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Early Embryonic Mesoderm Development
Virginia E. Papaioannou
Introduction
Mesoderm—from the Greek µεσο-ζ
′
(middle) + δερµα
′
(skin).
This simple descriptive name belies a multifaceted role for the
middle of the three embryonic germ layers. It is the youngest
layer, in evolutionary terms, and is a hallmark of the development of all complex metazoans. The mesoderm layer provided
the solution to more sophisticated functions than the simple
protective outer ectoderm and the absorptive inner endoderm.
As organisms became larger and more complex, the mesoderm assumed functions of support, movement, circulation,
and reproduction, working closely with internalized, ectoderm-derived neural and neural crest tissue as well as providing a supporting role and providing for intricate elaborations
of the protective and absorptive functions of the ectoderm and
endoderm. With increasingly complex modes of reproduction,
all three germ layers were called into play to form novel tissues for the adaptation to different modes of oviparity, ovoviviparity, and eventually viviparity. The repertoire of tissues
formed by the mesoderm is complex and varied, with many
precursor cell and stem cell populations developing during
embryonic life and some persisting into adulthood. The mesoderm plays a role throughout the development of the mammalian embryo, beginning from the initiation of gastrulation,
the process whereby the three embryonic germ layers are
formed. Here is a brief developmental history of that remarkable tissue in mammals, using the mouse as the prime example.
The mammalian embryo begins implantation into the uterus
at the late blastocyst stage, when it consists of an outer layer
of trophectoderm and a bilaminar inner cell mass (ICM) comprising the epiblast and the primitive endoderm (also called
the hypoblast) (Fig. 24–1A). The trophectoderm is a specialized ectoderm layer that mediates contact between the embryo
and the uterus and forms a major part of the placenta; it does
not, however, make any cellular contribution to the body of
the fetus. Similarly, the primitive endoderm layer of the ICM
forms the endoderm layer of the visceral and parietal yolk
sacs but does not contribute to the gut endoderm of the fetus.
It is the epiblast that is the origin of the entire body of the
fetus, including ectoderm-, endoderm-, and mesodermderived structures. These three primary layers of cells are
called the definitive germ layers and arise through the process
Handbook of Stem Cells
Volume 1
Copyright © 2004 by Elsevier Inc.
All rights of reproduction in any form reserved.
of gastrulation, a morphogenetic process that occurs within the
epiblast in a specialized structure called the primitive streak.
Primitive Streak as the Origin
of Mesoderm
The first mesoderm to appear in the mammalian embryo is
destined to be extraembryonic in nature. One can think of this
precocious appearance of mesoderm in the extraembryonic
region as an adaptation to viviparity, where a functioning
mesoderm-derived circulatory system is an early requirement
of successful intrauterine development. In the mouse, this
extraembryonic mesoderm arises from the primitive streak,
although in humans it appears prior to the formation of a
primitive streak. In human embryos, it is said to arise from
yolk sac cells,1 which consist at this time of cytotrophoblast
and hypoblast; however, the actual origin of this tissue in
humans is unknown because of the impossibility of carrying
out experimental analysis. Whatever its origin, it comes to
have a similar relationship to the other germ layers in the fetal
membranes as it does in other mammals, including the mouse.
Extraembryonic mesoderm forms and lines the extraembryonic
coelom or exocoelomic cavity, providing the mesoderm component of the amnion, chorion, yolk sac, and allantois.
The primitive streak of the mouse first appears about
5.5 days postcoitus (dpc) in the epiblast at the junction
between the embryonic and extraembryonic areas of the
mouse egg cylinder (Fig. 24–1B), marking the posterior pole
of the future embryonic body. The streak is an area in which
cells of the epiblast undergo an epithelial to mesenchymal
transition and commence ingressing, eventually taking up a
position and spreading between the visceral endoderm and the
epiblast in the embryonic region and between the visceral
endoderm and the extraembryonic ectoderm in the extraembryonic region (Fig. 24–1C). The primitive streak gradually
elongates with the growth of the embryo until at its maximum
length, it extends from the embryonic–extraembryonic boundary to the most distal tip of the egg cylinder. The anterior end
of the primitive streak is a specialized area known as the node
(or Hensen’s node), which appears as a bilaminar depression
at the distal tip of the egg cylinder (Fig. 24–1D). The node has
organizing capabilities in that it can induce a secondary neural
axis in heterotopic grafting experiments.2 The streak regresses
posteriorly toward the latter part of gastrulation, becoming
relatively shorter until at 9.5 dpc it is replaced by the tail bud
as the source of new mesoderm in the most posterior part of
the embryo.
