In the fetus, and after birth in the case of vitamin B12 or folic acid deficiency, the cells of the erythrocytic line are formed by maturation along the megaloblastic pathway.
The cells of the megaloblastic lineage are formed from the promegaloblast in the bone marrow when there is a shortage of vitamin B12 or folic acid.
The difference between the cells of this lineage and the cells of the erythroblastic lineage is in the disturbance of the cell maturation. All cells of the megaloblastic lineage are larger than their equivalents of the erythroblastic lineage.
All cells of the megaloblastic lineage may be found in the patients’ bone marrow. In the peripheral blood, megalocytes and rare megaloblasts can be found.