The polychromatophilic erythroblast shows both acidophilic and basophilic staining of cytoplasm.
The staining reactions of the polychromatophilic erythroblast may blend to give an overall gray or lilac color to the cytoplasm, or distinct pink (acidophilic) and purple (basophilic) regions may be resolved in the cytoplasm. The nucleus of the cell is smaller than that of the basophilic erythroblast, and coarse heterochromatin granules form a checkerboard pattern that helps identify this cell type.
The orthochromatophilic erythroblast is recognized by its increased acidophilic cytoplasm and dense nucleus.
The next named stage in erythropoiesis is the orthochromatophilic erythroblast (normoblast). This cell has a small, compact, densely stained nucleus. The cytoplasm is eosinophilic because of the large amount of hemoglobin (Fig. 10.20). It is only slightly larger than a mature erythrocyte. At this stage, the orthochromatophilic erythroblast is no longer capable of division.
The polychromatophilic erythrocyte has extruded its nucleus.
The orthochromatic erythroblast loses its nucleus by extruding it from the cell; it is then ready to pass into the blood  sinusoids of the red bone marrow. Some polyribosomes that can still synthesize hemoglobin are retained in the cell. These polyribosomes impart a slight basophilia to the otherwise eosinophilic cells; for this reason, these new cells are called polychromatophilic erythrocytes (Fig. 10.21). The polyribosomes of
FIGURE 10.20 • Electron micrograph of an orthochromatophilic erythroblast (normoblast). The cell is shown just before extrusion of the nucleus. The cytoplasm contains a group of mitochondria located below the nucleus and small cytoplasmic vacuoles. The cytoplasm is relatively dense because of its hemoglobin content. The fine, dense particles scattered in the cytoplasm are ribosomes. 10,000. (Courtesy of Dr. Dorothea Zucker-Franklin.)
FIGURE 10.21 • Electron micrograph of a polychromatophilic erythrocyte (reticulocyte). The nucleus is no longer present, and the cytoplasm shows the characteristic fimbriated processes that occur just after nuclear extrusion. Mitochondria are still present, as are early and late endosomes and ribosomes. 16,500. (Courtesy of Dr. Dorothea Zucker-Franklin.) the new erythrocytes can also be demonstrated with special stains that cause the polyribosomes to clump and form a reticular network. Consequently, polychromatophilic erythrocytes are also (and more commonly) called reticulocytes. In normal blood, reticulocytes constitute about 1% to 2% of the total erythrocyte count. However, if increased numbers of erythrocytes enter the bloodstream (as during increased erythropoiesis to compensate for blood loss), the number of reticulocytes  increases.
Kinetics of Erythropoiesis
Mitoses occur in proerythroblasts, basophilic erythroblasts, and polychromatophilic erythroblasts.