Blood is regarded as a connective tissue, fluid in character, and consists of formed elements and plasma. Red blood cells (erythrocytes), white blood cells (leukocytes), and thrombocytes (platelets) constitute the formed elements. Collectively, they make up 45% of the blood volume. Red blood cells transport and exchange oxygen and carbon dioxide. They constitute 99% of the blood cells. White blood cells are categorized as agranulocytes and granulocytes. The agranulocytes are further classified as lymphocytes and monocytes. The granulocytes, so named for the character of the granules that they contain in their cytoplasm, consist of neutrophils, eosinophils, and basophils. Each type of white cell has a specific role in immune and protective responses in the body. They typically leave the circulation and enter the connective tissue to perform their specific role. In contrast, red blood cells function only within the vascular system. Blood platelets are responsible for blood clotting and consequently have an essential role in incidents of small vessel damage. Blood smears are utilized for microscope examination and identification of relative numbers of white cells in circulating blood. The blood smear is prepared by placing a small drop of blood on a microscope slide and then smearing it across the slide with the edge of another slide. When properly executed, this method provides a uniform, single layer of blood cells that is allowed to air dry and then stained. Wright’s stain, a modified Romanovsky stain, is generally utilized. In examining the specimen under the microscope, it is useful to use a low magnifica-tion to find areas in which the blood cells have a uniform distribution like that seen in the smear on the adjacent page. Once this is accom-plished, by switching to a higher magnification, one can identify the various types of white blood cells and, in fact, determine the relative number of each cell type. A normal cell count is as follows: neutrophils, 48.6–66.7%; eosinophils, 1.4–4.8%; basophils, 0–0.3%; lympho-cytes, 25.7–27.6%; monocytes, 8.6–9.0%.
Blood smear, human, Wright’s stain, ×200.
This low magnification photomicrograph shows part of a blood smear in which the blood cells are uniformly distributed. The greater majority of cells are red blood cells. Because of their biconcave shape, most of the red blood cells
Neutrophils, blood smear, human, Wright’s stain, ×2,200.
Neutrophils exhibit variation in size and nuclear morphology that is associated with age of the cell. The nucleus seen on the left is that of a neutrophil that has just passed the band stage and has recently entered the blood stream. The cell is relatively small; its cytoplasm exhibits distinctive, fine granules. The middle neutrophil is considerably larger and its cytoplasm contains more
Eosinophils, blood smear, human, Wright’s stain, ×2,200.
The eosinophils seen in these micrographs similarly represent different stages of maturity. The eosinophil at the left is relatively small and is just beginning to show lobulation. The cytoplasm is almost entirely filled with eosinophilic granules that characterize this cell type. The lighter stained area, devoid of granules, probably represents the site of the Golgi apparatus (arrow). The
Basophils, blood smear, human, Wright’s stain, ×2,200.