside the crypts, where cells of the immune system often invade the epithelium, it will be difficult to find the boundary between epithelium and lymphoid tissue. Tissue preservation is not that great, but with a little bit of patience you should be able to find high-endothelial venules in the lymphoid tissue.
Sketch the organisation of the tissue at low magnification.