Histology of the Tonsils

Overview and Waldeyer's Ring

1. Surface Epithelium

• The palatine tonsil is covered by non-keratinized stratified squamous epithelium (SSE), which is continuous with the oropharyngeal mucosa.
• A defining histological feature is the infiltration of this epithelium by lymphocytes - often so dense that the epithelial-connective tissue boundary is completely obscured on H&E sections.
• Where infiltration is less marked, the basal cells (BC) of the stratified squamous epithelium are clearly recognizable, along with a thin band of collagen fibers (CF) at the epithelial-lamina propria interface.
• In heavily infiltrated zones, lymphoid nodules literally grow into the epithelium, leaving only isolated islands of epithelial cells (Ep) within a sea of lymphocytes, and the collagen band appears fragmented.

SSE = stratified squamous epithelium; TC = tonsillar crypts; L = lymphatic tissue; MG = mucus-secreting glands. (Histology: A Text and Atlas, Plate 14.1)

2. Tonsillar Crypts

• The surface epithelium dips deep into the underlying connective tissue, forming 10-15 blind-ending, often highly branched crypts that penetrate the full thickness of the tonsil.
• The crypt walls are lined by the same stratified squamous epithelium and are closely associated with underlying lymphoid nodules.
• Crypts serve as antigen-sampling sites - their epithelium is in intimate contact with mucosal lymphocytes, facilitating immune surveillance.
• The base of crypts may contain mucus-secreting glands (MG), visible at low magnification.
• Clinically, crypts can trap debris and bacteria, predisposing to recurrent tonsillitis.

3. Lymphoid Nodules and Germinal Centers

• The bulk of the tonsil consists of numerous lymphatic nodules packed within the lamina propria/mucosa.
• Because of their density, adjacent nodules tend to merge with one another.
• Many nodules contain well-developed germinal centers (GC) - lighter-staining central zones visible on H&E. These are sites of B-cell proliferation, somatic hypermutation, and affinity maturation following antigen stimulation.
• The germinal center is surrounded by a darker mantle zone of naive B lymphocytes.
• The overall architecture differs from a lymph node - there is no distinct cortex-medulla organization and no capsule separating lobules.

Top panel (x47): N = nodules; GC = germinal centers; SE = surface epithelium; Ly = lymphocytes invading epithelium; S = submucosa. Bottom panel (x365): BC = basal cells; CF = collagen fibers at epithelial boundary; Ep = islands of epithelial cells within lymphoid tissue; C = disrupted collagen. (Histology: A Text and Atlas, Plate 14.1)

4. Submucosa and Capsule

• Beneath the lymphoid nodules lies the submucosa (S), consisting of dense irregular connective tissue that is continuous with the connective tissue of the tonsillar bed.
• The deep (lateral) surface of the palatine tonsil is bounded by a fibrous hemicapsule, which separates the tonsil from the superior constrictor muscle and allows the surgical plane of cleavage during tonsillectomy.
• Unlike lymph nodes, the tonsil does not have a complete fibrous capsule on its mucosal side.
• Trabeculae extend inward from this hemicapsule, incompletely dividing the lymphoid tissue.

5. Associated Glands

• Minor mucous salivary glands are present in the connective tissue beneath and around the crypts.
• These are seromucinous glands whose secretions drain into the crypts, contributing to local mucosal defense.

6. Vascular Supply (Histologically Relevant)

• Blood vessels enter via the hemicapsule.
• The external palatine (paratonsillar) vein descends lateral to the hemicapsule - surgically important as a source of post-tonsillectomy hemorrhage.

7. Comparison: Three Tonsil Types

Key Histological Landmarks Summary