Definition: Originally, a term denoting all forms of human red blood cells containing a nucleus, both pathologic (megaloblastic) and normal (normoblastic). The pathologic or megaloblastic series is observed in pernicious anemia in relapse. The term megaloblast is also used to indicate the first generation of cells in the red blood cell series that can be distinguished morphologically; hence, with this usage, megaloblast denotes both a normal and an abnormal cell. In the erythroblastic series of maturation four stages of development can be recognized: 1) proerythroblast, 2) basophilic erythroblast, 3) polychromatic erythroblast, and 4) orthochromatic erythroblast. In the megaloblastic series of maturation, stages similar to those found in the normoblastic series are seen: 1) promegaloblast, 2) basophilic megaloblast, 3) polychromatic megaloblast, and 4) orthochromatic megaloblast. In the normal series of maturation, after loss of the nucleus, young erythrocytes are called reticulocytes; these cells may be recognized with supravital stains such as brilliant cresyl blue; ultimately the reticulocytes become erythrocytes, or mature red blood cells.
[erythro- + G. blastos, germ]
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