The metamyelocyte is the stage at which neutrophil, eosinophil, and basophil lines can be clearly identified by the presence of numerous specific granules.
A few hundred granules are present in the cytoplasm of each metamyelocyte, and the specific granules of each variety outnumber the azurophilic granules. In the neutrophil, this ratio of specific to azurophilic granules is about 2 to 1. The nucleus becomes more heterochromatic, and the indentation deepens to form a kidney bean–shaped structure. Theoretically, the metamyelocyte stage in granulopoiesis is followed by the band stage and then the segmented stage. Although these stages are obvious in the neutrophil line, they are rarely if ever observed in the eosinophil and basophil lines in which the next easily recognized stages of development are the mature eosinophil and mature basophil, respectively.
In the neutrophil line, the band (stab) cell precedes development of the first distinct nuclear lobes.
The nucleus of the band (stab) cell is elongated and of nearly uniform width, giving it a horseshoelike appearance. Nuclear constrictions then develop in the band neutrophil and become more prominent until two to four nuclear lobes are recognized; the cell is then considered a mature neutrophil, also called a polymorphonuclear neutrophil or segmented neutrophil. Although the percentage of band cells in the circulation is almost always low (0% to 3%), it may increase in acute or chronic inflammation and infection.
Kinetics of Granulopoiesis
Granulopoesis in the bone marrow takes about two weeks.
The mitotic (proliferative) phase in granulopoiesis lasts about a week and stops at the late myelocyte stage. The post-mitotic phase, characterized by cell differentiation—from metamyelocyte to mature granulocyte—also lasts about a week. The time it takes for half of the circulating segmented neutrophils to leave the peripheral blood is about 6 to 8 hours. Neutrophils leave the blood randomly—that is, a given neutrophil may circulate for only a few minutes or as long as 16 hours before entering the perivascular connective tissue (a measured half-life of circulating human neutrophils is only 8 to 12 hours).
Neutrophils live for 1 to 2 days in the connective tissue, after which they are destroyed by apoptosis and are subsequently engulfed by macrophages. Also, large numbers of neutrophils are lost by migration into the lumen of the gastrointestinal tract from which they are discharged with the feces.
Bone marrow maintains a large reserve of fully functional neutrophils ready to replace or supplement circulating neutrophils at times of increased demand.
In normal conditions, the bone marrow produces more than 1011 neutrophils each day. As a result of the release of neutrophils from the bone marrow, approximately 5 to 30 times as many mature and near-mature neutrophils are normally present in the bone marrow as are present in the circulation. This bone marrow reserve pool constantly releases  neutrophils into the circulation and is replenished by maturing cells. The reserve neutrophils can be released abruptly in response to inﬂammation, infection, or strenuous exercise.
A reservoir of neutrophils is also present in the vascular compartment. This reserve consists of a freely circulating pool and a marginated pool, with the latter contained in small blood vessels. The neutrophils adhere to the endothelium much as they do before leaving the vasculature at sites of injury or infection (see page 275). The normally marginated neutrophils, however, loosely adhere to the endothelium through the action of selectin and can be recruited very quickly. They are in dynamic equilibrium with the circulating pool, which is approximately equal to the size of the marginated pool.