Intrinsic lymphatics of the thymus in the rat: a detailed light and electron microscopic study using serial sections

ABSTRACT

The morphology and distribution of intrinsic lymphatics of the thymus gland of adult Swiss-Albino rats was studied by light and transmission electron microscopy. Lymphatic channels were traced, in serial semithin sections, progressively from the hilar region into the medullary region. The results show the presence of true lymphatic vessels in the connective tissue septa and close to blood vessels. When traced internally, these lymphatics were only identified in the cortico-medullary junction, to a lesser extent in the outer medulla, but never in the inner medulla or cortex. Afferent lymphatics to the thymus were not observed. Transmission electron microscopy, frequently showed concentric layers of small lymphocytes in the perivascular connective tissue spaces. Perivascular lymphocytes were smaller and more pachychromatic than those in the thymic parenchyma. They were enclosed by long mesenchymal/fibroblastic processes which formed a system of irregular "extra-lymphatic" channels or spaces located close to lymphatic vessels which contained many small lymphocytes closely resembling perivascular lymphocytes. These "extra-lymphatic" channels may be pre-lymphatic that may be concerned in the passage of thymocytes and interstitial fluid from the deep regions of the medulla to the nearest lymphatic in the perivascular connective tissue. This may provide an additional route for the migration of lymphocytes from the rat thymus gland supplementing the venous route. Further timed injection studies to trace the continuity of these "pre-lymphatic channels" will offer more precise results: