Light and electron microscopic observations of hepatic hematopoiesis of human fetuses. II. Megakaryocytopoiesis.

Liver tissues obtained from 78 human fetuses between 5 weeks and 10 months of gestation were observed with light and electron microscope. In the third month of gestation hepatic megakaryocytopoiesis reached the maximum level and then appeared to decline. At the maximum level immature to mature megakaryocytes were usually smaller than those from adult bone marrow. They were observed between growing liver cells, within hepatic sinusoids and in the mesenchyme surrounding the liver parenchyma in the given order of frequency. Immature megakaryocytes were characterized by the presence of both demarcation vesicles or tubules and a few small round specific granules. The nucleus was mononuclear but frequently indented. With cell maturation, the megakaryocytes were stained intensely by PAS-technique. The nuclei were multilobulated. Large cytoplasm was dominated by numerous specific granules and well-developed demarcation membrane system. Some megakaryocytes passed through the sinusoidal wall and possibly to hepatic sinusoids. These observations indicate that the main site of hepatic megakaryocytopoiesis may be extravascular and more likely between liver cells. Small megakaryocytes in fetal livers are discussed in relation to the occurrence of similar megakaryocytes in various blood dyscrasias.