How are platelets released into peripheral circulation from the bone marrow?
a. Mature platelets squeeze between sinusoid endothelial cells.
b. Megakaryocyte cytoplasmic fragments extend through the endothelial cells, lining the sinusoids, into blood and then shed platelets.
c. Platelets adhere to white cells and exit with them.
d. Megakaryocytes exit the marrow with white cells and then platelets fragment off once they reach the blood.
ANS: B
Cytoplasmic extensions elongate and pierce between the endothelial cells that line the marrow sinuses; then they extend into venous blood. These extensions release platelets once they enter blood. The cytoplasmic extensions are called proplatelet processes.
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