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:
- cells in
the tonsils are supported by a fine network of reticular fibres and
- 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.
![thumbnail]( "The tonsils are")
Title:
The tonsils are
by:
om
at
2013-02-15T03:29:00+07:00
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The tonsils are