The ultrastructure of megakaryopoietic cells of the yolk sac and liver in mouse embryo.

Megakaryopoietic cells in the yolk sac and liver of mouse embryos were examined by electron microscopy. At 10 days' gestation, yolk sac vitelline vessels contained a few free megakaryopoietic cells. On the basis of the development of demarcation membranes and granules, yolk sac megakaryopoietic cells were classified into three types: YM1, YM2, and YM3. The YM1 cells, which comprised 75% of the yolk sac megakaryopoietic cells, had poorly developed demarcation membranes and few granules in the cytoplasm. The YM2 cells had a developed demarcation membrane system around the nucleus and comprised 24% of the yolk sac megakaryopoietic cells. The YM3 cells, the rarest type, had an eccentrically located nucleus and a cluster of demarcation membrane structures. In the liver of 11-day embryos, immature megakaryopoietic cells were present in both the sinusoidal lumen and the hepatic cords. Most of the hepatic megakaryopoietic cells of 11-day embryos had ultrastructural features similar to those of YM1 cells or intermediate between YM1 and YM2 cells.