Epithelial-cell architecture during involution of the human thymus.

One hundred thymus glands were assessed histologically as to their degree of involution. Epithelial cells were demonstrated by an immunoperoxidase method using a monoclonal antibody against cytokeratin. The distribution of these cells was studied in the medulla, the cortico-medullary junction, the cortical parenchyma and the subcapsular cortex. As involution proceeds, the loss of cells from the thymus is almost totally confined to the lymphoid-cell elements. The architecture of the epithelial-cell network remains largely intact although there is extensive collapse of the structure due to the loss of the intervening lymphocytes. Even when involution is apparently complete; sheets of epithelial cells can be demonstrated in the thymic remnant.