THYMIC MICROENVIRONMENT AT THE LIGHT-MICROSCOPIC LEVEL

The thymus is a primary lymphoid organ that serves the immune system b y providing an optimal microenvironment for developing T cells to rear range the genes encoding the T-cell receptor and to undergo positive a nd negative selection in shaping the peripheral T-cell repertoire. The microenvironment of the organ is peculiar among lymphoid organs, as t he supporting stroma consists of reticular epithelial cells. Bone marr ow-derived interdigitating cells and macrophages are the main accessor y cell populations. The epithelium, interdigitating cells, and macroph ages each contribute to the T-cell selection process. During the last decade knowledge has been gathered that these cell populations show a considerable heterogeneity, as documented for subcellular features and immunologic phenotype. This heterogeneity may reflect various stages in differentiation, but may otherwise be linked to the functional acti vity of the cells. The authors survey the major cell populations, i.e. , epithelial cells and lymphocytes. Macrophages and interdigitating ce lls are briefly discussed. Emphasis is given to functional aspects of histologic/ cytologic features. (C) 1997 Wiiey-Liss, Inc.