Thymus histology


The lymphatic system is an essential part of the immune system and it consists of a network of lymphatic vessels, tissues, and organs.

The lymphatic vessels drain interstitial fluid or lymph from peripheral tissues back into the blood.

Lymphoid tissue and organs contain a lot of lymphocytes and other white blood cells.

The primary lymphoid organs include the thymus and bone marrow.

And the secondary lymphoid organs include the tonsils, lymph nodes, spleen, and mucosa-associated lymphoid tissue or MALT for short.

The thymus is a flat encapsulated lymphoid organ located in the anterior superior mediastinum, right behind the sternum.

During embryonic development, the thymus originates from the embryo’s third pair of pharyngeal pouches.

This organ is most active during childhood, reaching its largest size around puberty, with a weight of approximately 30 to 40 grams.

After puberty, the thymus will begin to slowly involute or decrease in size, with less lymphatic tissue and an increase in adipocytes.

The thymus plays an important role in the maturation of T cells, which includes negative selection or central tolerance.

This process, along with regulatory T cells help prevent autoimmunity.

This is a low power image of a neonatal thymus.

At this magnification, the thin collagenous capsule is a little hard to see, but if we zoom in a little further, we can see the capsule more clearly, as well as the connective tissue that extends inward from the capsule into the thymus, forming incomplete lobules.

When compared to an adult’s thymus, we can see that the adult thymus has noticeably more fatty infiltrate, which is seen by the white spaces scattered throughout the organ.

The lobules of both adult and neonatal thymi have inner regions that stain light purple and pink, which is called the thymic medulla; and the outer regions of the lobules are more basophilic or dark purple, which represents the cortex of the thymus.

The clear distinction between the inner medulla and outer cortex of each lobe is typically more prominent in early childhood, similar to the image on the right of the neonatal thymus.

If we look at the thymic cortex at high magnification, we see that the dark purple color of the cortex is mainly from densely packed and very basophilic T lymphoblasts, which are also called thymocytes.