Cellular Components of Blood

Switch to the 100x lens and examine the red blood corpuscles. They are biconcave discs approximately 7.2 µm in diameter. The cells appear darker at the periphery and light in the center. The color of red blood cells is due to the eosinophilia of hemoglobin. Mature erythrocytes are anucleated and lack organelles.

Platelets (2–5 µm) in diameter are fragments of cytoplasm surrounded by a plasma membrane. The cytoplasm stains blue and contains azurophilic granules.The platelets can occur singly or in clumps.

Examine an electron micrograph of platelets.

Neutrophils can be recognized by their segmented nuclei and the presence of abundant, small, pale staining granules in their cytoplasm. Often the individual granules are barely distinguishable. Examine a number of neutrophils under oil immersion until you can quickly identify them. In good preparations, you may be able to see that there are two types of granules present, the more abundant, smaller specific granules which stain light pink and the larger, non-specific azurophilic granules which stain red-purple. Under normal conditions, neutrophils constitute 60–70% of the total leukocyte count. Study the E.M. of a neutrophil.

Now examine your slide for an eosinophil. If you are in doubt, it is usually a neutrophil and not an eosinophil. The specific granules of the eosinophil are large and distinctive. These may be located even under low power by their large bright red-staining, refractile granules. The nucleus of the eosinophil is also segmented, but it is usually bi-lobed and paler staining than the neutrophil nucleus. The granules may be seen very clearly in cells which have had their cell membranes ruptured during preparation. The granules will then be spread apart and are easily seen to be large and oval. Eosinophils constitute up to 3% of the leukocytes. Examine an E.M. of an eosinophil.

Basophils make up less than 0.5% of the leukocytes and are difficult to find. The granules are very large, purple staining and not of uniform size. The nucleus, which is often difficult to see clearly because of the granules, maybe segmented. Because they are relatively rare, they may not be on every slide. Study the E.M. of a basophil.

Strictly speaking, there are probably no white cells totally devoid of granules. Azurophilic (non-specific) granules can also be found in lymphocytes and monocytes. However, these cells do not contain specific granules. The lymphocytes vary in size from 6 µm (slightly smaller than an RBC) to large cells up to 15 µm in size. The small lymphocytes have only a thin rim of sky-blue cytoplasm. Their nuclei of densely-stained chromatin are generally round or slightly indented on one side. Medium and larger lymphocytes have larger, round nuclei centrally located in a sky-blue cytoplasm. A few azurophilic granules may be present in the cytoplasm. Lymphocytes normally constitute 20–30% of the total leukocyte count, with small lymphocytes predominating. Study the E.M. of a lymphocyte.

The monocyte is usually the largest leukocyte present (15–20 µm). The nucleus of the monocyte, which is usually bean or U-shaped and is eccentric, may have a "lumpy" appearance which is seen by focussing up and down. The chromatin appears as a fine lacy network. The cytoplasm is gray in color and opaque and usually contains fine granules. The monocyte can sometimes be confused with a medium or large lymphocyte or with an immature neutrophil. A medium lymphocyte usually contains denser chromatin and sky-blue cytoplasm. A young neutrophil (called a band), contains a U-shaped nucleus with condensed chromatin and a cytoplasm filled with small granules. Study the E.M. of a monocyte.