The tonsils are masses of lymphoid tissue and form an important part of our immune system located at the gateway of respiratory and digestive tract. They act as the first line of defense against ingested or inhaled pathogens. Four types of tonsils are arranged into a ring around the oropharynx and nasopharynx, known as Waldeyer’s ring of lymphoid tissue. This article will look at the anatomy of the tonsils including the blood supply and innervation, as well as some histological anatomy and embryological development. We will also discuss function and clinical relevance.
The tonsils are part of MALT (mucosa associated lymphoid tissue). MALT can also be found in the bowel, in Peyer’s patches. In general MALT is relatively undeveloped at birth with low cellularity. Tonsils start to develop around 14-15th week of embryonic life, while germinal cenres are absent at this stage. Palatine tonsils and tonsillar fossa are believed to be the derivatives of the 2nd pharyngeal pouch. The epithelial lining proliferates and forms buds, which form the primordium of the palatine tonsil.
Microscopically, the tonsil is a mass of lymphoid follicles supported on a connective tissue framework. In addition, the center of each of these nodules is densely packed with lymphocytes, and is referred to as the germinal center. The tonsillar crypts (except the pharyngeal tonsil) will penetrate from the surface, almost down to the very center of the tonsil follicle. The luminal surfaces of the tonsils are coated in non-keratinising stratified squamous epithelium, which is the same tissue of the surrounding oropharynx.
They have antigen presenting cells on their surface that alert the underlying B and T cells, which are part of the adaptive immune response. In addition, the B cells produce antibodies, mainly IgA, which act to provide immune protection on mucosal surfaces.
Types of tonsils
These are located between the palatoglossal arch anteriorly and the palatopharyngeal arch posteriorly. They are located in the isthmus of the fauces (a cavity bound laterally by the palatoglossal arches, superiorly by the soft palate and by the tongue underneath). Laterally they are attached to the wall by a fibrous capsule, and are covered in stratified squamous epithelium on the pharyngeal side. The tonsil is penetrated by 15-20 crypts. The lumen of the crypts contain lymphocytes, bacteria and desquamated epithelial cells.
The palatine tonsils receive their blood supply from the tonsillar branches of five arteries:
- ascending palatine branch of the facial artery
- tonsillar branch of the facial artery
- ascending pharyngeal branch of the external carotid
- dorsal lingual branch of the lingual artery
- lesser palatine branch of the descending palatine artery
Venous drainage is to the internal jugular vein via the peritonsillar plexus of lingual and pharyngeal veins.
The nerve supply to the palatine tonsils arises from the maxillary division (V2 division) of the trigeminal nerve (cranial nerve V) as well as the tonsillar branches of the glossopharyngeal nerve (cranial nerve IX). The glossopharyngeal nerve will also continue on to supply taste to the posterior one third of the tongue as well as sensation.
These are small round elevations that sit on the most posterior part of the tongue base. They are considered a collection of lymphoid tissue which varies greatly in size and shape. They are covered by stratified squamous epithelium which invaginates to form a single crypt.
The blood supply to these tonsils is from the lingual artery, the tonsillar branch of the facial artery and the ascending pharyngeal branch of the external carotid artery.
The nerve supply is from the tonsillar branches of the glossopharyngeal nerve (cranial nerve IX).
These tonsils are located just posterior to the opening of the Eustachian tube (the torus tubaris) in the nasopharynx.
These are the most superior tonsils that lie in the superior part of the nasopharynx. It is attached to the periosteum of the sphenoid bone by connective tissue. The pharyngeal tonsils are covered with ciliated pseudostratified columnar epithelium, having ciliated, basal and goblet cells. The covering capsule is thinner compared to the palatine tonsils and the adenoids have no crypts. The epithelium is thrown into folds, with the lamina propria containing a mass of lymphatic tissue with numerous lymphatic nodules.