The thymus is a bilobed, encapsulated organ which is situated immediately dorsal to the heart. In the embryo, it is one of the first tissues to become lymphoid and is responsible for the maturation of T-lymphocytes. It is well developed at birth, achieves its greatest weight at puberty, and thereafter undergoes progressive involution.

Thymus, early human (H&E) [#42]: A thin capsule surrounds the two lobes of the thymus. Each lobe is incompletely divided into lobules by thin trabeculae. Each lobule has a cortex packed with lymphocytes (thymocytes) that mask other types of cells, and a medulla which is paler because it contains fewer lymphocytes. Branching irregular reticular endothelial cells are present in the medulla but are difficult to distinguish in this slide. A characteristic feature of the thymic medulla is the presence of many Hassall’s (thymic) corpuscles, which are encapsulated structures serving no known function.