Temporal Characteristics of the Differentiation of Embryonic Erythroid Cells in Fetal Peripheral Blood of the Syrian Hamster: (erythroid differentiation/embryonic erythroid cells/morphological change/image-processing analysis/syrian hamster).

The morphological changes in erythroid cells and their nuclei in the circulation of fetuses of the Syrian hamster were investigated by use of an image-processing system. The analysis included monitoring of nuclear condensation, nuclear periphralization (access of the nucleus to the cell membrane), enucleation, density of cells, and changes in cell size from day 9 of gestation to day 5 after birth. The yolk-sac-derived erythroid cells made rapid progress in nuclear condensation on day 11, while this process proceeded at a much lower rate after day 12 of gestation. The peripheralization of nuclei started on day 10 and reached a maximum on day 11. The frequency of enucleated cells was below 2% on day 11, while it increased to 30% on day 12. Extruded nuclei, most of which were accompanied by a small quantity of cytoplasm, appeared in the circulation on day 12. The most frequently observed diameter of enucleated erythrocytes, which was 10-10.5 μm on day 12, fell gradually to 8-9 μm on day 14. By contrast, the shift from fetal liver erythrocytes to adult erythrocytes occurred in a discontinuous manner. Adult-type erythrocytes were detected after birth with diameters of 5.5-6 μm. Our data allows us to present the schedule of morphological changes during embryonic erythropoiesis and show that the developmental behavior of "primitive" yolk-sac-derived erythroid cells is more closely correlated with that of the "definitive" fetal liver cells than has been considered to be the case to date.