Effect of Neonatal Thymectomy on Hemopoietic Tissue in Mice

The influence of neonatal thymectomy on the behavior of hemopoietic stem cells of bone marrow and spleen was investigated by studying the colony-forming capacity (CFC) of these cells. Bone marrow cells (2 x 10⁴) from neonatally thymectomized C3H mice, injected into lethally irradiated (850 R) recipients, showed a reduced CFC in relation to that of intact control donors. This decrease, which was more evident in female than in male mice, started early in life after thymectomy, and persisted throughout all the ages investigated. In most of the experiments a concomitant decrease in the total number of nucleated bone marrow cells was observed following thymectomy. When spleen cells (10⁵) from neonatally thymectomized mice were tested under similar experimental conditions, they manifested a CFC equivalent to that of intact controls. Neonatal thymectomy increased the number of spontaneous colonies appearing in the spleen of mice after a sublethal (650 R) total-body irradiation. In contrast, when neonatal splenectomy was tested, a moderate increase in the CFC of bone marrow and in the total number of bone marrow cells was observed. The present results suggest that the thymus plays a role in the proliferative pattern of hemopoietic stem (progenitor) cells located in the bone marrow, whereas it does not influence the behavior of more mature (precursor) hemopoietic cells populating the spleen.