Blood is composed of specialized cells that circulate in an extracellular fluid called plasma.
Plasma typically consists of about 92% water, 7% protein, and 1% is a combination of other
solutes. The three main types of blood cells are the red blood cells or erythrocytes, white blood
cells or leukocytes, and platelets, which are also called thrombocytes. The total blood volume
of an average adult is about 6 L. Leukocytes and thrombocytes only comprise about 1 percent of the
blood volume; whereas plasma, which is the largest portion of the blood, is about 55%. Hematocrit or
HCT for short, is the percent of packed red blood cells (or RBCs) in blood, by volume. Normally,
the hematocrit is between 39 to 50 percent in males and 35 to 45 percent in females.
The morphology and characteristics of blood cells can be analyzed histologically by using
a common technique called a blood smear or blood film. A drop of blood is placed on a glass slide,
then literally smeared or spread across the slide from left to right, creating a thin
layer of cells similar to this image of a blood smear stained with Wright’s stain.
This stain is a mixture of eosin, which is an acidic dye, methylene blue, which is a basic dye,
and azures, which are also basic dyes. It’s commonly used as a differentiating stain
for blood smears, bone marrow, and blood parasites. The head of the blood smear is
where the drop of blood was applied to the slide. When the blood was spread from the
head to the right side of the slide, the thin layer of blood gradually became even thinner.
The tail of the smear is the last portion of the smear that visibly tapers even more. The tail
of the smear is not used for examination because the morphology of the cells can appear distorted.
The cells are often abnormally grouped together, and red blood cells will have a loss of central
pallor, which could be mistaken for spherocytosis. If we take a closer look at the blood smear on the
left side, there are a lot more cells, but their morphology is also distorted. In this image,
we can see that the cells are overlapping and packed so tightly together that it’s hard to
differentiate the individual cells from each other. As a result, the best area of the smear
to examine is typically within the body of the smear, but further to the right, near the tail.
This zone will have some cells that overlap, but the majority of the cells will be evenly
spaced apart, allowing the cells to be easily counted and differentiated from one another.
Now let’s take a closer look at the erythrocytes. With Wright’s stain, the erythrocytes stain bright
pink or red because they contain a large amount of hemoglobin, which is eosinophilic.
Erythrocytes normally lack organelles or nuclei and they’re typically shaped like a biconcave
disc, which provides a large surface-to-volume ratio that helps facilitate gas exchange.
As a result, the center of erythrocytes are thinner, which is why they appear to have a
pale center when they’re positioned parallel to the microscope slide.
Erythrocytes normally survive in the circulation for about 120 days.
The small basophilic or purple discs that are seen throughout this image
are platelets. Platelets also don’t have nuclei and are normally only about 2 to 4 μm in diameter.
Whereas the average erythrocyte is about 7 to 8 μm in diameter, which is often used by histologists
as an internal standard to estimate other nearby cells or structures, such as the
diameter of this small capillary that’s only slightly larger than a single erythrocyte.
Many abnormalities in erythrocytes can be seen with a blood smear,
including spherocytes which lack a distinct pale center; fragmented RBCs called schistocytes;
sickle cells and target cells in patients with sickle cell anemia;
and many other abnormalities such as howell-jolly bodies, & bite cells.
Leukocytes in the blood can be classified into two groups, granulocytes and agranulocytes.
The two groups can be differentiated using light microscopy since the granulocytes have
prominent granules that look like small dots or grains in their cytoplasm, whereas agranulocytes
don’t have these granules. Neutrophils, eosinophils, and basophils are classified
as granulocytes; and both monocytes and lymphocytes are classified as agranulocytes.
Lymphocytes can then be further subdivided into B cells and T cells.
Let’s first take a closer look at the granulocytes. Neutrophils are also called
polymorphonuclear leukocytes or PMNs because of their distinct nuclei, which have 2 to 5 lobes.
The number of lobes will increase as the cell ages, so a young cell may only have 2 lobes,
while an older cell is more likely to have 5 lobes. The diameter of neutrophils are about 12
to 14 μm and they contain an abundant amount of two major types of cytoplasmic granules:
Lysosomes (often called azurophilic granules in blood cells) and specific granules that
bind neutral, basic, or acidic stains and have specific functions. Basophils have nuclei that
are either S-shaped or have two lobes and have diameters that range from 14 to 16 μm.
Basophils also have large basophilic or purple granules, which help to differentiate them from
eosinophils, which have large eosinophilic or pink granules instead. Eosinophils are often
larger than neutrophils with a diameter between 12 and 17 μm and nuclei that have two lobes.
Moving on to the agranulocytes, the lymphocytes are round or oval cells that have large round
nuclei that are densely stained dark purple. Their diameter can range from 6 μm all the way to 15 μm.
Lymphocytes with a diameter under 10 μm are considered small lymphocytes. In this image,
we can see that smaller lymphocytes only have a very small amount of cytoplasm that
forms a small rim surrounding the nucleus. Even though larger lymphocytes have more cytoplasm,
their large nuclei still occupy most of the cell’s body. Monocytes are the largest type of white
blood cell that has a diameter that ranges from 12 to 20 μm. They have a large nucleus that’s usually
found along the outer edge with a horseshoe or indented shape. Monocytes also have small
purple lysosomes and vacuoles in the cytoplasm that give the cells a frosted-glass appearance.
Alright, as a quick recap. When stained with Wright’s stain, erythrocytes are bright pink or
red with a pale center, due to their biconcave shape. Their diameter is about 7 to 8 μm
and they don’t have a nucleus. Leukocytes are divided into granulocytes and agranulocytes. The
granulocytes include neutrophils, basophils, and eosinophils; and agranulocytes include lymphocytes
and monocytes. Neutrophils have light purple azure granules and distinct nuclei with 2 to 5 lobes.
Basophils have either S-shaped or two lobed nuclei, and their cytoplasm contains basophilic or
dark purple granules. Eosinophils have nuclei with two lobes and many large eosinophilic or dark pink
granules. Lymphocytes are round or oval cells with large round nuclei that are
densely stained purple. These cells have a relatively small amount of cytoplasm.
And finally, monocytes are the largest type of white blood cell with large nuclei that
are either horseshoe-shaped or appear indented. Their cytoplasm also has a
frosted-glass appearance due to the presence of small purple lysosomes and vacuoles.