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Learning objectives

After completing this study unit you will be able to:

  1. Understand the histological structure of the thymus and its parts.
  2. Identify these under the microscope.

Browse atlas

The thymus is an encapsulated primary lymphoid organ. Histologically, it is divided into subcapsular cortical, cortical and medullary regions within each lobule, created by the intervening connective tissue septae extending from the capsule.

Each lobule has a peripheral dark zone called cortex and middle lighter zone called medulla. The capsule is made up of inner and outer layers of collagen and reticular fibres, lymphocytes are found in between.

The cortex contains a large number of small densely packed precursors of T lymphocytes (thymocytes). It also contains epithelial reticular cells and macrophages. The blood vessels of the thymus also lie within this network of epithelial reticular cells. The cortex is where the very early stages of thymocyte development take place, and where the rearrangement of the genes for the receptors on the surface of T cells takes place.

The medulla is the central portion of the thymus and marks the area where the network of reticular endothelial cells is denser and where the lymphoid cells are fewer. There are also a number of concentric bodies known as Hassall’s corpuscles. They are flattened epithelial reticular cells concentrically arranged and filled with keratin filaments. Also within these corpuscles is a central mass of a few granular cells. Some thymocyte development (later stages) equally takes place in the medulla. 

Ready to review all these structures in further detail? Browse our image gallery below:

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