Granulocytes

Granulocytes, as their name implies, contain distinctive cytoplasmic granules that are easily visualized in peripheral blood smears stained with Wright’s stain. There are three types of granulocytes: neutrophils, eosinophils, and basophils. These cells are approximately 12-15 microns in diameter, and their nuclei are either segmented or bilobed. In the case of eosinophils and basophils, their nuclei may be obscured by their abundant, intensely stained specific granules.

Neutrophils are the most abundant granulocyte, representing 50-70% of the WBC population, and are therefore the most readily identifiable leukocyte in a peripheral blood smear. They contain many lysosomes or azurophilic granules and pale staining specific granules, making it easy to visualize their segmented nuclei. Neutrophils are among the first cells to arrive at the site of injury or infection, exiting the vasculature by the process of diapedesis. They enter into the connective tissue where they function as phagocytes, engulfing and killing bacteria and releasing cytokines that modulate inflammation.

Eosinophils represent about 1-4% of white blood cells in a peripheral smear and are more difficult to locate. They possess distinctive, intensely stained pink granules that may obscure their bilobed nucleus. Eosinophils are involved in immune responses to helminthic infections and allergic reactions.

Basophils are the least abundant granulocyte, representing 0.5-1% of the WBC count. They possess abundant, basophilic specific granules and a bilobed or sometimes S shaped nucleus. They possess cell surface receptors for IgE, and binding to IgE triggers secretion of the contents in their specific granules. Basophils modulate immune responses and are involved in allergic and hypersensitivity reactions.

• Use the images below to help you identify the granulocytes in these slides of peripheral blood smears (sample 1 and sample 2). Note their size compared to red blood cells, nuclear morphology, and the appearance of their granules.
• To review the ultrastructural features of granulocytes in TEM images, please see Figures 12-8 through 12-10 in Junqueira’s Histology 14e.

Clinical note: A large accumulation of neutrophils, especially dead and dying neutrophils that have engulfed many bacteria, constitutes “pus.” Pus formation is a normal part of an infection that occurs within tissues.  

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