Which immature blood cell is typically found in circulating blood?

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Immature red blood cells that can be found circulating in the blood are reticulocytes. During erythropoiesis, cells progress from early precursors in the bone marrow to a stage where the nucleus is expelled. Once the nucleus is gone, the cell becomes a reticulocyte and is released into the bloodstream, where it continues to mature into a fully mature red blood cell over the next day or two. Reticulocytes still contain some residual RNA, which is why they can be identified with special stains and are typically slightly larger than mature erythrocytes. In healthy individuals, only a small fraction of circulating red cells are reticulocytes, reflecting normal bone marrow activity. Erythroblasts and normoblasts are earlier marrow precursors and usually stay in the marrow, while megaloblasts are large abnormal precursors seen in certain anemias; they are not normally circulating under standard conditions.