Seiji Kato

Enzyme-Histochemical Identification of Lymphatic Vessels by Light and Backscattered Image Scanning Electron Microscopy

The walls of lymphatics are characterized by strong 5-nucleotidase activity, whereas those of blood capillaries reveal significantly lower or no activity. Alkaline phosphatase activity, on the other hand, is markedly higher in blood capillaries than in lymphatic vessels. On the basis of such characteristics, lymphatics and blood capillaries were distinguished histochemically in rat stomach using 5-nucleotidase-alkaline phosphatase double staining. The distribution and intensity of lead-demonstrated 5-nucleotidase activity in lymphatic vessels could be determined by comparing the images of the same histochemically stained cryostat section as seen by light and backscattered image scanning electron microscopy. The specificity of the 5-nucleotidase reaction was obtained by inhibiting nonspecific alkaline phosphatase by including L-tetramisole in the 5-nucleotidase incubation medium. The products of the 5-nucleotidase activity were deposited on the outer surface of the plasma membrane of the lymphatic endothelial cells.

Intralobular lymphatic vessels and their relationship to blood vessels in the mouse thymus

SummaryThe spatial distribution and fine structure of the lymphatic vessels within the thymic lobules of normal and hydrocortisone-injected mice were studied by light- and electron microscopy. The lymphatic vessels of the cortex and medulla of normal thymus are irregularly shaped spaces closely associated with branches of the intralobular artery and vein. The overall distribution of these vessels in the greatly involuted thymus of hydrocortisone-treated mice is essentially the same as in the normal thymus. The wall of the lymphatic vessels consists of only a layer of endothelial cells supported by underlying reticular cells. The luminal surface of the endothelial cell is smooth, but trabecular processes are often seen. There are three morphological types of intercellular contacts between contiguous cells, namely, end-to-end, overlapping and interdigitating. The lymphatic vessel has anchoring filaments and collagen fibrils, but a basal lamina is either absent, or if present, is discontinuous. This is in contrast to the continuous basal lamina of the venule. The perivascular space surrounding the postcapillary venule opens into a terminal lymphatic vessel at the cortico-medullary junction and in the medulla. Lymphocytes are seen penetrating the lymphatic endothelium, particularly in acutely involuted thymuses. These findings suggest that the intralobular lymphatic vessels may originate from the vacuities that surround the postcapillary venules, and the lymphatic system may function as a pathway for the migration of lymphocytes into or out of the lymphatic circulation.