Thymus and tonsils under the microscope !
- Organ
The thymus is where T cells mature and it has no follicles. The tonsils are lymphoid nodules under the lining of the throat, with deep crypts.
Updated 2026-11-01
See it in 3D: Thymus · Palatine tonsils
What it is
The thymus is a primary lymphoid organ divided into lobules, each with a dark cortex packed with thymocytes and a pale medulla with Hassall's corpuscles; its framework is made of epithelial cells, not reticular fibres. The tonsils are mucosa-associated lymphoid tissue with no complete capsule, and follicles with germinal centres just under the epithelium. This page describes what you see under the microscope; the immune function of both is covered in the Immunology section.
Structure
- Thymus: a thin capsule with septa that divide it into incomplete lobules; the medulla of neighbouring lobules is continuous.
- Thymic cortex: dark because thymocytes are densely packed, with thymic epithelial cells and macrophages that engulf dying thymocytes.
- Thymic medulla: paler, with fewer thymocytes and with Hassall's corpuscles (concentric layers of epithelial cells, sometimes keratinized at the centre).
- Blood-thymus barrier in the cortex: continuous capillaries wrapped by epithelial cells.
- With age the thymus involutes: its lymphoid tissue is replaced by fat, although islands of thymic tissue remain.
- Palatine tonsil: non-keratinized stratified squamous epithelium dipping into deep crypts; under it, follicles with germinal centres; a partial capsule on the deep side only.
- Pharyngeal tonsil (adenoids): ciliated pseudostratified columnar respiratory epithelium, with folds rather than deep crypts.
- Lingual tonsil: at the base of the tongue, stratified squamous epithelium with simple crypts.
Clinical notes
- The crypts of the palatine tonsil trap debris and bacteria; they are where tonsillitis and peritonsillar abscess begin.
- Group A streptococcal pharyngitis and infectious mononucleosis enlarge the tonsils.
- The crypt epithelium is a common site of origin of HPV-associated oropharyngeal carcinoma.
Test yourself
Think of the answer, then tap the question to see it.
Which structure in the medulla identifies the thymus under the microscope?
Hassall's corpuscles, made of concentric layers of epithelial cells.
Why does the thymic cortex stain darker than the medulla?
It holds many more tightly packed thymocytes, whose nuclei take up the hematoxylin.
Which epithelium lines the palatine tonsil?
Non-keratinized stratified squamous epithelium that dips down to form crypts.
What happens to the thymus with age?
It involutes: its lymphoid tissue is gradually replaced by fat.
See it in 3D
The structures of Durgical's 3D anatomy where it is found.
Related
- Lymph node and spleen under the microscope
- Simple, stratified, pseudostratified and transitional epithelium
- Adipose tissue: white and brown fat
- Oral cavity, tongue and teeth
- Enteroendocrine cells and GALT
Related conditions
Microorganisms
Educational content on normal microscopic structure and its clinical relevance; no substitute for studying slides or for a health professional's advice.
Sources: 21.1 Anatomy of the Lymphatic and Immune Systems (OpenStax, Anatomy and Physiology 2e, 2022); Thymus (NCI SEER Training Modules, 2026); Components of the Lymphatic System (NCI SEER Training Modules, 2026).
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Representative model, for educational purposes only.