Download Embryogenesis Handout Part 2

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Block 2 Unit 1
Evolution of the Mesoblast
Terminology:
Extraembryonic
Mesoblast
Intraembryonic
Mesoblast
is formed already during the 2nd week; is involved in the construction
of the placenta and the other appendage organs.
the third germinal layer; arises in the 3rd week via the immigration of
cells at the primitive streak; out of this develop the various tissues and
organs of the embryo.
In the beginning, the cells of the mesoblast (mesodermal cells) build a thin, widely
meshed layer on both sides of the median line, between the ectoderm and the endoderm. While
the notochord is forming—it grows to the same extent that the primitive streak recedes—the
intraembryonic mesoderm cells multiply on both sides of the median line and so form 3
structures in the shape of longitudinal columns: the paraxial mesoderm, the intermediate
mesoderm, and the lateral plate mesoderm. This process begins at the cranial pole and continues
up to the 4th week in the caudal direction [see pictures below].
Mesoderm After the Completed Mesoblast Immigration: Day 25
Evolution of Mesoblast: Day 25
1
2
3
4
5
6
1 Paraxial mesoderm
2 Intermediate mesoderm
3 Lateral plate mesoderm
4 Chordal process
5 Intraembryonic coelom
6 Amnion
7 Endoblast
8 Ectoblast
9 Somatopleural
(mesoderm, ectoderm)
10 Splanchnopleural
(mesoderm, endoderm)
11 Neural groove
12 Neural plat
Paraxial mesoderm
Intermediate mesoderm
Lateral plate mesoderm
Neural groove
Ectoblast
Endoblast
Simple Rendering
Detailed Rendering
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Block 2 Unit 1
Paraxial Mesoblast
Terminology:
Somitomeres
Somites
Pharyngeal Arches
Sclerotome
Myotome
Dermatome
indistinct aggregates of paraxial mesoderm along embryo’s rostralcaudal axis; adjacent to neural tube.
segmented; 1-7 form the striated muscles of the face, jaws, and throat;
more caudal somites give rise to sclerotome, myotome, & dermatome.
derived from somites 1-7; contribute to development of head and neck.
somite precursor of vertebral column.
somite precursor of striated muscle of neck, trunk, and limbs.
somite precursor of subcutaneous tissue and skin.
The paraxial mesoblast comes from the epiblast
cells that migrated into the region of the primitive node
(or the cranial portion of the primitive streak). It forms a
pair of cylindrical, epithelially-organized mesenchymal
segments that are in the immediate vicinity of the neural
tube and the notochord. After the start of the 3rd week,
these cylinders become segmented from the cranial to the
caudal end into somitomeres [see pictures to right].
Except for the somitomeres (1 to 7) that form no
somites—but are involved in the formation of the
pharyngeal arch mesoblast—the others form somites in
the cranio-caudal direction. The somite pairs are formed
along the neural tube and range from the cranial region up
to the embryo’s tail. In the end, the human embryo will
have approximately 35-37 somite pairs. The number of
somites is used to roughly determine the embryo's age
in this developmental stage.
The somites are formed through the
segmentation of the paraxial mesoderm. They organize
themselves without cell differentiation. They are
responsible for the segmental organization of the
embryo and contribute to its restructuring. They will
develop into three distinct regions: the sclerotome, the
myotome, and the dermatome. The segmental partitioning
of the spine, the neural tube, the trunk wall and the thorax
(ribs) depends on the ordered arrangement of the somites.
Intermediate Mesoblast
The intermediate mesoblast is found between the
paraxial mesoblast and the lateral plate mesoblast. This
longitudinal, dorsally lying crest is called the urogenital
crest and serves as the origin of the kidney and gonads.
Intraembryonic Coelom: Day 23
1
2
3
4
5
6
Paraxial mesoderm
Intermediate mesoderm
Lateral plate mesoderm
Chordal process
Sectional edge of amnion
Intraembryonic coelom
Somitomere Appearance: Day 25
7 Endoblast
8 Ectoblast
9 Somatopleure with ectoblast
10 Splanchopleure with endoblast
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Block 2 Unit 1
Lateral Plate Mesoblast
Terminology:
Intraembryonic
Coelom
Somatopleure
Splanchopleure
represents the future serous cavities of the trunk (i.e., the pleural,
peritoneal, and pericardiac cavities).
consists of somatic mesoderm and ectoderm; dorsally located; gives
rise to connective tissue and some vascular smooth muscle.
consists of visceral/splanchnic mesoderm and endoderm; ventrally
located; gives rise to smooth muscle of the GI tract and cardiac muscle.
The lateral plate
mesoderm is composed of
two thick layers that
surround a cavity, called
the
intraembryonic
coelom
(the
coelom
represents the future serous
cavity of the trunk:
peritoneal, pleural and
pericardial cavities). The
somatopleure, which is
close to the ectoderm, is
involved in the formation
of the lateral and ventral
walls of the embryo. The
splanchnopleure, which
lies on the endoblast, takes
part in the formation of the
wall of the digestive tube.
