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Cell Signaling During Primitive Hematopoiesis
Hematopoietic Progenitor
Unipotent or multipotent cells that give rise to committed blood precursors that in turn
develop into fully differentiated blood cells.
Primitive Hematopoiesis
The Frst wave of hematopoiesis that occurs during development that generates large
numbers of primitive erythrocytes and some primitive leukocytes.
Defnitive Hematopoiesis
The hematopoiesis that permanently replaces primitive hematopoiesis during fetal
stages to supply all fetal and adult blood lineages.
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Primitive hematopoiesis is the Frst wave of blood development that occurs in
development, and it produces the Frst red and white blood cells of the embryo.
Although primitive hematopoiesis produces a largely transient population of
primitive blood cells, some blood progenitors that function later in development
to produce deFnitive blood cells also develop in the primitive hematopoietic site.
Studies of mouse, chick, zebraFsh, and
Xenopus
embryos have revealed that many
of the molecular events that regulate primitive hematopoiesis are conserved among
vertebrates. In addition, roles for nonhematopoietic tissues in regulating primitive
hematopoiesis have been discovered. In this article, we review what is known about
vertebrate primitive hematopoiesis at the tissue, cellular, and molecular levels.
1
Primitive Hematopoiesis
Hematopoiesis
is the generation of blood
cells (erythrocytes and leukocytes) from
a common precursor, the hematopoietic
stem cell (HSC). During development,
two types of hematopoiesis, primitive
and deFnitive, supply the embryo with
blood cells (±ig. 1). The Frst wave of
hematopoiesis is termed primitive because
it produces blood cells that are prim-
itive in character in that they do not
fully resemble their adult counterparts ei-
ther functionally or molecularly. Primitive
blood cells exist transiently, primarily serv-
ing the embryo’s early requirement for
oxygen, until the second and Fnal wave of
hematopoiesis, deFnitive hematopoiesis,
occurs. DeFnitive hematopoiesis contin-
uously produces all the fetal and adult
blood lineages, including the myeloid
(red cells, macrophages neutrophils, mast
cells, platelets) and lymphoid (B-cells and
T-cells) lineages throughout the life of
the animal. Although there are anatom-
ical differences in where both waves of
hematopoiesis occur among vertebrate
species, there is a striking conservation
of the genes required for these processes.
The most numerous cells generated dur-
ing primitive hematopoiesis are primitive
erythrocytes, large nucleated cells that ex-
press embryonic globins and accumulate
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