The Growth and Development of Lambs Thymectomized in Utero

Summary The effects of in utero thymectomy on the subsequent growth rate, the levels of circulating white blood cells, the levels of lymphocytes in lymph and the size and development of the peripheral lymphoid organs have been investigated in lambs. Consistent and severe depressions were observed in the circulating lymphocyte counts in both the blood and the lymph of thymectomized lambs of 3 months of age; by 10 months of age, although these levels were still significantly below normal, the differences in the circulating lymphocyte counts were much less. Histological observations showed that the lymphoid tissues were depleted of cells to varying degrees. The tissues of lambs thymectomized at 80–90 days post‐conception were generally less depleted than lambs which were thymectomized at 60–70 days post‐conception. The “wasting syndrome” described to occur following thymectomy in mice was not seen in lambs. These results in the lamb snggest that, while the thymus is a source of lymphocytes, it is not the only source and a considerable proportion of the lymphocytes present in normal animals is not derived directly from the thymus.