1. Megakaryocytes were demonstrated in central venous blood of each of 23 patients who underwent cardiac catheterization. Cell counts ranged from 0.7 to 5.9 megakaryocytes per ml. of blood; the equivalent of one-third of these cells were considered to contain a full complement of cytoplasm. It has become evident that megakaryocytes are normal constituents of blood.
2. In an attempt to quantify megakaryocyte migration from the bone marrow it was calculated that from 20-50 per cent of the mature megakaryocyte population enters the blood and ultimately reaches the lungs. The possibility that all megakaryocytes migrate from the marrow is not precluded with certainty by these studies.
3. It was estimated that from 7-17 per cent of the body’s platelets are released in the pulmonary capillaries. If all megakaryocytes migrate from the bone marrow, then as much as 33 per cent of the platelet population is delivered to the blood in the lungs.