FROM MEGAKARYOCYTES TO PLATELETS - PLATELET MORPHOGENESIS TAKES PLACEIN THE BLOOD-STREAM

We studied megakaryocyte processes formed in rat bone marrow and splee n, using both the transmission and scanning electron microscopes. Some processes were bulky, others slender and beaded. The bulky megakaryoc yte processes developed a specialized arrangement of organelles at the site at which they entered the lumen: filaments present around the ou tside of the process seemed to support a central cylinder in which org anelles flowed along microtubules. Megakaryocyte processes were presen t in platelet-rich plasma from both human and rat blood. When followed in living preparations, bulky processes developed pointed tips, elong ated, and became slender and beaded. Fusiform proplatelets also were p resent in the platelet rich plasma, with pointed tips at both ends of what appeared to be single ''beads''; we assume that the long, beaded megakaryocyte processes would have fragmented were we to have had prop er culture conditions. The straight, shorter fusiform proplatelets in living preparations underwent characteristic curving and bending motio ns, eventually forming disk-shaped cells which sometimes had appendage s. This behaviour suggests that the entire process of platelet morphog enesis takes place in plasma: megakaryocyte processes first elongate, then bead and fragment, and then curve and fuse to form disk-shaped pl atelets. This interpretation is strengthened by finding in freshly iso lated plasma many of the shapes seen in the transformations studied in living cell preparations. The megakaryocyte processes and the proplat elets seemed to appear in plasma with a periodicity related to light a nd dark cycles - that is, with a circadian rhythm. In particular, mega karyocyte processes appear in human blood within a few hours after sun rise; we argue that this might be related to similar peak periods for heart attacks.