FIGURE 10.2  Erythrocyte morphology. a. Photomicrograph of three capillaries (Cap) joining to form a venule (V ), as observed in adipose tissue within a full-thickness mesentery spread. The erythrocytes appear in single file in one of the capillaries (the other two are empty). The light center area of some of the erythrocytes results from their biconcave shape. Erythrocytes are highly plastic and can fold on themselves when passing through very narrow capillaries. The large round structures are adipose cells (A). 470.
b. Scanning electron micrograph of erythrocytes collected in a blood tube. Note the concave shape of the cells. The stacks of erythrocytes in these preparations are not unusual and are referred to as rouleau. Such formations in vivo indicate an increased level of plasma immunoglobulin. 2,800.
This unique cytoskeletal arrangement contributes to the shape of the erythrocyte and imparts elastic properties and stability to the membrane. The cytoskeleton is not static; it undergoes continuous rearrangement in response to various protein 4.9 protein 4.1
FIGURE 10.3  Erythrocyte membrane organization. The rectangle in the sectioned erythrocyte (upper left) represents the area of the membrane in the larger diagram. The large diagram shows the arrangement of the peripheral and integral membrane proteins. The peripheral proteins form a cytoskeletal lattice on the interior surface of the plasma membrane; the predominant protein is spectrin. The lattice is anchored to the plasma membrane by a number of protein complexes.
physical factors and chemical stimuli as the cell moves through the vascular network. Any defect in the expression of genes that encode these cytoskeleton proteins can result in abnormally shaped and fragile erythrocytes. For instance, hereditary spherocytosis is caused by a primary defect in spectrin gene expression that results in spherical erythrocytes. Hereditary elliptocytosis is caused by a deficiency in band 4.1 proteins that results in elliptical erythrocytes. In both conditions, erythrocytes are unable to adapt to changes in their environment (e.g., osmotic pressure and mechanical deformations), which results in premature destruction of the cells, or hemolysis.
Erythrocytes contain hemoglobin, a protein specialized for the transport of oxygen and carbon dioxide.
Erythrocytes transport oxygen and carbon dioxide bound to the protein hemoglobin (68 kilodaltons). A monomer of hemoglobin is similar in composition and structure to myoglobin, the oxygen-binding protein found in striated muscle. A high concentration of hemoglobin is present within erythrocytes and is responsible for their uniform staining with eosin and the cytoplasmic granularity seen with the TEM. The disc shape of the erythrocyte facilitates gas exchange because more hemoglobin molecules are closer to the plasma membrane than they would be in a spherical cell. Thus, gases have less distance to diffuse within the cell to reach a binding site on the hemoglobin.
 FOLDER 10.1
Clinical Correlation: ABO and Rh Blood a life-threatening complication, blood for transfusion must