Morphological changes in erythroblasts during erythropoietin-induced terminal differentiation in vitro

Immature murine erythroblasts infected with the anemia-inducing strain of Friend virus (FVA cells) differentiate in vitro under the influence of erythropoietin (EP). These cells were used as a model for the examination of morphological changes occurring during terminal erythroid differentiation. FVA cells differentiate more completely in vitro in response to EP than continuous erythroleukemia cell lines do in response to chemical induction. Because they can be isolated in much greater numbers and in much higher purity than bone marrow or spleen cells explanted from anemic mice, FVA cells are an attractive alternative for studies of mammalian terminal erythroid differentiation. FVA cells cultured with EP followed a sequence of differentiation events that included a progressive decrease in cell size, disappearance of nucleoli, condensation of nuclei, and accumulation of hemoglobin. After 45 h of culture most FVA cells enucleated, giving rise to vacuolated reticulocytes and free nuclei that were surrounded by a thin layer of cytoplasm and a plasma membrane. The ratio of nuclear to cytoplasmic volumes increased significantly by 24 h of culture but did not change significantly from 24 through 36 h of culture. Variation in the morphology of enucleating FVA cells indicated that not all cells proceeded through a rigorously defined series of morphological stages prior to enucleation. These results are discussed in terms of previous studies of erythroblast maturation.