By seven days of gestation, the yolk sac of the mouse has a sheet of mesoderm adjacent to the basement membrane separating it from the endodermal epithelium. Localized proliferations of this mesoderm produces thickened cellular regions which transform into the angioblastic cords; all of these developmental cells are attached by tight junctions and desmosomes. By eight and onehalf days, lumina appear within the angioblastic cords; the peripheral cells become attenuated and form endothelial cells which will line the primitive vessels while the more central cells become the primitive erythroblasts of the blood island.
The process of vasculogenesis and lumenization occurs between eight and onehalf and nine days of gestation and has been correlated with the reduction of cellular junctions between angioblasts and fixed primitive erythroblasts, a loss of the visceral basement membrane and the formation of wide intercellular channels between endodermal epithelial cells. The primitive erythroblasts comprising the blood islands have abundant polysomes, sparse rough endoplasmic reticulum and possess coated vesicles and ferritin aggregates in their cytoplasm and coated invaginations of their plasma membrane. By nine days of gestation, the primitive erythroblasts lose their attachments and become free in the vitelline vessels. Mitochondria of the primitive and free erythroblasts are slightly enlarged and have lighter matrices than angioblasts and mesodermal cells. By 10 to 11 days of gestation, as differentiation proceeds, coated vesicles and invaginations become more numerous and the developing erythroblasts gradually decrease in both cell and nuclear size. Concomitant with these changes is the decrease in the number and size of the mitochondria, a decrease in polysomal numbers and an increase in hemoglobin and cytoplasmic density.