The relative roles of the spleen and bone marrow in platelet production in mice.

The relative importance of the spleen and bone marrow in platelet production is not known. In an attempt to delineate these roles in mice, the spleen was removed by surgical excision or the bone marrow was ablated with an injection of strontium-89 (89Sr). Platelet production in these mice was compared with that in untreated mice, 88Sr (nonradioactive strontium)-treated mice, sham-splenectomized mice, and whole-body irradiated mice. Platelet production, platelet counts, and platelet sizes were similar in strontium-89 marrow-ablated mice and whole-body irradiated mice, as were RBC counts, WBC counts and hematocrit values, until the spleens of the 89Sr-treated mice became larger (150% of normal by day 9) and apparently began to produce platelets to compensate for the thrombocytopenia that resulted from the loss of the bone marrow. Splenectomy did not affect the ability of mice to maintain normal platelet production and platelet counts, or RBC counts, WBC counts, and hematocrit values. Therefore, under normal conditions the spleen appears to contribute little or nothing to platelet production in mice, but after bone marrow removal, platelet production by the spleen may occur as a compensatory mechanism. The platelets produced by the compensating spleen resemble in size those platelets that are normally produced by the marrow after thrombocytopenia, with above average sizes. After splenectomy, platelets were were smaller than normal indicating that older, smaller platelets were being retained in the circulation; loss of the spleen could also cause a temporary deregulation of the bone marrow resulting in the production of small platelets.