Cell Signaling During Primitive Hematopoiesis
431
Primitive hematopoietic mesoderm
(yolk sac, ventral blood island,
intermediate cell mass)
Definitive hematopoietic mesoderm
(aorta-gonads-mesenephros region)
Primitive erythrocytes
Primitive leukocytes
Definitive progenitors
HSCs
Definitive erythrocytes, macrophages,
mast cells, neutrophils, platelets
B-cells, T-cells
Mesoderm
Fig. 1
Embryonic hematopoiesis. This simplifed illustration depicts the diFFerentiated cell
types that are produced during the frst waves oF primitive and defnitive hematopoiesis,
respectively. There are many steps and intermediate cell types in these pathways that are
discussed within the text.
hemoglobin during maturation. In con-
trast, defnitive erythrocytes are smaller,
contain Fetal or adult globins and are enu-
cleated (e.g. in mammals) or have diFFerent
nuclear morphology (e.g. in
Xenopus
). In all
vertebrates, primitive hematopoiesis is ini-
tiated during gastrulation in the embryonic
or extraembryonic mesoderm, depending
upon the species. In mammals and birds,
primitive hematopoiesis occurs extraem-
bryonically within the yolk sac mesoderm,
whereas in
Xenopus
(AFrican clawed Frog)
and zebrafsh, primitive hematopoiesis
occurs embryonically, in a subset oF ven-
trolateral mesoderm known as the
ven-
tral
blood
islands
and
intermediate
cell
mass
, respectively. In the mouse, non-
hemoglobinized primitive erythroblasts
(immature primitive erythrocytes) are frst
morphologically evident during late gas-
trulation stages, around embryonic day (E)
7.5 in the yolk sac mesoderm, and they
enter the embryo via the circulatory sys-
tem at E8.25. AFter E9, primitive erythroid
cells are no longer produced by the yolk
sac. Within the embryonic blood vessels,
primitive erythroid progenitors complete
their diFFerentiation in circulation through
E13, undergoing mitosis, nuclear conden-
sation, accruement oF hemoglobin, and
a decrease in cell size. Prior to E12 in
the mouse, the only red cells in cir-
culation are derived From the yolk sac.
At E12, some liver-derived defnitive ery-
throblasts (that are smaller in size and
express adult globins) enter circulation,
and within a Few days, the primitive ery-
throcytes are no longer detectable. In the
chick, yolk sac mesoderm diFFerentiates
into primitive erythroblasts by 1.5 days
postFertilization and the erythroblasts en-
ter circulation by day 2, and subsequently
mature. In
Xenopus
, ventral blood island-
derived primitive erythrocytes enter cir-
culation at stage 35/36, just a Few hours
aFter the heart starts beating, and they
are the predominant circulating cells until
they are replaced by defnitive erythrocytes
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