The morphology of perivascular spaces in the thymus.

The perivascular spaces (PVS) are a functional component of the thymus gland that allows cells or possibly soluble products to reach or leave the medulla without the necessity of passing through the cortex. They are connective tissue compartments, continuous with the tissue around the gland, that extend to the cortico-medullary junction where they become merged with the medulla. Around the gland and in their course to the cortico-medullary junction they are delimited by a continuous layer of type-1 epithelial cells (subcapsular/perivascular cells) that rest on a basal lamina. They contain the major blood vessels to and from the medulla, and capillary loops that enter the cortex from the medulla are also ensheathed by the type-1 cells. Capillary loops in the medulla may not be ensheathed. Nerves run in the tunica adventitia of the blood vessels and may also be found separate from the vessels as small unmyelinated fibres that enter the cortex. Lymphatic vessels are formed either in the medulla or at the cortico-medullary junction and leave the gland along the main PVS between thymic lobes. PVS frequently contain lymphocytes, plasma cells and myeloid cells. Lymphocytes free in the connective tissue are commonest at the cortico-medullary junction: macrophages, plasma cells, eosinophils are frequent in the connective tissue of all PVS, and mast cells are closely associated with the veins of PVS and the capsule around the lobes. Some possible functional correlates of this morphological arrangement are discussed.