Blood is a fluid connective tissue that circulates through the cardiovascular system. It consists of formed elements, cells and their derivatives, and a protein-rich fluid matrix called plasma. The formed elements include red cells (erythrocytes), white cells (leukocytes), and platelets. Cells constitute approximately 45% of the blood volume, and red blood cells constitute about 99% of the cells. Red blood cells function in the transport and exchange of oxygen and carbon dioxide in the other tissues of the body. White cells consist of agranulocytes and granulocytes, and each white cell type has a specific function in immune and protective responses in the body. Most white cells leave the circulation to perform their functions in the connective tissue, whereas red cells function within the vascular system. Platelets are essential in blood clotting.
The relative distribution of cells in blood is usually studied in a blood smear. In this preparation, a small drop of blood is placed on a microscope slide and is smeared across the slide with the short edge of another slide. The preparation is then air-dried and stained with a mixture of dyes (e.g., Wright's stain, a modified Romanovsky-type stain). In examining a blood smear, it is useful to survey the smear with a low-power objective in order to ascertain which parts of the preparation show an even distribution of blood cells. In general, the periphery of the smear should be avoided, since the cells are either distorted, too close to each other (at the end where the drop of blood was placed), or too widely dispersed (at the opposite end, where the smear ended). Furthermore, the edges of a blood smear do not show a true percent distribution of white cells.
Figure 1, blood smear, human, Wright's stain x400.
This is a low-magnification photomicrograph of a smear with the blood cells well distributed. Most of the cells are erythrocytes (RBC). They are readily identified because of their number and lack of a nucleus. Scattered among the red cells are three white blood cells that can be distinguished from the erythrocytes without difficulty by their larger size and their staining characteristics. Interspersed among the cells are numerous small speck-like objects. These are blood platelets that have aggregated into small groups and, thus, can be readily observed even at this low magnification. They can be visualized better at higher magnification in Figures 2 to 4 (arrows).
Erythrocytes have a biconcave shape. They measure about 8.0 jxm in diameter in the circulating blood, about 7.5
Figures 2-7, blood smear, white blood cells, human, Wright's stain x1800.
Figures 2 to 4 illustrate characteristic features of lymphocytes. The nucleus stains intensely, generally has a rounded shape, and, as illustrated in Figure 2, may possess a slight indentation. These cells measure about 8, 10, and 12 fxm in diameter, respectively (circulating lymphocytes range from 6 to 12 p,m). In a small lymphocyte (Fig. 2), only a small amount of cytoplasm is evident, and the nucleus seems to constitute most of the cellular volume. In large lymphocytes of circulating blood (Figs. 3 and 4), there is a larger amount of cytoplasm. (Large lymphocytes of circulating blood are equivalent to the medium-sized lymphocytes of lymphatic tissue. The large lymphocytes of lymphatic tissue are not a characteristic feature of circulating blood except in certain abnormal conditions.) The cytoplasm of lymphocytes may stain a pale blue; sometimes, a p.m in blood smears, and 6 to 10 pjn, depending on the method used for preserving the tissue, in sectioned material. (A size of 7 p,m for the erythrocyte in sectioned material is useful to remember. It enables one to estimate the size of other structures in a histologic section by comparison with the RBC without resorting to a micrometer.) Erythrocytes stain uniformly with eosin, a component of the usual dye mixture (e.g., Wright's) used to stain blood smears. Because of the biconcave form of the erythrocyte, however, its center is thinner and appears lighter than the periphery.
To distinguish the different kinds of leukocytes in a blood smear, it is advantageous to use the highest available magnification, usually an oil-immersion lens. This enables one to use the morphologic features of the cytoplasm in addition to nuclear morphology and cytoplasmic staining in identifying the cell type.
distinct, lightly stained Golgi area is evident. In addition, lymphocyte cytoplasm may contain azurophilic granules, as shown in Figures 3 and 4.
Figures 5 to 7 show characteristic features of monocytes. These cells measure approximately 12 and 15 p,m in diameter, respectively (monocytes range from 9 to 18 |xm). The nucleus is somewhat less "compact" than the nucleus of lymphocytes. The cytoplasm, like that of lymphocytes, stains lightly but tends to have a grayer or duller blue tint. Azurophilic granules are also present in the cytoplasm. Lymphocytes and monocytes are classified as agranulocytes; i.e., they are usually free of specific cytoplasmic granules. Moreover, their nuclei are nonlobed, although monocyte nuclei may occasionally show a deep indentation (Fig. 6). Thus, they are distinguished from granulocytes, which possess specific cytoplasmic granules as well as a lobed or segmented nucleus.
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