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From megakaryocytes to platelets: platelet morphogenesis takes place in the bloodstream
1University of Copenhagen, Department of Medical Anatomy, Panum Institute, Denmark.
Abstract:
We studied megakaryocyte processes formed in rat bone marrow and spleen, using both the transmission and scanning electron microscopes. Some processes were bulky, others slender and beaded. The bulky megakaryocyte processes developed a specialized arrangement of organelles at the site at which they entered the lumen: filaments present around the outside of the process seemed to support a central cylinder in which organelles flowed along microtubules. Megakaryocyte processes were present in platelet-rich plasma from both human and rat blood. When followed in living preparations, bulky processes developed pointed tips, elongated, and became slender and beaded. Fusiform proplatelets also were present 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 proper culture conditions. The straight, shorter fusiform proplatelets in living preparations underwent characteristic curving and bending motions, eventually forming disk-shaped cells which sometimes had appendages. This behaviour suggests that the entire process of platelet morphogenesis takes place in plasma: megakaryocyte processes first elongate, then bead and fragment, and then curve and fuse to form disk-shaped platelets. This interpretation is strengthened by finding in freshly isolated plasma many of the shapes seen in the transformations studied in living cell preparations. The megakaryocyte processes and the proplatelets seemed to appear in plasma with a periodicity related to light and dark cycles--that is, with a circadian rhythm. In particular, megakaryocyte processes appear in human blood within a few hours after sunrise; we argue that this might be related to similar peak periods for heart attacks.
Insights
Megakaryocyte processes in blood plasma form platelets through elongation, beading, and fragmentation. This process, observed in rats and humans, may follow a circadian rhythm and potentially link to heart attack timing.
Area of Science:
- Hematology
- Cell Biology
- Microscopy
Background:
- Megakaryocytes are bone marrow cells responsible for platelet production.
- Platelet formation (thrombopoiesis) is a complex process involving megakaryocyte fragmentation.
Purpose of the Study:
- To investigate the in vivo morphology and behavior of megakaryocyte processes and proplatelets.
- To elucidate the dynamic process of platelet morphogenesis in plasma.
- To explore potential circadian rhythms in megakaryocyte process formation.
Main Methods:
- Transmission and scanning electron microscopy of rat bone marrow and spleen.
- Observation of living human and rat platelet-rich plasma preparations.
- Analysis of cell morphology and dynamic behavior in real-time.
Main Results:
- Megakaryocyte processes exhibit varied morphology (bulky, slender, beaded) and specialized organelle arrangement.
- In vivo, megakaryocyte processes elongate, bead, and fragment into proplatelets.
- Proplatelets undergo dynamic movements, forming disk-shaped platelets, suggesting plasma-based morphogenesis.
- Megakaryocyte processes and proplatelets show potential circadian periodicity, peaking after sunrise.
Conclusions:
- Platelet morphogenesis is a dynamic plasma-based process initiated by megakaryocyte processes.
- The observed circadian rhythm in megakaryocyte process appearance may have clinical implications, potentially correlating with cardiovascular events.