The thymus can be thought of as a chimeric organ composed of a central lymphoid compartment that lies within the true epithelial thymus and a peripheral lymphoid compartment located in the extrathymic perivascular space. At birth the thymic epithelial component is filled with developing thymocytes, whereas the thymic perivascular space contains only vessels and scattered peripheral lymphoid and myeloid cells.
From early childhood onward the thymic perivascular space begins to accumulate peripheral lymphoid and myeloid cells, as well as gradually increasing numbers of mature adipose cells. The aging process within the thymus leads to progressive atrophy of the true epithelial thymus, loss of peripheral cells within the thymic perivascular space, and eventual filling of the perivascular space with adipocytes. An appreciation of the central and peripheral compartmentalization of the thymus and the age-related changes in these compartment is essential for the interpretation of disease-related alterations in thymic histology and function.