Thymic interdigitating reticulum cells demonstrated by immunocytochemistry.

The lymphoid interdigitating cell (IDC) was investigated in human and rat thymus. A glial protein, S-100, was demonstrated in IDCs found in the human and rat thymic medulla by light microscopic immunocytochemistry. This marker served to distinguish IDCs from conventional macrophages of the thymic cortex. IDCs were S-100+, lysozyme-. Cortical macrophages were S-100-, lysozyme+. Thymic epithelial cells and lymphocytes possessed none of these markers. Examination of human fetal thymic tissue revealed that the IDC is present within the thymus very early in embryogenesis. Ultrastructural analysis of a developmental series of the rat thymus identified IDCs and macrophages. Medullary IDCs possessed many of the morphologic features of the epidermal Langerhans cell including the Birbeck granule. Cortical macrophages contained many inclusion bodies and lysosomes indicative of active phagocytosis. Some changes in IDC shape and structure were noted during maturation in the rat that may reflect the migration of this cell into the thymic parenchyma.