Cellular Components of Blood
To recognize the cellular elements of blood.
Blood consists of cellular elements suspended in a complex solution called
plasma. Blood is usually classified as one of the connective tissues. The cellular
components of blood include red corpuscles (erythrocytes), platelets (thrombocytes),
and five types of white corpuscles (leukocytes). Erythrocytes and thrombocytes
are anucleated cells which perform most of their functions within the blood.
Leukocytes (neurophils, eosinophils, basophils, monocytes and lymphocytes) are
nucleated cells which migrate out of postcapillary venules into connective tissue
or lymphatic tissue. They perform their functions both within and outside the
blood stream. Blood smears are used to examine the size, shape and maturity
of blood cells. They are also used to determine the relative percent of each
type of white blood cell.
Blood: (Slide #24, normal blood smear)
Examine slide #24, (smear of normal blood). The smear is composed of erythrocytes,
interspersed with leukocytes and platelets. The leukocytes are difficult to
examine if the smear is too thick, and hard to find where the smear is too thin.
Examine the blood smear using the low power (10x) objective. The ideal place
to examine both erythrocytes and leukocytes is where there is only slight overlapping
of the erythrocytes.
Scan this area on your slide and observe the idiosyncracies of your own preparation.
If your background light is yellowish, use a blue filter. To quickly determine
if your slide is well-stained, examine the red blood cells. Are the erythrocytes
stained pink? If so, then other eosinophilic structures will be properly stained.
If not, you will have to make allowances for cells which may look a little more
basophilic than they should, or cells which do not show their granules as well
as they should.
Switch to the 100x lens and examine the red blood corpuscles. They
are biconcave discs approximately 7.2 Ám 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 (25 Ám) 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.
Granulocytes (polymorphonuclear leukocytes)
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.
Nongranular leukocytes (mononuclear leukocytes)
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 Ám (slightly smaller
than an RBC) to large cells up to 15 Ám 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
Ám). 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.