AG RAN U LOCYTE S AN D R E D MAR ROW KEY A, adipocytes BN, band neutrophil E, eosinophils Ey, erythrocytes M, megakaryocyte 306 Erythropoiesis is a process by which the concentration of erythrocytes in the peripheral blood stream is maintained under normal condi-tions in a steady state. Stimulation of erythroid stem cells (ErP or CFU-E) by hormonal action results in a proliferation of precursor cells that undergo differentiation and maturation in the bone marrow. The earliest recognizable precursor of the red blood cell is the proerythroblast. These cells lack hemoglobin. Their cytoplasm is basophilic and the nucleus exhibits a dense chromatin structure and several nucleoli. The Golgi apparatus, when evident, appears as a light staining area. The basophilic erythroblast is smaller than the proerythroblast, from which it arises by mitotic division. Its nucleus is smaller. The cytoplasm shows strong basophilia due to the increasing number of ribosomes involved in hemoglobin synthesis. The accumulation of hemoglobin in the cell gradually changes the staining reaction of the cytoplasm so that it begins to stain with eosin. The recognizable presence of hemoglobin in the cell by virtue of its staining signifies its transition to the polychro-matophilic erythroblast. The cytoplasm in the earlier part of this stage may exhibit a blue-grey color. With time, increasing amounts of hemoglobin are synthesized and concomitantly, decreasing numbers of ribosomes are present. The nucleus of the cell is smaller than that of the basophilic erythroblast and the heterochromatin is much coarser. At the end of this stage, the nucleus has become much smaller and the cytoplasm more eosinophilic. This is the last stage in which mitosis occurs. The next definable stage is the orthochromatophilic erythrob-last, also called normoblast. Its nucleus is smaller than earlier stages and is extremely condensed. The cytoplasm is considerably less blue leaning more to a pink or eosinophilic coloration. It is slightly larger than a mature erythrocyte. At this stage, it is no longer capable of division. In the next stage, the polychromatophilic erythrocyte, also more commonly called a reticulocyte, has lost its nucleus and is ready to pass into the blood sinusoids of the red bone marrow. Some ribosomes that can still synthesize hemoglobin are present in the cell. These ribosomes create a very slight basophilia to the cell. Comparison of this cell to typical mature erythrocytes in the marrow smear reveals a slight difference in coloration.
Polychromatophilic erythroblast, bone marrow smear, human, Giemsa, ×2,200.
Two polychromatophilic erythroblasts are seen in this micrograph. The larger and less mature cell exhibits a pronounced clumping of its chromatin. The cytoplasm is basophilic, but is considerably lighter in color than that of the basophilic erythroblast. The cytoplasm also exhibits some eosinophilia, which is indicative of hemoglobin production. The smaller cell represents a later stage of a polychromatophilic erythroblast. Note how much more dense the chromatin appears as well as how much smaller the nucleus has become. Also, the cytoplasm now favors an eosinophilia. However, some basophilia is still evident.
Orthochromatophilic erythrocyte, bone marrow predominantly eosinophilhic, but still possesses a degree of basophilia. Over-smear, human, Giemsa, ×2,200. all the cell is only slightly larger than a mature erythrocyte. At this stage, the cell is no longer capable of division.