Abstract
Ontogenetic development of monocytic cell series in fetal and bone marrow hematopoiesis of mice was investigated by electron microscopy, ultrastructural peroxidase (PO) cytochemistry, and morphometric analysis. At 10 days of gestation, a few mononuclear cells similar in localization pattern of PO activity to bone marrow promonocytes or monocytes developed in blood islands of the yolk sac and were called fetal promonocytes or monocytes. However, they showed prominent nuclear indentation or lobulation, their PO-positive granules were less in number, and the intensity of PO activity in rough endoplasmic reticula, nuclear envelope and Golgi complexes of the fetal promonocytes was weaker than that of bone marrow promonocytes. In a 3 day-culture of the cell suspensions from the yolk sac, colony-forming cells showed similar ultrastructural and cytochemical features of the fetal promonocytes and monocytes developing in the yolk sac. In the liver anlage, similar fetal promonocytes and monocytes were found in the early stage of hepatic hematopoiesis and subsequently they changed into promonocytes or monocytes seen in bone marrow hematopoiesis. The number and size of PO-positive granules in the fetal promonocytes and monocytes in the colonies growing in culture of the suspensions from the hepatic anlage were intermediate between those in the colonies from the yolk sac and those from the bone marrow.
The above findings indicate that the fetal promonocytes or monocytes first appearing in the yolk sac show incomplete ultrastructural features of bone marrow promonocytes or monocytes and gradually become mature and complete during the stage of hepatic hematopoiesis.