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Epiblast
A.
Primitive endoderm (hypoblast)
Trophectoderm
B.
Extraembryonic ectoderm
Visceral
endoderm
Primitive streak
Epiblast
C.
Extraembryonic coelom
Primitive streak
Pr o a m n i o t i c
c avi t y
D.
C h or i o n
Yo l k s a c
E.
A l l a nto i s
Amnion
Head fold
Pe r ic ar dial
c av it y
P r i m i t i ve s t r e a k
Node
Hear t
Node
Cephalic
m es oder m
Somites
Figure 24–1. Diagrammatic representation of midsagittal sections of mouse embryos from 4.5 to 8.5 dpc. (A) A 4.5-dpc blastocyst, just prior
to implantation. (B) Early gastrula at 5.5 dpc, with the primitive streak at the posterior pole. (C) Midstreak stage embryo at 6.5–7.0 dpc, with the
amniotic fold pushing across the proamniotic cavity. Intracellular spaces within the extraembryonic mesoderm coalesce to form the extraembryonic
coelom. (D) A 7.5-dpc embryo with fetal membranes and allantois showing the left lateral wing of the embryonic mesoderm, which spreads
between the epiblast and the visceral endoderm. (E) Headfold stage embryo at 8.5 dpc, showing embryonic and extraembryonic mesoderm.
In parts B–E, stippled area represents the mesoderm, and triangles indicate the boundary between the embryonic and the extraembryonic regions.
In parts C–E, the placenta and parietal yolk sac, which would surround the entire embryo, are not shown.
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24. Early Embryonic Mesoderm Development
Epiblast cells mostly converge on the streak, undergo the
epithelial to mesenchymal conversion, ingress, and then move
away from the streak between the epiblast and the endoderm.
However, during the life of the streak, there is evidence that
some cells have properties of stem cells for a limited time. In
the anterior streak, fate mapping studies have indicated that
some cells not only have progeny that differentiate as mesoderm
and move away from the streak but also have progeny that
remain in the streak, continuing to self-renew and retaining
the potential to form new mesoderm.3
Fate Map of the Primitive Streak
Different types of mesoderm begin to appear upon departure
from the primitive streak. The timing and order of their
appearance is a closely coordinated choreography that has
been revealed mostly by fate mapping studies relying on
cell grafting or marking and short-term embryo culture
(reviewed by Lawson4 and Tam and Quinlan5). The differentiation of the different types of mesoderm into morphologically
and functionally distinct cell types is presaged by differential
gene expression along the length of the streak.6 Although
there is considerable overlap at the edges of the fate map
boundaries, different parts of the primitive streak produce
different types of mesoderm (Fig. 24–2), and the types change
over time.7
The first mesoderm to emerge is the extraembryonic mesoderm from the posterior part of the early streak followed by
the cardiac and cranial mesoderm from a more anterior part of
the steak. The extraembryonic mesoderm pushes across the
proamniotic cavity, carrying with it the overlying layer of
extraembryonic ectoderm (Fig. 24–1C). As it does so, intercellular cavities form within the layer and coalesce into the
extraembryonic coelom, effectively separating the mesoderm
into two layers. This extraembryonic mesoderm will form the
yolk sac mesoderm and the mesoderm of the amnion and
chorion. Later, the allantois will bud out of the most posterior
part of the streak and move across the extraembryonic coelom
Extraembryonic mesoderm
Lateral plate
mesoderm
Ectoderm
Primitive streak
Somites
Notochord/head process
Node
Figure 24–2. Representative fate map of the mid- to late streak stage
epiblast of the mouse embryo. The differently shaded areas represent the
progenitors of the tissues indicated in the epiblast prior to ingression through
the node or streak. The boundaries are not as sharp as indicated; rather, they
have considerable margins of overlap.
to establish a connection with the chorion (Fig. 24–1D),
forming the umbilicus.
