Pulmonary embolization-induced thymus hyperplasia in rabbits

The effects of pulmonary embolization on the thymus glands of rabbits were studied morphologically, morphometrically and immunohistochemically. Pulmonary embolization was induced by an intravenous injection of 0.4 ml of Sephadex bead suspension (particle size; 150 to 300 microns, about 2,000 per ml). Both mean weight and volume of the thymus of rabbits killed at 2 weeks after embolization, were about 1.5 times more than those in control animals treated with physiologic saline. Histological examinations showed enlargement of the cortex and medulla of the thymus, and the embolized Sephadex beads in the branches of pulmonary arteries of the lung. The area ratios of medulla/cortex, in the embolization group and in control, were not significantly different. The cells with immunohistochemically positive staining of anti-nuclear antigen of monoclonal antibody of Ki-67, were found in both portions of the medulla and cortex. These data suggest that pulmonary embolization in the rabbit induces true thymic hyperplasia. An intravenous injection of India ink into the right highest intercostal artery revealed the distribution of bronchial arteries, which send the branches to the right lobe of the thymus. In 2 out of 4 animals killed 2 weeks after pulmonary embolization, the left lobe of the thymus as well as the right were stained with the injected ink. As it is known that pulmonary vascular obstruction caused a marked increase in the bronchial blood flow, these data suggest that the thymus blood supply from the bronchial arteries increases in the conditions of pulmonary embolization, which might contribute to thymus hyperplasia.