Development of megakaryocytes in bone marrow of the rat: an analysis by electron microscopy and high resolution autoradiography

The fine structure of immature megakaryocytes was studied by the use of high resolution autoradiography to identify DNA synthesizing cells. Approximately 8.5 % of all megakaryocytes were labelled, and these cells were of three main types. The most immature were small cells with bilobed nuclei and sparse cytoplasm. Demarcation tubule formation from the cell surface was seen in these cells. The other two types of cell had three or more nuclear lobes, and a greater amount of cytoplasm containing dense granules and demarcation tubules. In the most mature labelled cells, tubules were numerous. Microtubules were seen in some labelled cells. Megakaryocytes in mitosis had similar cytoplasmic characteristics, with dense granules and demarcation tubules. No cells corresponding to the megakaryoblast described in light microscopic studies were observed. Possible reasons for this finding are discussed, and it is suggested that the megakaryocyte possesses a lobulated nucleus and shows cytoplasmic differentiation from the time of its first endomitosis.