Identification of the Bone Marrow-Derived Ia Positive Cells in the Rat Thymus: A Morphological and Cytochemical Study

Ia+ cells of bone marrow origin were studied in the thymus of normal and bone marrow-reconstituted radiation chimeras of rats and characterized on morphology and acid phosphatase (APh)- and Ia-staining pattern in-vivo and in-vitro. In-vivo, the majority of the bone marrow-derived Ia+ cells were present in the medulla. They were large, had cell processes extending between surrounding thymocytes, and frequently weak APh activity. In-vitro, the Ia staining was demonstrated on the cell surface in variable intensity. Ia+ cells showed a frayed outline with cell processes between adherent thymocytes and had weak cytoplasmic APh activity, frequently in a central spot. They had an elongated, usually invaginated, nucleus and a rather pale cytoplasm, in which Birbeck granules sometimes were observed. The characteristics of the bone marrow-derived Ia+ cells demonstrate these cells are an equivalent of the interdigitating cells present in thymus medulla. They distinguish themselves from a population of Ia-, strongly APh+ macrophages predominantly localized in the cortex, and from the Ia+, APh- thymic reticular epithelial cells.