Myeloblasts are the earliest stage of development and are characterized by their large size, round nucleus, and prominent nucleoli. Promyelocytes are slightly more mature than myeloblasts and have a more indented nucleus. Myelocytes are further along in development and have a horseshoe-shaped nucleus. Metamyelocytes are the most mature stage of immature granulocytes and have a bean-shaped nucleus.
Immature granulocytes are released from the bone marrow into the bloodstream when they are mature. They then circulate in the blood for several days before migrating into tissues where they can fight infection.