Megaloblasts are abnormal large nucleated erythroid precursors which develop due to deficiency of vitamin B12 or folic acid
Deficiency of vitamin B12 and folic acid causes ineffective erythropoiesis in which cell maturation is effected
Megaloblasts are named according to the stage of the erythroid precursor like promegaloblast, early, intermediate and late megaloblasts
Megaloblasts differ from normoblast in having
Increased cell size and amount of cytoplasm
Nuclear chromatin of megaloblasts is sieve- like or stippled (open).
Nuclear – cytoplasmic asynchrony is seen i.e, Nuclear maturation (condensation of nuclear chromatin) falls behind the cytoplasmic maturation (hemoglobinization)
Megaloblasts also exhibit
Howel Jolly bodies in cytoplasm
Nuclear border irregularity due to nuclear budding and nuclear bridging
Abnormal mitosis
Reference
Shirish M Kawthalkar. Essentials of Hematology. First edition, 2006