Autophagy of mitochondria in rat bone marrow erythroid cells. Relation to nuclear extrusion.

Late erythroblasts and reticulocytes from bone marrow of male Wistar rats were studied by electron-microscopic stereology. Late erythroblasts with morphological signs of nuclear extrusion (EN + erythroblasts) and late erythroblasts without these signs (EN-erythroblasts) were analysed separately. The volumes of mitochondria, autophagosomes, autophagocytosed mitochondria, autophagocytosed cytoplasm and degraded material inside autophagosomes were calculated per unit volume of cytoplasm. The results demonstrate that (1) the volume density of mitochondria in the cytoplasm decreases by 34% during maturation from (EN-)- to (EN +)-erythroblasts (P less than 0.001) and by 60% during differentiation from (EN +)-erythroblasts to reticulocytes (P less than 0.001), (2) a fivefold increase in the volume density of autophagosomes in the cytoplasm is noted during maturation from (EN-)- to (EN +)-erythroblasts (P less than 0.01), whereas the value of this parameter remains essentially unchanged during the subsequent differentiation to reticulocytes, (3) no mitochondria are found inside autophagosomes of (EN-)-erythroblasts, whereas mitochondria occupy 26% and 35%, respectively, of the autophagosomal volume in (EN +)-erythroblasts and in reticulocytes. Our results show that autophagocytosis of mitochondria starts at the moment of nuclear extrusion and continues in the bone marrow reticulocytes.