The respective role of the spleen or of the bone marrow in the regeneration of the haemopoietic progenitor compartment of heavily-irradiated mice has been investigated. Splenectomy was used to this end in animals injected with exogenous isogenic cells or regenerating from endogenous spleen or marrow cells. Analysis of the data as a function of time shows that the presence of the spleen affects marrow CFU repopulation only at the early post-irradiation stages. The expansion of the marrow progenitor pool proceeds, however, rather independently of the spleen and marrow CFU remain eventually as the main source of haemopoietic cells in the surviving mice. Thus the reaction of the spleen may be envisaged as a fast, important but transient contribution to the overall haemopoietic function of heavily-irradiated animals.

Preliminary pulse radiolysis data indicate that both e-aq and

The model of the heterochronic parabiosis of CBA mice aged 4–5 and 24–28 months was used for the study of the possible mechanisms of immunological aging. The exchange of humoral factors between the parabionts began on the very first day after the operation. Later on the vascular anastomoses developed which provided an exchange of the cellular elements and mutual lymphoid organs cellular population. Studies on the primary immune response to SRBC 2–2.5 months after the operation showed that old organism inhibited the immune system function of young animals. At the same time the stimulatory effect of the old organism by the young one was poorly pronounced. The appearance in the old animals of the factors which actively inhibit the immune response of the young animal is discussed.

Measurements were carried out on mice of various ages of some haematological values in the circulatinng blood (number of erythrocytes, leukocytes, platelets; mean erythrocytic volume; haematocrit; haemoglobin) and of some cellular parameters in the bone-marrow and the spleen (total number of cells; transplantibility; total number of haemopoietic stem-cells). Although in some cases significant statistical correlations were found as a function of age, the changes appear biologically of doubtful significance. It is concluded that the potential for division and differentiation of the haemopoietic cells is probably greatly in excess of that required to support the life of the animal and that the problem of ageing must therefore be solved at the cellular rather than at the whole-body level.

By the use of a 59Fe tracer technique esti mates were obtained of the colony-forming units (units ratio and of the total number of CFU in the spleen and the peripheral blood of mice during pre- and post-natal life. The present data, together with others previously reported on the liver, allowed a semi-quantitative comparative evaluation of the haemopoietic function in the fetal and adult life of the mouse, based on the number of progenitor cells in the various haemopoietic districts.