Disruption of microtubules in vivo by vincristine induces large membrane complexes and other cytoplasmic abnormalities in megakaryocytes and platelets of normal rats like those in human and Wistar Furth rat hereditary macrothrombocytopenias.

Abnormal organization of platelet microtubules is associated with abnormal platelet formation in hereditary macrothrombocytopenias such as the gray platelet syndrome, May-Hegglin anomaly, and Epstein's syndrome, and that of the Wistar Furth rat, suggesting that aberrant microtubule organization may contribute to defective platelet formation in these clinical entities. Here, we examined the consequence of microtubule disruption on the organization of megakaryocyte cytoplasmic organelles using the microtubule depolymerizing agent, vincristine (VCR). Wistar rat bone marrow was fixed and processed for transmission electron microscopy after VCR administration alone, after 5-fluorouracil (5-FU) administration alone, or after 5-FU followed by intravenous injection of 0.1-1.0 mg/kg VCR for intervals of 30 min to 8 hr. 5-FU was given to increase megakaryocyte frequency to facilitate ultrastructural evaluations. VCR alone or in combination with 5-FU caused formation of large membrane complexes in the cytoplasm of Wistar rat megakaryocytes at all dosages studied, identical to those found in megakaryocytes of human hereditary macrothrombocytopenias and the Wistar Furth rat. The proportion of megakaryocytes with these large membrane complexes increased with time after 5-FU and VCR, and was maximal (approximately two-thirds of megakaryocytes) at VCR dosages of 0.75-1.0 mg/kg. The majority of megakaryocytes displayed other abnormalities, including blebbing of plasma membranes, an increased number of dense compartments, dilated demarcation membrane (DMS) channels, which contained dense material immunocytochemically identified as secreted alpha-granule proteins, and an increased incidence of emperipolesis. Rats administered 5-FU alone did not demonstrate these abnormalities, with the exception of an increase in dense compartments. Platelets from rats treated with VCR alone or 5-FU and VCR also showed abnormalities including membrane complexes, rounded shape, formation of tubulin paracrystals, development of membrane blebs, and the presence of proteinaceous material within the cisternae of the surface-connected canalicular system (SCCS). The membrane complexes in platelets of 5-FU-, VCR-treated Wistar rats as well as untreated Wistar Furth rats were composed of elements of both the SCCS and dense tubular system; membrane complexes in megakaryocytes of 5-FU-, VCR-treated rats were composed of both DMS and smooth endoplasmic reticulum. We conclude that intact microtubules play a major role in the organization of the megakaryocyte DMS and may contribute to the stability of megakaryocyte alpha-granules.