ABSTRACT
The thymus is a primary lymphoid organ which is located in the neck and extends into the thorax within the cranial mediastinum. Epithelial cells instead of connective tissue form the framework of this organ which is seeded with developing lymphocytes from the bone marrow. The organ is comprised of lobes covered by a connective tissue capsule. The capsule extends into the lobes as septa that further divide the parenchyma into lobules. Each lobule has a cortical and a medullary region.
Blood vessels traverse in the fine reticular connective tissue which surrounds the lobules. The thymus is most prominent in young animals and regresses later in life.
The parenchyma of the involuted thymus is mostly replaced by adipose and connective tissues.
