Elements of structure and ultrastructure of the blood-thymus barrier in ACTH involuted thymus

The ACTH influence upon the thymus may be a reliable model for stress involution. In this case, the cortical lymphocytic depletion is accompanied by mast cell accumulation and increased caliber of the blood vessels. The blood-thymus barrier which has an active role in involution shows an enriched transport activity of the endothelial cells, great enlargement of the basement membrane, increased macrophage activity within the perivascular space with elevated values of acid phosphatase activity, and thickening of the fibrillar network. The epithelioreticular cells show plenty of vacuoles in their cytoplasm, the mitochondria undergo swelling processes, and their cristae are diminished. The ultrastructural data show that lymphocyte depletion is carried out by macrophage-mediated lymphocytolysis. But by counting the peripheral blood cells an earlier mechanism is revealed; i.e. migration through the enlarged but more permeable blood-thymus barrier. The epithelioreticular cells do not seem to have an active, direct implication in any of the phenomena.