The Arrangement of Collagen Fibrils in the Lymphatic Sinus Wall of the Rabbit Appendix

Abstract

The lymphatic sinus of the rabbit appendix was examined by scanning and transmission electron microscopy. Luminal surfaces of the sinus were lined by squamous endothelial cells, some of which, however, were blebby in surface relief due to the presence of electron dense granules in the cytoplasm. Many clusters of lymphocytes were present on the endothelial surface, mainly along the blood vessels of the trabeculae and of the sinus wall. Regional differences in collagen fibrillar arrangement of the sinus wall were demonstrated after the removal of the endothelial cells by the NaOH maceration method (OHTANI, 1987). In the upper region of the sinus and the lymphatic plexus of the inter-nodular thymus dependent area (TDA), the fibrillar layer was a delicate network of interwoven thin fibrils. Furthermore, transmission electron microscopy revealed frequent transmural passages of lymphocytes, which suggest the existence of pores or fenestrations in the fibrillar layer of these regions. The fibrillar layer in the lateral and basal sinus was formed by densely inter-woven thick bundles or sheets which consisted of thin collagen fibrils. No obvious space was seen between fibrils. These observations suggest that the upper region of the sinus and the lymphatics in the TDA may be active sites of cell migration from the reticular tissue.