Cell Signaling During Primitive Hematopoiesis
435
thymocytes is also detected postmeta-
morphosis, when the bulk of deFnitive
hematopoietic cells are derived from the
dorsolateral plate. Thus, cells derived from
the site of primitive hematopoiesis appear
to make at least a temporary contribution
of deFnitive progenitors to the developing
embryo.
In summary, primitive hematopoiesis
occurs in a unique site and produces
the Frst circulating red cells and leuko-
cytes of the developing embryo. Although
differentiated primitive blood cells play
only a transient role in development,
numerous lines of evidence point to
a lasting contribution of deFnitive pro-
genitors and, potentially, some deFni-
tive HSCs that are generated in the
primitive hematopoietic site. In the fol-
lowing sections, we will review what
is known about the molecular regula-
tion of primitive hematopoiesis includ-
ing the speciFcation and maturation of
primitive hematopoietic cells as well as
tissue interactions that influence these
events.
2
Specifcation, Expansion and DiFFerentiation
oF the Primitive Erythroid Lineage
As discussed above, one of the main func-
tions of primitive hematopoiesis is the
generation of large numbers of primi-
tive erythrocytes. In this section, we will
briefly review what is known about the
molecular regulation of primitive erythro-
poiesis, including the signals that regu-
late ventral patterning of the mesoderm,
from which primitive blood is derived,
the speciFcation of hematopoietic fate
within this mesoderm, and the subsequent
production, expansion, survival, and dif-
ferentiation of progenitors into primitive
erythrocytes (summarized in ±ig. 2). We
have elected to focus on primitive ery-
throcyte differentiation because relatively
Mesoderm
Ventral
mesoderm
Hematopoietic
mesoderm
Committed
erythroid
progenitor
Bmp
2, 4, 7
Bmp4
cloche
Scl
Lmo2
Gata2
bloodless
moonshine
Proerythroblast
Gata1
Retinoic acid, TGF
β
1
Epo/EpoR, Jak2,
SCF/c-Kit
Primitive
erythrocytes
Cytokines
cloche
Bmp4
Non hematopoietic
cells
±ig. 2
Primitive erythropoiesis. Genes required for different steps of the
erythroid development pathway areshowninp
la
intext,andzebraFsh
mutants that block erythroid development at various steps are shown in
italicized text. ±or details, see text in Sects. 2 and 3.
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