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Migration of entire megakaryocytes through the marrow--blood barrier
Abstract:
Electron microscopic evidence is presented to indicate that entire megakaryocytes traverse the marrow--blood barrier and enter the circulation. Passage occurs through apertures of 6 micrometer in diameter, located in the parajunctional areas of the marrow sinus endothelium. Serial sectioning indicates that these apertures are transendothelial rather than interendothelial. The cytoplasm of these megakaryocytes form sinuating elongated projections which may release their platelets in the sinus lumen or when reaching the pulmonary circulation. In the extravascular compartments, megakaryocytes are preferentially located in the subendothelial region. In this location they can send numerous organelle-free projections into the lumen. These projections distinguishable from less numerous organelle-containing projections could serve to 'anchor' the cell to the endothelium. They could also serve to 'monitor' the circulation and to receive information as to the requirement of body for platelet formation.
Insights
Entire megakaryocytes, the cells that produce platelets, pass through the marrow-blood barrier via specific endothelial apertures. This passage allows for platelet release and potential monitoring of circulation needs.
Area of Science:
- Hematopoiesis and Cell Biology
- Vascular Biology and Endothelial Function
Background:
- Megakaryocytes are large bone marrow cells responsible for platelet production.
- The precise mechanism of megakaryocyte transit across the marrow-blood barrier remains incompletely understood.
Purpose of the Study:
- To provide electron microscopic evidence of entire megakaryocyte translocation across the marrow-blood barrier.
- To elucidate the structural pathways and cellular interactions involved in megakaryocyte circulation.
Main Methods:
- High-resolution electron microscopy was employed to visualize megakaryocytes within the bone marrow and sinus endothelium.
- Serial sectioning techniques were utilized to determine the nature of endothelial apertures and cell passage.
- Morphological analysis of megakaryocyte projections and their interaction with the endothelium was performed.
Main Results:
- Electron microscopy confirmed that whole megakaryocytes traverse the marrow-blood barrier through 6-micrometer transendothelial apertures.
- These apertures are located in parajunctional regions of the marrow sinus endothelium.
- Megakaryocytes form elongated cytoplasmic projections that may facilitate platelet release in the circulation or pulmonary vasculature.
Conclusions:
- Megakaryocytes actively migrate through specific endothelial channels, rather than between endothelial cells.
- Extravascular megakaryocytes utilize organelle-free projections to interact with the endothelium, potentially for anchoring and sensing circulatory demands.
- This study reveals a novel mechanism for megakaryocyte circulation and its implications for platelet production and body-wide regulation.