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

The relative importance of the spleen and of the bone marrow in platelet production is not known for mice. 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 and platelet counts in ⁸⁹Sr marrow-ablated mice were similar to platelet production and platelet counts in whole-body irradiated mice, as were RBC counts, WBC counts, and hematocrit values, until the spleen became larger (150% of normal by day 9) and apparently began to produce platelets to compensate for the loss of the bone marrow. Splenectomy did not affect the capability of mice to maintain normal platelet production and platelet counts, or RBC counts, WBC counts, and hematocrit values. Therefore, under normal conditions the spleen contributes 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 normally produced by the marrow after thrombocytopenia, with above average sizes. After splenectomy, platelets were of decreasing average size indicating that older, smaller platelets were being retained in the circulation since the spleen was no longer present to serve as a platelet trapper.