Tonsils
Tonsils belong to the mucosa associated lymphoid tissue (MALT),
but because they are incompletely encapsulated, they are considered organs and
will be studied apart from the MALT. The tonsils constitute a lymphoid tissue
that lies beneath, and in contact with, the epithelium of the initial portion
of the digestive tract. Depending on their location, tonsils in the mouth and
pharynx are called palatine, pharyngeal, or lingual.
Palatine
Tonsils
The two palatine tonsils are located in the lateral walls of
the oral part of the pharynx. They are lined with a squamous stratified
epithelium that often becomes so densely infiltrated by lymphocytes that it may
be difficult to recognize. The lymphoid tissue in these tonsils forms a band
that contains free lymphocytes and lymphoid nodules, generally with germinal
centers. Each tonsil has 10–20 epithelial invaginations that penetrate the
tonsil deeply, forming crypts, whose lumens contain desquamated
epithelial cells, live and dead lymphocytes, and bacteria. Crypts may appear as
purulent spots in tonsillitis. Separating the lymphoid tissue from subjacent
structures is a band of dense connective tissue, the capsule of the
tonsil. This capsule usually acts as a barrier against spreading tonsillar
infections.
Pharyngeal
Tonsil
The pharyngeal tonsil is a single tonsil situated in the
superior— posterior portion of the pharynx. It is covered by ciliated
pseudostratified columnar epithelium typical of the respiratory tract, although
areas of stratified epithelium can also be observed.
The pharyngeal tonsil is composed of pleats of mucosa and
contains diffuse lymphoid tissue and lymphoid nodules. It has no crypts, and
its capsule is thinner than the capsule of the palatine tonsils. Hypertrophied
pharyngeal tonsils resulting from chronic inflammation are called adenoids.
Lingual
Tonsils
The lingual tonsils are smaller and more numerous than the
palatine and pharyngeal tonsils. They are situated at the base of the tongue
and are covered by stratified squamous epithelium. Each lingual tonsil has a
single crypt.
Waldeyer's Ring
of Lymphoid Tissue
The lymphoid
tissue that surrounds the opening into the respiratory and digestive systems
forms a ring. The lateral part of the ring is formed by the palatine tonsils
and tubal tonsils (lymphoid tissue around the opening of the auditory tube in
the lateral wall of the nasopharynx). The pharyngeal tonsil in the roof of the
nasopharynx forms the upper part, and the lingual tonsil on the posterior third
of the tongue forms the lower part.
REFERENCES
Junqueira's
Basic Histology: Text and Atlas, 13th Edition
Wheater's
Functional Histology: A Text and Color Atlas, 5th Edition