The fine structure of haemopoiesis in the human fetal liver. II. Origin and differentiation of the megakaryocyte.

The differentiation of the megakaryocyte was studied at the ultrastructural level in the liver of human fetuses of between 49 and 134 mm crown-rump length. The development of the cells was traced from lymphoid elements with the features of haemopoietic stem cells and was divided on the basis of nuclear morphology into three stages. Granule formation commenced during the first stage and demarcation membrances could be demonstrated in the perinuclear cytoplasm early in the second stage. Late stage 2 cells often contained more than one nucleus, and the possibility that this was due to cellular fusion is discussed. The third stage was characterized by the appearance of cytoplasmic zoning and by the gradual extension of the demarcation system throughout the cytoplasm. There was evidence that the demarcation membrances were initially formed directly from the Golgi apparatus, but that their further development was due to the incorporation of elements of the agranular endoplasmic reticulum. The surface projections associated with platelet release were observed only in fully developed cells, and the formation of a zone of clear cytoplasm at the periphery was related to events occurring during the later stages of platelet release.