138
UNIT 2
Basic Tissues
Erythrocytes and Platelets
CLINICAL CORRELATION
Figure 8-2C.
Sickle Cell Anemia, Blood Smear.
Wright
stain,
3
1,035
Sickle cell anemia
is an autosomal recessive disorder character-
ized by the production of defective hemoglobins, which aggregate
and polymerize when deoxygenated. The red blood cells become
longer and curved, similar to a “sickle.” Sickle cells block blood
vessels, causing
ischemia
of the tissues and severe pain. Symptoms
and signs, which start in early childhood, include anemia, vasooc-
clusive complications, and chronic
hyperbilirubinemia
. This dis-
ease is more common in people of African, Turkish, Arabian, and
Mediterranean ancestry. Dehydration, infection,
hypertonicity,
and decreased pH can trigger an onset. In a sickle cell patient, the
average life span of red blood cells is 17 days, as compared to 120
days in normal persons. Bone marrow transplants can cure a small
number of people.
Normal
and
sickle cell
forms are shown here.
Sickle-shaped
red blood cells
Inmature red
blood cell
Normal red
blood cell
C
Figure 8-2A.
Erythrocytes (red blood cells), blood smear.
Wright stain,
3
1,576
Erythrocytes
are the most abundant cells in blood. They have a
unique
biconcave
disk appearance (Fig. 8-3), are
anucleate
(without
nuclei), and have no organelles after they mature. Here, red blood
cells are seen as
pink circles with pale centers
in a Wright stain.
Erythrocytes are produced in the red bone marrow and are trans-
ported into the blood circulation through the walls of sinusoidal
capillaries in the marrow. Their life span is about 120 days. Aged
erythrocytes are destroyed by macrophages in the spleen, liver, and
bone marrow. Erythrocytes contain highly concentrated hemoglo-
bin (Hb). They appear bright red in color when oxygen content is
high and are more purple when they are depleted of oxygen. Their
function is to transport oxygen to peripheral tissues and carry car-
bon dioxide out of tissues. Hemoglobin binds with oxygen to form
oxyhemoglobin when the O
2
level is high (lung) and binds with CO
2
to form carbaminohemoglobin when the CO
2
level is high (tissue).
D. Cui
Erythrocytes
Erythrocyte
A
D. Cui
Granulomere
Hyalomere
Platelets
Platelets
B
Figure 8-2B.
Platelets (thrombocytes), blood smear.
Wright
stain,
3
1,576; inset
3
1,570
Platelets
, also called
thrombocytes
, are very small, lens-shaped frag-
ments of cells. They have some functional characteristics of whole
cells, even though they do not have nuclei. Each platelet has a surface
membrane covering cytoplasm that contains microtubules, micro-
± laments, mitochondria, and several types of granules. The central
region where granules stain purple is termed the
granulomere
; the
peripheral region, which stains light blue, is called the
hyalomere
.
The two main types of granules in platelets are
alpha granules
and
delta
granules
(
dense bodies
). These play a role in the adhesion and
aggregation of platelets in blood coagulation. If damage occurs to
the vascular endothelium, platelets adhere to the vessel wall, releas-
ing granules and aggregating to stop bleeding.
The absence of alpha granules can cause
gray platelet
syndrome
,
whereas reduced numbers or absence of delta granules will lead
to
storage pool def
ciency
.
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