Like all blood cells, granulocytes are produced in the extravascular space of the bone marrow through the differentiation of hemopoietic stem cells that can also produce red cells and platelets. Stem cells differentiate into progenitor cells committed to the production of granulocytes and macrophages. These progenitor cells, although morphologically not recognizable, can form granulocytic colonies in culture and, hence, are known as colony-forming units in culture. They further differentiate into recognizable myeloblasts, which first develop primary granules and then specific neutrophilic granules. When they are fully mature, they crawl to the abluminal surface of specialized vascular structures, known as sinusoids. Here, on the body's demand and in the presence of appropriate stimuli, they penetrate the cytoplasm of sinusoidal endothelium, entering the lumen and thence into the circulation. Humoral factors such as corticosteroids, immune mediators (eg, complement components), and possibly lymphokines (eg, interleukins) may function as the mediator of their egress.
Tavassoli M, Kataoka M, Collier B. The Birth of Granulocytes. JAMA. 1984;251(23):3141. doi:10.1001/jama.1984.03340470067032
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