Fetal erythropoiesis in steel mutant mice I. A morphological study of erythroid cell development in fetal liver

A method of definitive identification of mutant (S1S1d) and wild-type (+/+) mouse embryos in segregating litters is described, based on the total number of circulating erythrocytes in a unit volume of embryonic blood and the relative proportion of nonnucleated vs. nucleated red blood cells. Evidence is presented that from days 13–17 of gestation, S1S1d embryos have many fewer fetal liver derived nonnucleated erythrocytes whereas the number of yolk sac-derived nucleated red blood cells is similar between S1S1d and +/+. Erythroid precursor cells at various stages of maturation in mutant fetal livers are studied by light and electron microscopy, and their fine structure is found to be identical to those present in normal embryos. The number of hemoglobin-containing mature erythroblasts in mutant fetal livers is far fewer than that of the normal, whereas the number of immature erythroid precursors present in a unit area of fetal liver is not significantly different between S1S1d and +/+. It is suggested that the mutant S1 gene product(s) interferes with or fails to support the differentiation of immature erythroid precursors into hemoglobin synthesizing cells.