NEURAMINIDASE-INDUCED THROMBOCYTOPENIA IN MICE - EFFECTS ON THROMBOPOIESIS

Previous studies to examine the effects of thrombocytopenia on thrombopoiesis have generally utilized immune-mediated platelet depletion. We have developed a nonimmune model to exclude the possibility that adverse immune-mediated effects have been misinterpreted as the physiological response to stimulation of thrombopoiesis. Thrombopoiesis was examined in mice after induction of thrombocytopenia with a single injection of the nonimmunologic agent neuraminidase (Ndase). Utilizing electron microscopy, we examined platelets and megakaryocytes (MK) obtained 8, 12, 24, 48, 72, 96, and 120 hr after administration of Ndase. Eight to 48 hr after induction of acute, severe thrombocytopenia (mean platelet count < 50,000/mu-l), the medians of the platelet sectional area distributions, as measured morphometrically, were significantly greater than the median platelet sectional area of pooled controls. The maximum median value for platelet sectional area was observed at 24 hr. The largest platelets in these samples contained more profiles of endoplasmic reticulum and Golgi cisternae, and a lower concentration of surface-connected canalicular system, as compared with normal platelets. By 72 hr post-injection of Ndase, virtually all platelets exhibited normal size and organelle complement. Mean platelet volumes, determined by electrical impedance analysis, paralleled the serial changes in platelet sectional areas. MK frequency and ploidy, measured by two-color fluorescence activated flow cytometry, were unchanged 12 and 24 hr following Ndase. At 48 hr, total MK frequency increased significantly (P < 0.01) from 0.11% to 0.17%, and MK ploidy distribution shifted with a reduction in 16N MK (P < 0.005) and an increase in 32N MK (P < 0.01). MK ploidy was maximally altered from normal at 72 hr with increased 32N MK frequency (32.0%, P < 0.001) and increased 64N MK frequency (2.4%, P < 0.005). Morphologic and morphometric examination of MK at all time points did not reveal detectable changes from normal in cytoplasmic appearance or size, respectively. Therefore, we have demonstrated marked alterations of morphology and size of platelets, and of MK ploidy, using this nonimmunologic model. These studies further support our previous observations that megakaryocyte ploidy and platelet volume are independently regulated in response to depletion of the circulating platelet mass, and they show that these changes are not dependent upon the mechanism of thrombocytopenia.