Granulocytes originate and mature in the bone marrow and then migrate to the blood. In physiological stages the number of the cells moving from the bone marrow to the blood is equivalent to the number of the cells moving from the blood to tissue.
Proliferation, differentiation, maturation, and storage in the bone marrow are well known for neutrophilic but not for basophilic granulocytes. By proliferation from myeloblast to myelocyte a total mass of granulocytes is formed, and by maturation from myeloblast to segmented granulocyte, during what granules are formed, granulocyte becomes qualifiable for functional maturity. The main factor stimulating the proliferation of precursors, as well as differentiation and proliferation of directed stem cells, is the granulocyte colony-stimulating factor (G-CSF).
The cells of granulocytopoiesis have characteristic cytologic features in regard to the nucleus and the cytoplasm that contains three various types of granules.
In the bone marrow, the proliferation happens in the mitotic compartment where the mass of granulocytes develops after 4-5 divisional cycles. Most of the divisional cycles, about 3, happen on the level of myelocytes. Maturation period from the myelocyte to mature granulocyte of the peripheral blood is about 5-7 days. Myeloblast, promyelocyte, and myelocyte are cells that divide and mature. The matured part of granulocytes in the bone marrow consists of the cells that do not divide any more, just mature; they are metamyelocytes.
During the maturation of the cell the color of the cytoplasm changes, granules appear in the cytoplasm, depending on the type of granulocytes, and the shape of the nucleus changes. At the end the nucleus takes a horseshoe appearance in the metamyelocyte, and rod-like appearance in non-segmented granulocyte. Finally, the nucleus becomes segmented in the segmented granulocyte.
Granulocytes are supposed to stay in the peripheral bloodstream for about 6-8 hours. Passing into peripheral tissues, where granulocytes live for about 2-3 days, the life-span of white blood cells is over.
In physiological conditions the exact fate of granulocytes has not known yet.
Granulocytes migrate from the bloodstream to the lungs, gastrointestinal tract, liver and spleen; they may be lost from mucosal surfaces, “die in these tissues”, i.e., they are phagocytized by tissue macrophages.
In pathological conditions it is possible to find some or all morphological stages of this lineage in the peripheral blood. Under stress and during the maturation time may be shortened, divisions may be skipped, so immature forms of the precursors may appear in the peripheral blood.