Ultrastructure of MKs exposed to high shear rates leading to platelet formation. Effluent cell suspensions (A-E) or alternatively cells present on the coverslips (F) were processed for electron microscopy as described in “Electron microscopy.” Mature MKs were elongated, extending an often unique long cytoplasmic filopod enclosing parallel longitudinal microtubules (inset). These proplatelets exhibited regular swellings containing cytoplasmic organelles. A large spherical cytoplasmic fragment, probably a detached proplatelet (pp) was located nearby (A). The nuclear lobes (N) containing dense chromatin were elongated and located at one pole of the cell. Naked nuclei with an oval shape and compact nuclear lobes containing dense chromatin, which are normally absent from MK cultures, were retrieved in the effluents (B). Proplatelets (pp) filled with cytoplasmic organelles appeared as large cytoplasmic fragments, devoid of nuclei, roughly spherical, dumbbell-shaped, or elongated with slender extremities (C-D). Several isolated platelet-sized fragments (P) were observed (E). Panel F shows the section of the cell monolayer present on a VWF-coated coverslip removed from the chamber after exposition to shear. It shows 2 platelet-sized fragments (P) located close to an adhering MK, displaying α granules and surface-connected canalicular system characteristic of platelets, and whose distribution pattern resembles that of the putative mother cell (MK). Scale bar represents 2 μm.