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U N I T 3
Hematopoietic Function
to two types of multipotent stem cells: the hematopoietic
stem cells, which remain in the bone marrow, and the
lymphopoietic stem cells, which migrate to the thymus,
lymph nodes, spleen, and mucosa-associated lymphoid
tissues.
The Bone Marrow and Hematopoiesis
The bone marrow consists of hematopoietic or blood-
forming cells and stromal tissue that provides support
for the blood-forming cells. The blood-forming popula-
tion of bone marrow is made up of three types of cells:
pluripotent stem cells, multipotent stem cells, and com-
mitted progenitor cells that develop into the various
types of blood cells
1–4
(Fig. 11-1). The pluripotent stem
cells give rise to two types of multipotential stem cells,
the common lymphoid and the common myeloid stem
cells. The common lymphoid stem cells, in turn, differ-
entiate into lineage-specific precursor cells that develop
into T lymphocytes (T cells), B lymphocytes (B cells),
and natural killer (NK) cells. From the common myeloid
stem cells arise precursor cells capable of differentiating
along the erythrocyte/megakaryocytic and granulocyte-
monocyte pathways.
Several levels of differentiation lead to the develop-
ment of committed unipotential cells, which are the
progenitors for each of the blood cell types. These cells
are referred to as
colony-forming units
(CFUs).
3
These
progenitor cells only have a limited capacity for self-
renewal but retain the potential to differentiate into
lineage-specific precursor cells. These precursor cells
have morphologic characteristics that permit them to
be recognized as the first cell of a particular cell line.
They have lost their ability for self-renewal but undergo
cell division and differentiation, eventually giving rise
to mature lymphocytes, monocytes, granulocytes, mega-
karyocytes, or erythrocytes.
Hematopoietic Growth Factors
Under normal conditions, the numbers and total mass
for each type of circulating blood cell remain relatively
constant. This regulation of blood cells is thought to
be at least partially controlled by hormone-like growth
factors called
cytokines.
The cytokines are a family of
short-lived mediators that stimulate the proliferation,
differentiation, and functional activation of the various
blood cells.
Monocyte
B cell
progenitor
B cell
T cell
progenitor
Thymus
NK
progenitor
Myeloid stem cell
Pluripotent stem cell
Lymphoid stem cell
Monoblast
Granulocyte
Megakaryocyte
Erythrocyte
Reticulocyte
Megakaryocyte
Platelets
Basophil
Neutrophil
Eosinophill
Monocyte
Plasma
cell
T cell
NK cell
Erythrocyte
(Committed Stem Cell)
(Mature Cell)
FIGURE 11-1.
Major developmental stages of blood cells. NK, natural killer cell.