The cranial and the cardiac mesoderm push anteriorly
around the egg cylinder in the embryonic region, eventually
meeting in the anterior midline. The rest of the embryonic
mesoderm arises from the middle to the anterior part of the
streak, producing first the lateral plate mesoderm then the
intermediate and paraxial mesoderm from the midstreak
region. The axial notochord arises from the node as does the
head process, an anterior extension of the streak. The node
contributes cells to the cranial mesoderm and the most anterior somites. As the streak regresses, production of extraembryonic mesoderm ceases, the notochord continues to be
formed from the regressing node, and paraxial presomitic
mesoderm continues to be produced. By the time the tail bud
takes over the production of mesoderm, only lateral plate,
somitic mesoderm, and notochord are being produced.8
Extraembryonic Mesoderm
The extraembryonic mesoderm remains closely associated with
both the ectoderm and endoderm layers in the extraembryonic
region, but in so doing it splits into two layers, the somatic and
splanchnic layers, by the formation of a cavity between them
known as the extraembryonic coelom or exocoelom. Combined
with ectoderm, somatic mesoderm makes up the amnion, which
remains as a nonvascular, protective membrane surrounding the
fetus. Extraembryonic mesoderm makes up the mesothelial
lining of the chorion, which, combined with trophectoderm
derivatives, will later take part in the formation of the chorioallantoic placenta. Combined with the extraembryonic endoderm,
splanchnic mesoderm makes up the yolk sac, which will form
vascular endothelial cells throughout that will eventually
coalesce into the vitelline circulation (Fig. 24–1D). In egglaying vertebrates, extraembryonic mesoderm is also associated
with endoderm in the allantoic sac, whereas in mammals such
as mouse and human, the allantois is almost purely mesodermal
with only a rudimentary endodermal component, as the
allantois has lost its role as a waste retention sac. In placental
mammals, the primary function of the allantois is to provide
the vascular, umbilical link between mother and fetus, a role
fulfilled by the mesoderm on its own.
Within the yolk sac mesoderm, vasculogenesis takes place in
intimate association with hematopoiesis, or the formation of
blood cells, in specialized areas called blood islands. It is here
that the elusive hemangioblast is thought to reside. The hemangioblast is a common progenitor of both endothelial and
hematopoietic lineages and may also be the precursor of vascular smooth muscle cells, qualifying it as a stem cell for the vascular and hematopoietic lineages.9–11 From the yolk sac blood
islands, as well as from other extraembryonic areas such as the
vitelline and umbilical arteries, the hematopoietic stem cells
(HSC) arise and function for a limited period during mid- to late
gestation, producing the primitive blood cell lineages and seeding the embryonic hematopoietic system.12 The embryonic
hematopoietic system has its origin in multiple independent
sites, including some intraembryonic sites (see the next section).
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Embryonic Mesoderm
In the embryonic region of the gastrulating mouse embryo,
the mesoderm moves away from the primitive streak and
spreads in an anterior direction between the endoderm and
epiblast layers as two lateral spreading wings of mesenchymal
cells. These wings eventually meet in the anterior part of the
embryo, thus forming a continuous sheet of mesoderm
between the endoderm and the epiblast, which has begun to
differentiate as the ectoderm layer of the embryo. At the same
time, the node generates the head process, which moves anteriorly in the midline, and the notochord, which forms the axial
supporting rod of the vertebrate embryo from the level of the
developing forebrain to the tip of the tail.
Along the length of the embryonic axis, the mesoderm that
ingresses first through the primitive streak moves the farthest
from the midline and differentiates as the lateral plate. In a
manner similar to the formation of the extraembryonic
coelom, this mesoderm splits into two layers by the coalescence of intercellular cavities to form the intraembryonic
coelom. The layers are continuous with the extraembryonic
somatic and splanchnic layers of mesoderm, and the cavity is
continuous with the extraembryonic coelom.
