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Developmental Immunology
Volume 3, Issue 2, Pages 113-122

Epithelium-Free Area in The Thymic Cortex of Rats

1TNO Toxicology and Nutrition Institute, Zeist, The Netherlands
2National Institute of Public Health and Environmental Protection, Bilthoven, The Netherlands
3Departments of Internal Medicine and Pathology, University Hospital Utrecht, Utrecht, The Netherlands
4National Institute of Public Health and Environmental Protection, P.O. Box 1, Bilthoven, 3720 BA, The Netherlands

Received 11 August 1992; Accepted 21 October 1992

Copyright © 1993 Hindawi Publishing Corporation. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.


The histology of epithelium-free areas in the subcapsular region of the thymus was studied in Wistar rats. Lymphocytes in these areas were CD4/CD8 double-positive, TCR α/β positive in low intensity, and in CD5 labeling either negative or positive in low intensity. There was a high proliferative activity as assessed by bromodeoxyuridine incorporation in vivo and detected by immunohistochemistry. Various macrophage types were observed. They were either large and round to slightly dendritic, or small and dendritic. Most large cells were positive for MHC Class II, and labeled by the antimacrophage antibodies ED1 and ED2. A few cells were strongly positive for Sudan black, Oil red O, nonspecific esterase, and acid phosphatase; they resembled the large rounded macrophages in the corticomedullary zone, although their MHC Class II and ED2 staining was more intense. A few cells showed features of tingible body macrophages, as they contained cellular debris.

Serial sections showed that epithelium-free areas run from the subcapsular area to deep in the cortex, and often border the medulla. This opens the opportunity for immature lymphocytes to move into the medulla and corticomedullary zone without contacting and potential selection with cortical stromal elements other than macrophages in the epithelium-free areas. In this case, the epithelium-free areas may offer a separate intrathymic pathway for T lymphocytes.