are classiﬁ ed according to their structure and afﬁ nity for the
the three classes of leukocytes that have multilobed nuclei and
abundant membrane-surrounded granules. The granules of
one group take up the red dye eosin, thus giving the cells their
Cells of a second class have an afﬁ nity for
a blue dye termed a “basic” dye and are called
granules of the third class have little afﬁ nity for either dye and
are therefore called
Neutrophils are by far the
most abundant kind of leukocytes.
A fourth class of leukocyte is the
somewhat larger than the granulocyte and has a single oval or
horseshoe-shaped nucleus and relatively few cytoplasmic gran-
ules. The ﬁ nal class of leukocytes is the
contains little cytoplasm and, like the monocyte, a single rela-
tively large nucleus.
Like the erythrocytes, all classes of leukocytes are pro-
duced in the bone marrow. In addition, monocytes and many
lymphocytes undergo further development and cell division in
tissues outside the bone marrow. These events and the spe-
ciﬁ c leukocyte functions in the body’s defenses are discussed
in Chapter 18.
The circulating platelets are colorless, non-nucleated cell frag-
ments that contain numerous granules and are much smaller
than erythrocytes. Platelets are produced when cytoplasmic
portions of large bone marrow cells, termed
pinch off and enter the circulation. Platelet functions in blood
clotting are described later in this section.
Regulation of Blood Cell Production
In children, the marrow of most bones produces blood cells.
By adulthood, however, only the bones of the chest, the base
of the skull, and the upper portions of the limbs remain active.
The bone marrow in an adult weighs almost as much as the
liver, and it produces cells at an enormous rate.
All blood cells are descended from a single population
of bone marrow cells called
pluripotent hematopoietic stem
which are undifferentiated cells capable of giving rise
to precursors (progenitors) of any of the different blood cells.
When a pluripotent stem cell divides, its two daughter cells
either remain pluripotent stem cells or become committed to
a particular developmental pathway; what governs this “deci-
sion” is not known. The ﬁ rst branching yields either lymphoid
stem cells, which give rise to the lymphocytes, or myeloid stem
cells, the progenitors of all the other varieties (
At some point, the proliferating offspring of the myeloid stem
cells become committed to differentiate along only one path—
for example, into erythrocytes.
Proliferation and differentiation of the various progeni-
tor cells is stimulated, at multiple points, by a large number
of protein hormones and paracrine agents collectively termed
hematopoietic growth factors (HGFs).
poietin, the hormone described earlier, is an HGF. Others
are listed for reference in
. (Nomenclature can
be confusing in this area because the HGFs belong to a still
larger general family of messengers called cytokines, which are
described in Chapter 18.)
Numbers and Distributions of
Erythrocytes, Leukocytes, and
Platelets in Normal Human Blood
= 5,000,000 per mm
= 7000 per mm
Percent of total leukocytes:
= 250,000 per mm
Classes of blood cells.