The mesoderm that ingresses next and remains closer to
the midline is called the intermediate mesoderm; that ingressing later and remaining closest to the midline is known as the
paraxial or somitic mesoderm. The three subdivisions—
lateral plate, intermediate, and paraxial—along with the axial
notochord continue to be formed progressively in an anteriorto-posterior direction as the primitive streak regresses posteriorly. Lateral plate, paraxial, and axial mesoderm are produced
by the tail bud toward the end of gastrulation to make up the
mesodermal structures of the tail.
Gastrulation, during which the endoderm as well as the
mesoderm is produced by ingression through the primitive
streak, is a dynamic process, proceeding in an anterior-toposterior progression. While the more posterior mesoderm is
still young, the more anterior mesoderm has already begun
differentiating, taking on specific mesodermal identities commensurate with its axial level and position in the embryo. The
morphological subdivisions of mesoderm—cranial, cardiac,
paraxial, axial, lateral plate, and intermediate mesoderm—all
have distinct fates, and most interact with endoderm- or
ectoderm-derived tissue during subsequent stages of organogenesis. In addition, mesoderm throughout the embryo forms
endothelial cells as the basis for the circulatory system and the
definitive HSC arise from embryonic mesoderm from the
aorta–gonad–mesonephros region,13 providing blood cells in
addition to those derived from the extraembryonic region to
seed the fetal liver and eventually the adult sites of
hematopoiesis: the thymus, spleen, omentum, and bone
marrow.
CRANIAL AND CARDIAC MESODERM
In the head region, unlike other regions of the embryo where
bone is derived exclusively from mesoderm, the bones of the
cranium, face, and neck are formed from ectoderm-derived
neural crest cells as well as mesoderm. The muscles of the face
and neck, with the exception of the iris muscles, are derived
from the anterior paraxial mesoderm. The cardiac mesoderm
progenitors are initially in the most anterior region of the gastrulating embryo within the splanchnic mesoderm layer, but they
come to lie in a ventral position caudal to the cranial region with
the formation of the anterior head fold and the lateral body folds,
which shape the embryonic body (Fig. 24–1E). The pericardial
cavity, within which the heart develops, forms by the coalescence of extracellular spaces between mesoderm cells as the
endothelial heart tubes come together to form the primitive heart
tube. Although the heart is an exclusively mesoderm-derived
tissue, it is thought that its differentiation is induced by the associated anterior visceral endoderm (see Schultheiss and Lassar14
for review). As the vascular network forms throughout the
mesoderm of the embryo, it connects with the pulsating heart
and with the extraembryonic vascular network, eventually forming a complete circulatory system with intraembryonic and
extraembryonic components.
L ATERAL PL ATE
With the formation of the anterior, posterior, and lateral body
folds, the somatic and splanchnic layers of mesoderm with
their associated ectoderm and endoderm, respectively, gradually gather on the ventral side of the embryo to meet at the
umbilicus. This closes the connection between the extraembryonic coelom and the intraembryonic coelom, which later
forms the abdominal and pleural cavities within the embryo.
Thus the somatic and splanchnic mesoderm layers of the lateral
plate completely line the body cavities and form the serous
membranes.
In association with the endoderm, the splanchnic mesodermal layer of the lateral plate also takes part in the formation
of the gut-associated derivatives of the respiratory and digestive systems: It forms the vascular components, the supporting mesenteries, and the muscular components of the gut
along the length of the gut tube from the esophagus to the
colon. From its position surrounding and supporting the gut,
it also participates in the organogenesis of gut outpocketings
forming the connective and stromal tissue of organs such as
the trachea and lungs, the liver, and the pancreas.
The somatic mesoderm of the lateral plate, in association
with the overlying ectoderm, forms the ventrolateral body
wall, including the ventrolateral dermis. This somatic mesoderm is also the source of body wall muscles and the cells that
ossify into the sternum. In addition, the lateral plate mesoderm forms the two pairs of limb buds that push out from the
ventrolateral body wall. Complex interactions take place
between the lateral plate mesoderm and the overlying ectoderm to form and pattern the limbs.15,16 The bones of the limb
develop from the lateral plate mesoderm, whereas cells
migrate into the developing limbs from the somites to form
the limb musculature.
