The thymus The thymus functions to support the maturation and proliferation of T-cells. In the young human, the thymus is a two-lobed organ located in the upper mediastinum, and is derived from pharyngeal pouch endoderm during development. The thymus grows from birth until adolescence, but undergoes slow involution becoming infiltrated by adipose cells in the adult. The juvenile thymus is a highly lobulated organ divisible into two distinct zones, the cortical lobules that surround a medullary core . The juvenile cortical lobules are the site of T-cell maturation.
Lymphocytes arriving from bone marrow populate the outer margins of the cortex.
They replicate rapidly with cell cycles of 9 hours or less, and the newly generated lymphocytes are crowded deeper into the thymic lobules where they mature into naive T-cells. The cortical lobules are composed of densely packed immature T-cells and macrophages surrounded by epithelial reticular cells . The epithelial reticular cells, also known as nurse cells , are joined to one-another by desmosomes and function to isolate the cortex from the rest of the body. The epithelial reticular cells also produce paracrine hormones that promote T-cell maturation, as do hormones from the anterior pituitary. In a process known as positive selection , the TCR associates with class I and class II MHC proteins, and self antigens on the surface of the epithelial reticular cells.