Ultrastructure of the cells in the peripheral blood of human embryos at early gestational stages.

The presence of the different blood series in the peripheral blood of 10- to 18-week-old human embryos was followed by electron microscopy. It has been demonstrated that the earlier the stage of embryonic development, the more immature cells are found in the peripheral blood. This phenomenon may suggest migration of blood cell progenitors from earlier to more definitive sites of embryonic erythropoiesis.