Light and electron microscopic observations on the vascular pattern of the thymus of the mouse.

The thymus is one of the subjects that have received the most intensive investigation in recent years, and it has become clear that the thymus plays an important role in development of the immune system. The present state of the knowledge of the thymus studies is surveyed repeatedly and no repetition is needed (see, for instance: GOOD and GABRIELSEN 1964; DEFENDI and METCALF 1964). A remarkable progress in the studies of thymic function also appears to give a new impetus to the study of thymic structure. This is also the case with the vascular pattern of the thymus. The vascular pattern of the thymus was previously reported in detail, particularly for the mouse (SMITH, THATCHER, KRAEMER and HOLT 1952). Recently electron microscopy was also applied to the study of this subject. As pointed out by SMITH, THATCHER, KRAEMER and HOLT (1952), the thymic cortex is characterized by a fine network of numerous capillaries and the medulla by a preponderance of veins and fewer arteries. Therefore, in considering the vascular pattern of the thymus particularly in relation to the functional role, one should pay attention mainly to capillaries in the cortex and to veins in the medulla. In the cortex, the capillaries were examined with the electron microscope in particular connection with the question whether or not a blood-thymus barrier, if present, is morphologically confirmed. As an explanation of immunological unresponsiveness of the thymus to antigens in the blood stream the concept of a vascular barrier was suggested (MARSHALL and WHITE 1961). The electron microscopical findings of the thymic capillaries also were in favor of the concept of the barrier (CLARK 1963, WEISS 1963, GAUDECKER and HINRICHSEN 1965, KAMEYA and WATANABE 1965). More recently, however, the presence of the barrier was not confirmed electronmicroscopically (CLARK 1964), and further close investigation into this problem is required. In the medulla, the veins, particularly larger ones, are surrounded by a wide perivascular space which is considered to serve as passageway of thymic lymphocytes which are formed in the cortex and migrate to the medulla (SAINTE-MARIE and LEBLOND 1958, 1964; HINRICHSEN 1965), For this reason, relationship of the perivascular space to the surrounding medullary parenchyma is of special interest in connection with transfer of thymic lymphocytes. However, while the vascular pattern in the cortex has often been observed with the electron microscope, that in the medulla has been relatively neglected. For above reasons, it was decided to re-examine the vascular pattern of the mouse thymus by light and electron microscopy.