Path Image
The cortex is darker while the medulla is paler in color.

The thymus is composed of lobules separated by fibrous septae.

In the adult, the thymus is mostly replaced by fat.

A dense population background of thymocytes (thymus lymphocytes) is seen. Scattered larger epithelial cells with more abundant cytoplasm can be seen.

Hassall corpuscles can demonstrate degenerative changes such as cystic dilation and cellular debris in the lumen.

Yet another image of a Hassall corpuscle, composed of whorls of flattened epithelial cells which appear squamous.


The thymus weighs approximately 10-35 grams at birth and grows until puberty to reach 20-50 grams. After puberty, the organ involutes and is gradually largely replaced by fat (Kumar).

The thymus plays an important role in developing immunity. T lymphocytes mature in the thymus and B lymphocytes mature in the bone marrow. Thus, progenitor cells from the bone marrow migrate to the thymus to undergo positive and negative selection. The majority of T lymphocytes do not survive this selection and undergo apoptosis. Mature immunocompetent T lymphocytes are then released to circulate in the lymphovascular system. Errors in the thymic maturation of lymphocytes may contribute to the development of autoimmune diseases, as self-reactive T lymphocytes are not appropriately destroyed.

The thymus is constructed of lobules separated by fibrous septae. Each lobule consists of a cortex and medulla. These are two main cell populations, epithelial cells and thymocytes (thymus lymphocytes). The cortex looks darker because the thymocytes (dark round nuclei with a thin rim of cytoplasm) predominate with sparsely scattered epithelial cells. The medulla looks paler because the epithelial cells (pale vesicular nuclei with abundant light pink cytoplasm) predominate (Mills).

Other cells that can be found in the thmymus include macrophages, dendritic cells, B lymphocytes, rare neutrophils, eosinophils, myoid (muscle-like) cells, neuroendocrine cells and germ cells (still debated). The myoid cell may be related to the development of myasthenia gravis, although the exact mechanism has not yet been eludicated.

The most distinguishing feature of the thymus is Hassall corpuscles, which are swirls of epithelial cells within the medulla. Some of the corpuscles can undergo cystic dilation and dystrophic calcification.


Mills, S. Histology for Pathologists. 3rd Ed. Philadelphia, PA: Lippincott, Williams & Wilkins; 2007: 505-522.

Kumar V, Abbas AK, Fausto N. Robbins and Cotran Pathologic Basis of Disease. 7th Ed. Philadelphia, PA: Elsevier; 2005: 706-8.

Last updated: 2011-03-03
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