The overall architecture and structure of the perivascular space in the rat thymus were studied by light microscopy using silver-impregnated sections and sections stained immunohistochemically with anti-cytokeratin antibody, and by transmission and scanning electron microscopy (TEM and SEM). In silver-impregnated sections, the perivascular space was delimited by a thin sheath of delicate argyrophilic fibers from the thymic parenchyma in the cortico-medullary region and medulla. This space was continuous with the septal connective tissue, indicating that this was the connective tissue compartment rather than with the epithelial compartment of the parenchyma. In the medulla, the perivascular space widened at places, where the argyrophilic sheath was often discontinuous and the boundary between the perivascular space and parenchyma was indistinct. Lymphatics were located in the perivascular space of the corticomedullary region and sometimes in the wide perivascular space of the medulla.
The presence of a thymic epithelial sheath surrounding the perivascular space was confirmed by light microscopy of anti-cytokeratin antibody immunostained sections and by TEM. SEM observations revealed three-dimensionally that the epithelial sheath lined by collagen fibrillar (i. e., argyrophilic) layer form a rather continuous tubular structure in the cortico-medullary region, while it often interrupted in the medulla. These findings indicated that the perivascular space (i. e., the connective tissue compartment) is extensively open to the parenchyma (i. e., the epithelial compartment) in some portions of the medulla, where medullary lymphocytes are probably freely exposed to blood borne substances similar to the peripheral lymphoid tissues.