Mice Rendered Severely Deficient in Megakaryocytes through Targeted Gene Deletion of the Thrombopoietin Receptor c-Mpl Have a Normal Skeletal Phenotype

To explore whether a functional relationship exists between megakaryocytes and the cellular processes responsible for bone formation, we examined if Mpl ^−/− mice, which are severely megakaryocyte-deficient through c-Mpl gene deletion, have an abnormal skeletal phenotype compared to Mpl ^+/− and wild-type littermates. We also analyzed whether the osteogenic response to high-dose estrogen treatment is altered in Mpl ^−/− mice. Megakaryocyte numbers and skeletal indices were compared between Mpl ^−/− mice and littermate Mpl ^+/− and wild-type 12-week-old mice (six per group). Dual-energy X-ray absorbtiometry of whole body, excised tibias, and femurs was performed. Histomorphometric analyses of the proximal metaphysis and mid-diaphysis were carried out on longitudinal and transverse sections, respectively. Histomorphometry was performed on the proximal tibial metaphysis of four Mpl ^−/− and four wild-type mice following high-dose estrogen treatment (0.5 mg/animal/week) for 4 weeks. Mpl ^−/− mice had 10% the megakaryocyte number of Mpl ^+/− and wild-type littermates. Bone mineral density values in Mpl ^−/− mice were identical to those in Mpl ^+/− and wild-type mice for whole body, femur, and tibia. Histomorphometric analysis demonstrated that cancellous and cortical tibial bone parameters were similar across all genotypes. The osteogenic response to estrogen treatment was indistinguishable between Mpl ^−/− and wild-type mice. We found that mice severely deficient in megakaryocytes have a normal skeletal phenotype. Additionally, the deficiency did not diminish the osteogenic marrow response to high-dose estrogen treatment. These results represent the first in vivo evidence that severe megakaryocyte deficiency does not affect bone formation, suggesting that this process is not dependent on normal megakaryocyte number.