INTERMEDIATE MESODERM
The intermediate mesoderm comes to lie in parallel ridges in
the roof of the intraembryonic coelom on either side of the
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24. Early Embryonic Mesoderm Development
midline in the thoracic and abdominal regions. These ridges,
known as the urogenital ridges, later form both the excretory
and the reproductive organ systems.17,18 The development of
these two systems is closely interconnected. Early in development, the intermediate mesoderm in the extreme cranial end
of the embryo forms a vestigial and transitory excretory
system, the pronephros. Later, in the thoracic region, the
mesonephros and mesonephric ducts form in the urogenital
ridge. The mesonephros is transitory, but the mesonephric
duct persists in male embryos and becomes part of the genital
system. The definitive kidney, or metanephros, is induced in
posterior intermediate mesoderm by an outgrowth of the
mesonephric duct called the ureteric bud.
The gonads arise from the medial portion of the urogenital
ridges from swellings called the genital or gonadal ridges,
although the primordial germ cells (PGCs), the stem cells for
oogonia and spermatogonia, arise from a distant site. The
PGCs first appear in the posterior part of the primitive streak,
migrate into the base of the allantois, then reenter the embryo
to migrate along the dorsal mesentery and eventually into the
gonads.19
PARAXIAL MESODERM
Immediately adjacent to the axis, flanking the neural tube and
notochord is the paraxial mesoderm, which will form the
transitory, segmental blocks of tissue called the somites
(Fig. 24–1E). Throughout the process of gastrulation, the
paraxial mesoderm is continuously ingressing, segmenting to
form somites, and differentiating in an anterior-to-posterior
progression so that by the time the tail somites are newly
formed, the most anterior somites have already undergone differentiation into other structures. The regular segmentation of
presomitic paraxial mesoderm involves a molecular oscillator,
called the segmentation clock, which utilizes the conserved
Notch signaling pathway.20 After the initial epithelialization
into segmental blocks, the somites rapidly undergo regional
diversification into the dorsolateral dermomyotome and the
ventromedial sclerotome. As the name implies, the dermomyotome further differentiates into the dermal component of the
skin of the dorsolateral body, and into skeletal muscle, including the muscle of the limbs.21,22 The sclerotome undergoes a
further transition to mesenchymal cells and resegments
around the neural tube to form the vertebrae of the axial skeleton. The resegmentation of sclerotome to form vertebrae is out
of register with the original somite segmentation such that
cells from the anterior part of one somite combine with cells
from the posterior part of the preceding somites to form a vertebral segment.
In their differentiation, the somites are subject to the influences of multiple signals from various signaling pathways emanating from adjacent tissues. For example, Sonic
hedgehog signals from the notochord and floor plate of
the neural tube induce sclerotome differentiation, signals
from the dorsal neural tube act to delimit the dermomyotome (Wnt signals) and dermis (neurotrophin 3), and Wnt
and inhibitory bone morphogenetic protein signals from the
body wall and mesoderm, respectively, act together on the
dorsolateral somite to induce limb and body wall musculature.22–24
NOTOCHORD
The axis of the embryo is defined by the position of the
primitive streak at the posterior pole, with the epiblast
dorsally located and primitive endoderm ventrally located.
Cells that ingress through the node at the anterior end of the
primitive streak and move cranially differentiate as a solid rod
of cells called the notochord. These cells progress as far as the
precordal plate, an area near where the buccopharyngeal
membrane will later open as the mouth. As the primitive
streak regresses in later gastrulation, notochord cells continue
to ingress, forming a solid rod of mesoderm along the axis of
the entire trunk and tail. It is around this structure that
sclerotome-derived cells condense to form the vertebrae. The
notochord makes only a small cellular contribution to the
adult vertebral column, persisting only in the intervertebral
disc. However, it is an important signaling center and structural component during embryogenesis of all chordate
embryos.
ACKNOWLEDGMENTS
I wish to acknowledge support from the NIH (GMO 60561 and
HD33082).
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