Light micrograph of bone marrow cells showing precursor red (faint red) and white (purple) blood cells from a patient with megaloblastic anaemia. Megaloblastic anaemia is caused by a deficiency in vitamin B12 or folic acid. A megaloblastic cell is an abnormal form of any of the cells that are precursors to red blood cells. They may be unusually large and their nuclei fail to mature in the normal way. The oxygen carrying capacity of the red blood cells of an anaemic person is greatly reduced which results in tiredness, breathlessness, pallor and poor resistance to infections. Magnification: x100 at 35mm size.