Significance of electron‐dense deposits in the mitochondrial matrix of erythroid precursors in aplastic anaemia and myelodysplastic syndrome

Recently the number of long‐term survivors of aplastic anaemia has increased, with some of these cases evolving into myelodysplastic syndrome (MDS). Because it is difficult to discriminate between aplastic anaemia and hypoplastic MDS, it is unknown whether these patients have had MDS from the time of diagnosis of aplastic anaemia. Presence of ringed sideroblasts on an iron‐stained bone marrow smear is a characteristic of some cases of MDS. Amorphous electron‐dense deposits in the mitochondrial matrices of erythroid precursors observed with an electron microscope show ringed sideroblasts, and detection of this mitochondrial pathology is useful for confirming the presence of ringed sideroblasts because this mitochondrial pathology can be found not only in erythroblasts but also in reticulocytes, which is particularly useful in cases in which few erythroblasts are found. We found this mitochondrial pathology in two of nine children who had an initial diagnosis of aplastic anaemia and in three children with hypoplastic MDS. It is unknown at present whether the first two children had aplastic anaemia or hypoplastic MDS. Our results warrant further studies on more patients to confirm the significance of amorphous electron‐dense deposits in the mitochondrial matrices of erythroid precursors.