THYMUS

Origin: from the third (sometimes also the fourth) branchial (oropharyngeal) pouch.

Location: in the superior mediastinum anterior to the heart and great vessels.

Function: the site of programming, differentiation, and proliferation of T lymphocytes.

Composition:

Capsule - the external connective tissue that extends trabeculae to the margin of the cortex and medulla. It contains blood vessels, efferent lymphatic vessels, and nerves. Domains between trabeculae are called thymic lobules.
Epithelioreticular cells

Function: the supporting tissue, a framework for the thymic lymphocytes; may be a source of thymosin - the humoral factor that promotes the transformation of primitive or immature lymphocytes to T lymphocytes.

Cortex - the outer portion of parenchyma contains numerous closely packed small lymphocytes and macrophages.
Medulla - the inner portion of parenchyma that contains a lesser number of large lymphocytes and Thymic (Hassal's) corpuscles - isolated masses of concentrically arranged epithelioreticular cells with evidence of keratinization in center.