Normal Red Blood Cell¨ Discoid shape with 7-8 micron diameter¨ Can squeeze thru 3 micron capillary¨ As it ages, it loses water & surface area, impairing deformability¨ These changes are detected by the RES and trigger removal of the aged RBCs by macrophages¨ reticulocytes - Young erythrocytes recently released into blood that still contain ribosomal RNA that precipitates and stains as a sort of reticulum.
Life of a Red Blood Cell¨ Erythroid precursors undergo 4-5 divisions in bone marrow, extrude nucleus, become reticulocytes, enter peripheral blood, and survive ~100-120 days.¨ Must withstand severe mechanical & metabolic stress, deform to pass thru capillaries half their size, resist shearing force across heart valves, survive stasis-induced acidemia & substrate depletion, avoid removal by macrophages.
Figure 12-3. Scanning Electron Micrographof normal human erythrocytes. Note their biconcaveshape for maximum oxygen exchange. x 3300 In mammals, RBCs in peripheral blood lack a nucleus; whereas those of birds and reptiles have a nucleus.
Hematopoiesis¨ Hematopoiesis: making blood cells¨ Location of hematopoiesis: ¤ Embryo: yolk sac, then liver/spleen ¤ After birth: bone marrow¨ Types of hematopoiesis: ¤ Erythropoiesis: red cells ¤ Granulopoiesis: granulocytes ¤ Megakaryopoiesis: megakaryocytes
Erythropoiesis¨ The erythroblast develops into a proerythroblast, which is only slightly smaller than the blast, but has a more basophilic cytoplasm.¨ The basophilic erythroblast forms when the proerythroblast loses its nucleolus. These cells are much smaller than the blast cells and have an intensely basophilic cytoplasm that results from the accumulation of ribosomes.¨ The polychromatophilic erythroblast has a darkly staining nucleus and its cytoplasm stains a grayish-green color due to the accumulation of hemoglobin.
Erythropoiesis, cont…¨ In the orthochromatic erythroblast, or normoblast, the nucleus becomes smaller and darker and the cytoplasm becomes pinker. Nuclear expulsion occurs at the end of this stage through an asymmetric division of the orthochromatic erythroblast. The portion that contains the cytoplasm and organelles becomes the reticulocyte, while the portion containing the nucleus is destroyed by macrophages.¨ The reticulocyte contains cytoplasm, cytoplasmic organelles, and many ribosomes. It is released from the bone marrow and develops into a mature erythrocyte after spending 1 to 2 days in the peripheral blood.
Erythropoiesis, cont…¨ Pronormoblast: 13-18 microns, round / ovoid with thin rim of basophilic cytoplasm, large spherical nucleus with fine chromatin and 1-2 nucleoli; usually perinuclear halo; N/C ratio is 90%.
Erythropoiesis, cont…¨ Basophilic normoblast: 12-17 microns; increase in deeply basophilic cytoplasm compared to pronormoblast and slightly smaller nucleus with slight chromatin condensation; often perinuclear halo; no granules, no nucleolus; N/C ratio is 75%- 85%
Erythropoiesis, cont…¨ Polychromatophilic normoblast: 12-15 microns; round / ovoid with abundant, dull gray to gray- green, variegated cytoplasm due to polyribosomes (basophilic) and hemoglobin (eosinophilic); round, condensed and basophilic nucleus has coarse granules that give it a cart-wheel appearance; perinuclear halo present; no nucleolus; N/C ratio is 60-80%
Erythropoiesis, cont…¨ Orthochromatophilic normoblast: 8-12 microns; round / ovoid cells with pink-orange uniformly staining cytoplasm, dark and opaque nucleus that may be pyknotic or in the process of being extruded, no nucleolus
Erythropoiesis, cont…¨ Reticulocyte: 7-10 microns; cannot identify without supravital stain (new methylene blue, brilliant cresyl blue) that colors RNA deep blue and granular; must have at least 2 granules to classify as reticulocyte; cytoplasm is red to pale blue due to RNA, no nucleus is present; larger than mature erythrocyte and lacks central pallor, passes through the bone marrow sinus wall and endothelial cells into the circulating blood where it matures
Erythropoiesis, cont…¨ Erythrocyte: 7-8 microns; round / ovoid biconcave disc with orange-red cytoplasm, no RNA, no nucleus; survives ~120 days in circulation
Erythroid Progenitor…derived from a myeloidstem cell, that is destined to produce the most common bloodcell, the erythrocyte.
A basophilic erythroblast (#4) is the 2nd individually recognizable stage in erythroid maturationafter proerythroblast and is characterized by deeply basophilic cytoplasm and coarser nuclearchromatin without a distinct nucleolus. An Orthochromatic erythroblast (#5) is characterized byhemoglobinized cytoplasm and condensed darker nucleus which is about to be extruded from thecell upon maturation.
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