636
UNIT 4
Maintenance of the Body
17
an orthochromatic erythroblast has accumulated almost all of
its hemoglobin, it ejects most of its organelles. Additionally, its
nucleus degenerates and is pinched off, allowing the cell to col-
lapse inward and eventually assume the biconcave shape. Te
result is the
reticulocyte
(essentially a young erythrocyte), so
named because it still contains a scant
reticulum
(network) of
clumped ribosomes.
Te entire process from hematopoietic stem cell to reticu-
locyte takes about 15 days. Te reticulocytes, filled almost to
bursting with hemoglobin, enter the bloodstream to begin their
task of oxygen transport. Usually they become fully mature
erythrocytes within two days of release as their ribosomes are
degraded by intracellular enzymes.
Reticulocytes account for 1–2% of all erythrocytes in the
blood of healthy people.
Reticulocyte counts
provide a rough
index of the
rate
of RBC formation—reticulocyte counts be-
low or above this range indicate abnormal rates of erythrocyte
formation.
Regulation and Requirements for Erythropoiesis
Te number of circulating erythrocytes in a given individual is
remarkably constant and reflects a balance between red blood
cell production and destruction. Tis balance is important be-
cause having too few erythrocytes leads to tissue hypoxia (oxy-
gen deprivation), whereas having too many makes the blood
undesirably viscous.
±o ensure that the number of erythrocytes in blood remains
within the homeostatic range, new cells are produced at the in-
credibly rapid rate of more than 2 million per second in healthy
people. Tis process is controlled hormonally and depends on
adequate supplies of iron, amino acids, and certain B vitamins.
Hormonal Controls
Erythropoietin (EPO)
, a glycoprotein hor-
mone, stimulates the formation of erythrocytes
(Figure 17.6)
.
Normally, a small amount of EPO circulates in the blood at
all times and sustains red blood cell production at a basal rate.
Te kidneys play the major role in EPO production, although
the liver also produces some. When certain kidney cells become
Production of Erythrocytes
Blood cell formation is referred to as
hematopoiesis
(hem
0
ah-
to-poi-e
9
sis;
hemato
5
blood;
poiesis
5
to make). Hematopoi-
esis occurs in the
red
bone marrow
, which is composed largely
of a soF network of reticular connective tissue bordering on
wide blood capillaries called
blood sinusoids
. Within this net-
work are immature blood cells, macrophages, fat cells, and
retic-
ular cells
(which secrete the connective tissue fibers). In adults,
red marrow is found chiefly in the bones of the axial skeleton
and girdles, and in the proximal epiphyses of the humerus and
femur.
Te production of each type of blood cell varies in response
to changing body needs and regulatory factors. As blood cells
mature, they migrate through the thin walls of the sinusoids to
enter the bloodstream. On average, the marrow turns out an
ounce of new blood containing 100 billion new cells every day.
Te various formed elements have different functions, but
there are similarities in their life histories. All arise from the
he-
matopoietic stem cell
, sometimes called a
hemocytoblast
(
cyte
5
cell,
blast
5
bud). Tese undifferentiated precursor cells re-
side in the red bone marrow. However, the maturation pathways
of the various formed elements differ, and once a cell is
commit-
ted
to a specific blood cell pathway, it cannot change. Tis com-
mitment is signaled by the appearance of membrane surface
receptors that respond to specific hormones or growth factors,
which in turn “push” the cell toward further specialization.
Stages of Erythropoiesis
Erythrocyte production, or
eryth-
ropoiesis
(ĕ-rith
0
ro-poi-e
9
sis), begins when a hematopoietic
stem cell descendant called a
myeloid stem cell
transforms into
a
proerythroblast
(Figure 17.5)
. Proerythroblasts, in turn,
give rise to
basophilic erythroblasts
that produce huge num-
bers of ribosomes. During these first two phases, the cells divide
many times. Hemoglobin is synthesized and iron accumulates
as the basophilic erythroblast transforms into a
polychromatic
erythroblast
and then an
orthochromatic erythroblast
. Te
“color” of the cell cytoplasm changes as the blue-staining ribo-
somes become masked by the pink color of hemoglobin. When
Stem cell
Hematopoietic stem
cell (hemocytoblast)
Proerythroblast
Basophilic
erythroblast
Polychromatic
erythroblast
Orthochromatic
erythroblast
Phase 1
Ribosome synthesis
Phase 2
Hemoglobin accumulation
Phase 3
Ejection of nucleus
Reticulocyte
Erythrocyte
Committed cell
Developmental pathway
Figure 17.5
Erythropoiesis: formation of red blood cells.
Reticulocytes are released into
the bloodstream. The myeloid stem cell, the phase intermediate between the hematopoietic
stem cell and the proerythroblast, is not illustrated.
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