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236 CHAPTE R 9 I Blood first recognizable erythrocyte precursor, the proerythroblast.

The first recognizable precursor cell in erythropoiesis is called the proerythroblast

The proerythroblast is a relatively large cell measuring 12 to 20 /xm in diameter. It contains a large spherical nucleus with one or two visible nucleoli. The cytoplasm shows mild basophilia because of the presence of free ri-bosomes. Although recognizable, the proerythroblast is not easily identified in routine bone marrow smears.

The basophilic erythroblast is smaller than the proerythroblast, from which it arises by mitotic division

The nucleus of the basophilic erythroblast is smaller (10 to 16 pm in diameter) and progressively more hete-rochromatic with repeated mitoses. The cytoplasm shows strong basophilia because of the large number of free ri-bosomes (polyribosomes) that synthesize hemoglobin. The accumulation of hemoglobin in the cell gradually changes the staining reaction of the cytoplasm, so that it begins to stain with eosin. At the stage when the cytoplasm displays both acidophilia, because of the staining of hemoglobin, and basophilia, because of the staining of the ribosomes, the cell is called a polychromatophilic erythroblast.

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. 9.19). 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 a blood sinus 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. 9.20). The polyribosomes of 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 (new erythrocytes) 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

At each of these stages of development, the erythroblast divides several times. It takes about a week for the progeny of a newly formed basophilic erythroblast to reach the cir-

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