Proliferation and transformation of lymphocytes in the lung capillaries of the rat: ultrastructure, acid phosphatase and peroxidase cytochemistry.

After fixation by vascular perfusion numerous leukocytes, among which 91% lymphocytes - not only small but particularly medium- and large-sized categories - were observed in the lung capillaries of healthy Wistar rats. We are struck by the presence of lymphocytes belonging to different intermediate stages between small lymphocytes and plasma cells. Lymphocyte mitosis can be found in the lung capillaries, resembling 'blast cell mitosis'. The different kinds of lymphocytes can be observed with a variable amount of uranylophilic membrane-bound granules with a diameter ranging from 200 to 400 nm. Acid phosphatase (AP) and peroxidase (PO) reaction product was found in several granules. The distribution of azurophil granules corresponds to that of the AP and PO positive granules. Since staining with uranyl acetate increases the density of the azurophil granules and thereby allows a direct electron mircoscopical identification of azurophils, AP reaction azurophil granules in lymphocytes correspond to a from of lysosome containing AP and PO. Thus, it would appear that movement of blood cells to the lung capillaries is not random but preferentially favors cells of the lymphocytic series. The morphology and cytochemical reactions for AP and PO of the medium- and large-sized lymphocytes is characteristic of cells subjected to antigenic stimulation.