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PH-HE07

Thymic hyperplasia and tumours

How the thymus educates T cells, why it shows follicular hyperplasia in myasthenia and rebound hyperplasia after stress, and how thymomas and thymic carcinoma arise from its epithelial cells, invade the mediastinum and cause autoimmunity.

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  • 5 steps · 10 questions
  • 3D normal anatomy

By the end, you can

  • Describe the structure of the thymus and how its epithelial cells select self-tolerant T cells.
  • Distinguish thymic follicular hyperplasia from true (rebound) hyperplasia, and relate follicular hyperplasia to myasthenia gravis.
  • Explain the WHO classification of thymomas (types A, AB and B1–B3) and how thymic carcinoma differs.
  • Explain how a thymoma invades the mediastinum, what staging records, and why it causes paraneoplastic autoimmune disease.
  • Recognise the classic appearances (germinal centres, lobulated thymoma with perivascular spaces, thymic squamous carcinoma), the WHO types, Masaoka–Koga stage and key tests such as chest CT and acetylcholine receptor antibodies, and explain how thymectomy and the other treatments act.

What it is

Thymic hyperplasia is enlargement of the thymus or growth of lymphoid follicles within it, without a tumour. Thymic epithelial tumours arise from the thymus’s epithelial framework: thymomas, whose cells keep part of the gland’s ability to raise T cells and are classified by the WHO as types A, AB and B1–B3, and thymic carcinomas, whose cells are overtly malignant and have lost it.

4 types to tell apart
Follicular hyperplasia
Lymphoid follicles with germinal centres in a gland of normal size; typical of myasthenia gravis.
True (rebound) hyperplasia
A larger gland with normal structure, often regrowing after stress, chemotherapy or steroids.
Thymoma
An epithelial tumour that still nurtures immature T cells; often linked with autoimmunity.
Thymic carcinoma
An overtly malignant epithelial tumour, usually squamous, with no immature T cells; aggressive.

US spelling: tumours → tumors, tumour → tumor.

Normal function

The thymus: a school for T cells

The lies in the anterior mediastinum, behind the and in front of the , the and the . Each lobe is divided into lobules with a dark outer cortex packed with developing T cells (thymocytes) and a paler medulla containing Hassall corpuscles, whorls of keratinised epithelial cells.

Its framework is a network of thymic epithelial cells, the cells thymomas arise from. Cortical epithelial cells present self-MHC molecules, and thymocytes that recognise them survive (positive selection). Medullary epithelial cells, helped by the transcription factor AIRE, display proteins normally made only in other tissues; thymocytes that bind them strongly are deleted or become regulatory T cells (negative selection). Most thymocytes die, so only self-tolerant T cells reach the blood.

The thymus is largest relative to body size in childhood. After puberty it involutes as fat replaces the cortex, though some output continues. Stress and corticosteroids shrink it quickly, because cortical thymocytes die by apoptosis. A few B cells and muscle-like myoid cells, which carry acetylcholine receptors, live in the medulla.

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