The Role of the Thymic Cortex and Medulla in T Cell Differentiation

Abstract

The thymus has two main cellular zones, clearly distinguishable histologically; the major outer zone, the cortex, and the smaller central zone, the medulla. Although this morphological subdivision has been known for a long time, we still don’t know the respective roles of these two thymic compartments in the generation of peripheral T cells [reviewed in 1,2]. Our ignorance in this area is surprising since we have a lot of information concerning the nature of the lymphoid and non-lymphoid cells in the two compartments. Thus, isolated thymic lymphocytes fall into two major categories: “mature” cells binding low levels of peanut agglutinin, and expressing relatively little Thy 1 but a lot of H-2 antigen, and “immature” cells which are PNA^hi, Thy 1^h1 and H-2^low [reviewed in 3]. The mature cells are located mainly in the medulla and hence we will call them medullary-phenotype cells, while the major population (85%) of immature cells is located in the cortex and hence these are termed cortical-phenotype cells. We use the term cortical- and medullary-phenotype because a small number of cells of either phenotype could exist in the “wrong” region and could thus be, for example, of medullary phenotype but located in the cortex. We should stress, though, that most cells do occur in the “right” region, and exceptions (for which there is little hard evidence, see below) must be few in number [see ref. 3].