Megakaryocytes and the Heterogeneity of Circulating Platelets

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Abstract
Megakaryocytes mature to the point of cytoplasmic disintegration in 3 principal ploidy classes: 8n, 16n and 32n. Cells of each of these ploidy classes were identified, using both microdensitometry and measurement of cell volume and submitted to morphometric analysis. Mature megakaryocytes of the 3 ploidy classes differ in the concentration of cytoplasmic constituents which would be expected to relate to the buoyant density of their platelet progeny. Density separated platelets were similarly analyzed. Light platelets correspond with the 32n megakaryocytes and are more liberally endowed with surface connected canalicular system than the progeny of the common 16n megakaryocytes; it is proposed that they have functional characteristics related to this finding. Dense platelets, which are larger in size, correspond with 8n megakaryocytes and show a greater content of granules and mitochondria than platelets of average density. These platelets most probably show specialized function relating to release of granule constituents. Fragments of cytoplasm released from megakaryocytes represent one form of megathrombocyte equated by others with newly formed platelets. The differences in structure between these fragments and circulating platelets are emphasized; each such fragment must undergo further disintegration into a number of platelets. The heterogeneity of circulating platelets with respect to both size and density may stem from the origin of platelets of varying density from the different populations of megakaryocyte and their release in the form of large cytoplasmic fragments rather than as platelets.