Spleen regeneration in mice after gamma irradiation and administration of thymosin.

The effect of thymosin (thymic humoral factor isolated from calf thymus) on regeneration of the spleen in mice after whole-body gamma irradiation was studied. Thymosin, in varied dosages (0.1--2.0 mg/day) applied subcutaneously before and after radiation exposure, stimulated splenic regeneration as indicated by increased splenic weight, number of endogenous splenic colonies and 59Fe and 125IUdR incorporation into the spleen. A control extract of brain tissue (cerebrosin) isolated in the same way as thymosin was applied to mice to verify specificity of thymosin. After cerebrosin application, a mild increase also was observed. Whereas a near maximal effect of thymosin was reached at a dosage of 0.1 mg, a comparable response with cerebrosin required a dosage of 1.0 mg. These data suggest that administration of thymosin has both a specific and non-specific effect on splenic regeneration and proliferation of hematopoietic stem cells.