Changes with age in the hematopoietic system in a cesarean-derived, barrier-maintained colony of Wistar rats

In a colony of cesarean-derived barrier-maintained inbred W rats given saline injections into the right pleural cavities, histologic and cytologic changes due to age were examined in the bone marrow, lymphatic tissue, and lungs; hematologic trends were also evaluated. The thymus showed two changes with age: 1) atrophy, which was evidenced by plasma cells near the blood vessels and among the cords of epithelial cells; and 2) disruption of the normal architecture of the gland. In all available thymuses, changes showed a transition toward malignancy. As the bone marrow showed increasing cellularity with time, polymorphonuclear cells and precursors appeared to increase. In lymph nodes, lymphocytes in the paracortical area were excessive and resembled the lymphocytes of the "lymphoid bursts" found in the thymus. In the lungs, small parabronchial and paravascular collections of mononuclear cells, occurring with slightly increasing frequency with age, were consistently found; however, any lung disease associated with these cells was not evident. In the blood, absolute numbers of mononuclear and polymorphonuclear cells were altered.