1. The thymus. A. Section of a fetal thymus
The thymus. A. Section of a fetal thymus. The cortex, adjacent to the capsule, is densely packed with small thymocytes; medullary areas are far less cellular. B. Section of a fetal thymus. A septum, a short extension of the capsule, ends at the boundary between the cortex (right) and the medulla. C. Involution in an adult thymus. The overall cell density in this organ is low, and there are no obvious cortical and medullary areas. Epithelial reticular cells wrap around themselves and form large structures called Hassall's corpuscles, which facilitate identification of the organ in sections obtained by biopsy from an older person. D. High endothelial venule (HEV) in the medulla of a fetal thymus. The nearly cuboidal endothelial cells are diagnostic. A portion of a small Hassall's corpuscle is indicated (arrow). E. Development of T cells in the thymus. Progenitor cells from hematopoietic bone marrow enter the thymic medulla by exiting HEVs. Progenitor cells come in contact with epithelial reticular cells, become committed to T-cell development and proliferate, and the daughter cells move toward the capsule. T cells rearrange their receptor genes, express coreceptor molecules, and undergo positive selection by contact with epithelial cells. Negative selection to remove self-reactive cells occurs by contact with epithelial cells, macrophages, and dendritic cells. Ninety-eight percent of the cells die and are removed by macrophages; the surviving cells exit the thymus through HEVs.
Source: IMMUNE SYSTEM, The Big Picture: Histology
Citation: Ash R, Morton DA, Scott SA. The Big Picture: Histology; 2017 Available at: Accessed: November 06, 2017
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