Thymic lymphoid follicles in autoimmune diseases. II. Histological, histochemical and electron microscopic studies.

Abstract

Both histochemical and electron microscopic findings confirmed that thymic lymphoid follicles were identical in structure with lymphoid follicles in peripheral lymphoid tissues and not related to thymic epithelial cells. Histochemical demonstration of β-glucuronidase and other acid hydrolases showed that thymic medulla around the lymphoid follicles was infiltrated with the lymphocytes of the peripheral type corresponding to recirculating lymphocytes in paracortical areas of lymph nodes. Reticular cells around the thymic germinal center showed the same activity of alkaline phosphatase as those in lymph nodes.Electron micrographs revealed that the thymic germinal center contained reticular cells connected to each other with prominent interdigitation and desmosome-like structures. These cells differ from thymic epithelial cells in the absence of typical desmosomes, abundant tonofilaments and basement membranes.It could be assumed that reticular cells of thymic lymphoid follicles arise from mesenchymal cells of the perithymic tissue, and that the lymphocytes of the peripheral type occur in the thymus by mutation of thymic lymphocytes or by immigration of circulating lymphocytes.