|General Gross Description|
The thymus is a lymphoid organ that develops from the third and forth pharyngeal pouches.
It descends to the anterior part of the mediastinum, where it develops following influx of lymphocyte progenitors from the developing hemopoietic system.
In the adult, the thymocyte precursors come from the bone marrow.
The thymus is largest relative to the size of the individual at birth, weighing between 5 and 10 grams.
It increases in size to about 50 grams at the time of adolescence and subsequently involutes through adult life, until a fibro-fatty remnant is left behind in old age.
It is a roughly pyramidal, white, bi-lobed organ located anterior to the heart and great vessels and posterior to the sternum.
|General Microscopic Description|
Histologically, the thymus is divided into 2 lobes and several lobules by thin, fibrous septae.
The thymus is composed of 2 cellular components -- epithelial cells derived from the endoderm of the pharyngeal pouches and lymphocytes of hemopoietic (bone marrow) origin.
Histologically, the thymus has a distinct cortex and medulla.
The medulla has distinct, concentrically arranged nests of squamous epithelium, known as Hassels corpuscles.
The cortex has more densely packed lymphocytes, whereas the medulla has relatively less densely packed lymphocytes, with interspersed epithelial cells that form an intricate interlacing dendritic network.
Bloom and Fawcett: A textbook of Histology. 12th Edition. Chapman & Hall. 1994. pp 332 Gray: Gray's Anatomy. 15th Edition. Barnes & Noble Books. 1995. pp 966 et seq.
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||Synopsis by: T.V.Rajan, M.D., Ph.D. (T98000M00100)