Thymic Microenvironments: Development, Organization, and Function

The thymus is a primary lymphoid tissue that is unique in its ability to support a full program of T cell development. Thymic microenvironments consist of anatomically distinct cortical and medullary areas that shape the developing T cell repertoire in a step-wise fashion to become both self-tolerant and responsive to foreign antigens. Specialized epithelial cells in the cortex and medulla, together with mesenchyme populations that are becoming increasingly well defined, are key in providing the signals that control T cell development. Here, we outline our current understanding of the developmental pathways that give rise to distinct cortical and medullary epithelial cell lineages in the thymus. We also summarize knowledge of the functional heterogeneity of thymic stromal cells in relation to their roles in αβT cell development.