Proerythroblasts, also called pronormoblasts or rubriblasts, are the earliest precursors of erythropoiesis. They do not yet contain any hemoglobin. Their size ranges from 15 to 22 µm. The dark basophil, frequently lightly shaded cytoplasm is characteristic. The nucleus reveals a dense, fine honeycombed chromatin structure. As a rule, several, up to 5, pale blue nucleoli are present, which are difficult to define, however, and disappear on further maturation of the cell. In conjunction with all erythropoietic cells, proerythroblasts tend to produce multinucleated forms, but less so than plasma cells. A light area in the cytoplasm near the nucleus is typical which appears as finely granular by phase contrast microscopy. Hemoglobinization also commences at the nucleus: at first there is a lighter perinuclear zone (cells 1, 3, 8, 9), that eventually occupies the whole cell and directly passes to the polychromatic forms (cells 7–10). Simultaneously, the nucleus undergoes a characteristic change in structure: the nucleoli disappear. The chromatin stroma becomes coarser and transforms to the typical erythroblast nucleus.