FIGURE 10.1  Blood smear: Preparation technique and overview photomicrograph. a. Photograph showing the method of producing a blood smear. A drop of blood is placed directly on a glass slide and spread over its surface with the edge of another slide. b. Photomicrograph of smear from peripheral blood stained with Wright’s stain, showing the cells evenly distributed. The cells are mainly erythrocytes. Three leukocytes are present. Platelets are indicated by arrows. 350.
tissues and, in exchange, bind carbon dioxide for removal from the tissues. Their shape is that of a biconcave discs with a diameter of 7.8 m, an edge thickness of 2.6 m, and a central thickness of 0.8 m. This shape maximizes the cell’s surface area (140 m2), an important attribute in gas exchange.
7.8 m 0.8 m 2.6 m
The life span of erythrocytes is approximately 120 days, after which most (90%) of them are phagocytosed by macrophages in the spleen, bone marrow, and liver. The remaining aged erythrocytes (10%) break down intravascularly, releasing insignificant amounts of hemoglobin into the blood.
In H&E–stained sections, erythrocytes are usually 7 to 8 m in diameter. Because their size is relatively consistent in fixed tissue, they can be used to estimate the size of other cells and structures in tissue sections; in this role, erythrocytes are appropriately referred to as the “histologic ruler.”
Because both living and preserved erythrocytes usually appear as biconcave discs, they can give the impression that their form is rigid and inelastic (Fig. 10.2). They are, in fact, extremely deformable. They pass easily through the narrowest capillaries by folding over on themselves. They stain uniformly with eosin. In thin sections viewed with the transmission electron microscope (TEM), the contents of an erythrocyte appear as a dense, finely granular material.
The shape of the erythrocyte is maintained by membrane proteins.
The cell membrane of an erythrocyte is composed of a typical lipid bilayer and contains two functionally significant groups of proteins.
 Integral membrane proteins represent most of the proteins in the lipid bilayer. They consist of two major families: glycophorins and band 3 proteins. The extracellular domains of these integral membrane proteins are glycosylated and express specific blood group antigens. Glycophorin C, a member of the glycophorin family of transmembrane proteins, plays an important role in attaching the underlying cytoskeletal protein network to the cell membrane. Band 3 protein binds hemoglobin and acts as an additional anchoring site for the cytoskeletal proteins (Fig. 10.3).
 Peripheral membrane proteins reside on the inner surface of the cell membrane. They are organized into a two-dimensional hexagonal lattice network that laminates the inner layer of the membrane. The lattice itself, which is positioned parallel to the membrane, is composed mainly of cytoskeletal proteins including spectrin tetramers, actin, band 4.1 protein, adducin, band 4.9 protein, and tropomyosin (see Fig. 10.3) that form a network or mesh. The lattice is anchored to the lipid bilayer by the globular protein ankyrin, which interacts with band 4.2 protein as well as with band 3 integral membrane protein.
glycophorin C adductin spectrin ankyrin actin tropomyosin band 3 protein 4.2