Observations on the vascular barrier of the thymus in albino rat

A light and electron microscope study was performed to provide further information on the blood thymic barrier. Ten thymus glands were obtained from ten male albino rats ranging in age from 4 to 8 weeks. Paraffin sections were prepared. stained by Fix. and E. and examined by light microscope. Other specimens were processed for transmission electron microscope examination. The haematoxylin and eosin stained sections showed the thymic lobules consisting of cortex and medulla where the lymphocytes were condensely packed in the cortex but sparsely arranged in the medulla. Hassall's corpuscles appeared in the medulla and high endothelial venules were observed at the corticomedullary junction. The transmission electron microscope revealed lymphocytes in different sizes. The reticular cells appeared between the lymphocytes as large polygonal cells with large pale nuclei and vesicular cytoplasm. Macrophages also were observed. Two types of capillaries were detected. The first type showed less irregular endothelial lining with few internal projections; outer collagenous fibers were lacking and the endothelial cells were surrounded by retincular cells. The other type showed a highly irregular endothelial lining and an outer investment of a prominent collagenous coat. The epithelial investment of the first type and the collagenous coat of the other type were suggested to share in the hemato-thymic barrier around both types of thymic capillaries