The thymus is a specialized organ of the immune system. The thymus is derived
from the 3 pharyngeal pouches. The development of the thymus is a series of
epithelial/mesenchymal inductive interactions between neural crest-derived
arch mesenchyme and pouch endoderm. Unlike most other lymphoid structures,
the thymus grows rapidly and reaches its greatest size relative to the rest of the
body during fetal life and the first years after birth. Thereafter, it continues to
grow, but more slowly than the other organs. It can be said that the thymus
enlarges during childhood, and atrophies at puberty. The thymus of older people
is barely distinguishable from surrounding fatty tissue. As one ages, the thymus
slowly shrinks, eventually degenerating into tiny islands of fatty tissue.
The thymus is divided into two lobes, where the two lobes are divided into
lobules by connective tissue septa that are continuous with the outer fibrous
capsule. Each lobule is subdivided into an outer zone, the cortex, and the inner
zone, the medulla, where they are distinguishable.
The organ is composed principally of two types of cells, lymphocytes and
reticular cells. The reticular cells form a loose meshwork, while the spaces
between them are packed with lymphocytes. The cortex, characterized by its
heavy lymphocyte concentration, is the site of lymphocytic proliferation.
Proliferation of lymphocytes in the thymus is distributed evenly throughout the
cortex. T cells that are produced in the cortex migrate to the medulla, where they
enter the bloodstream through the m