Lateral Plate Mesoderm: Day 23
Lateral Plate Mesoderm: Day 25
1
2
3
4
5
6
7
8 Notochord
9 Splanchopleure with
endoderm
10 Somatopleure with
ectoderm
11 Amniotic cavity
12 Umbilical vesicle
Lateral plate mesoderm
Intermediate mesoderm
Paraxial mesoderm
Neural groove
Coelomic vacuoles
Intraembryonic coelom
Somites
The Intraembryonic Coelom
The intraembryonic coelom first appears in the lateral plate mesoderm as the lateral plate
mesoderm begins to split into the dorsal somative mesoderm (somatopleure) and the ventral
splanchnic mesoderm (splanchnopleure).Initially, it looks like several isolated vacuoles. During
the lateral unfolding of the embryo in the 4th week, these vacuoles fuse and form a U-shaped
cavity: the intraembryonic coelom. In the beginning, a connection exists between the intra- and
extraembryonic coeloms. With the progress of the unfolding, though, the merging of the
ectoblast layers along the medial line has the effect that the intraembryonic coelom is separated
from the extraembryonic one and remains enclosed in the lateral mesoblast.
The intraembryonic coelom will give rise to the futurer serous cavities of the trunk—
including the thoracic cavities (i.e., pleural and pericardial) and abdominal cavity (i.e.,
peritoneal).
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Block 2 Unit 1
Differentiation of the Mesoblast
NOTA BENE: SEE DR. CHIAIA’S LECTURE ON
THE “DEVELOPMENT OF THE MUSCULAR
SYSTEM” FOR ADDITIONAL INFORMATION.
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Induction of the Neural Plate
Terminology:
Neurulation
the formation of the embryonic neural plate and its transformation into
the neural tube.
Primary Neurulation process in human embryos by which cells of the neural plate invaginate
and pinch off to form the neural tube.
Neural Plate
a flat region of neuroectodermal cells which lies directly above the
notochord; the first-formed embryonic rudiment of the CNS; during
neurulation, it will become the neural tube.
Neural Tube
longitudinal tube of neural tissue formed by the rolling up of the neural
plate until the neural folds join in the mid-dorsal line and the epidermis
fuses above; precursor of the brain and spinal cord.
Neural Crest
located between the neural tube and epidermis during neural tube
formation; migrate immediately after neural tube closure; sometimes
referred to as the fourth germ layer.
Early in the development of an embryo, a
strip of specialized cells called the notochord
induces the cells of the ectoderm directly above
it to become the primitive nervous system (i.e.,
neuroectoderm) [A, see diagram].
The appearance of the neural plate
represents the first step in the genesis of the
nervous system. The neural plate is identifiable
as the medio-sagittal thickening of the ectoblast
rostral to the primitive streak. At the cranial
end, the neural plate is wider and gives rise to
the brain. At the caudal end, it is narrower and
gives rise to the spinal cord.
In the third week, the edges of the neural
plate rise up and become neural folds enclosing
the neural groove [B, see diagram]. As the tips
of the folds fuse together, a hollow tube (i.e., the neural tube) forms [C, see diagram]. The
neural tube is the precursor of the brain and spinal cord. Meanwhile, the ectoderm and
endoderm continue to curve around and fuse beneath the embryo to create the body cavity—
completing the transformation of the embryo from a flattened disk to a three-dimensional body.
Cells originating from the fused tips of the neuroectoderm (i.e., neural crest cells) migrate to
various locations throughout the embryo, where they will initiate the development of diverse
body structures [D, see diagram].
The neural crest cells form, so to speak, a 4th embryonic germinal layer. This contains
a partial segmentation that contributes to the formation of the peripheral nervous system
(neurons and glia cells of the sympathetic, parasympathetic and sensory nervous systems).
The neural crest cells are distinguished by a great migrating ability and phenotypic
heterogeneity, since numerous and various differentiated cell types will arise from them: PNS
nerve and glia cells; epidermal pigment cells (melanocytes); calcitonin cells of the thyroid gland;
cells of the adrenal medulla; some components of skeletal and connective tissue in the head area.
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Neural Plate: Day 23
Additional views of the
closure of the neural tube:
Neural Plate: Day 25
(follow the arrows in order
to trace the process by which
the neural tube develops)
1
2
3
4
Neural plate
Primitive streak
Primitive nodes
Neural groove
5 Somites
6 Cut section of amnion
7 Neural folds
Neural Plate and Neural Crest
A Neural plate stage
1 Epiblast
B Neural groove stage
2 Neural groove
3 Neural crest
Migrating Neural Crest
Cells
Neural Tube
1 Epiblast
2 Neural fold
3 Migrating neural crest
cells
4 Neuroepithelium
5 Central canal
6 Neural tube
Neural Tube: Day 28
1
2
3
4
Neural tube
Neural fold
Neural groove
Somites
Neural Tube: Day 29
5
6
7
8
Neural crest
Pericardium protrusion
Cranial neuropore
Caudal neuropore
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Block 2 Unit 1
Folding of the Germinal Disc
Terminology:
Pericardium
one of three serous cavities which develops in the embryo/fetus.
Septum Transversum thick mass of cranial mesenchyme which gives rise to parts of the
thoracic diaphragm and the separates the intraembryonic coelom into
thoracic and abdominal cavities.
Cardiac anlage
the initial clustering of embryonic cells that serves as a foundation
from which the heart develops
Allantois
a pre-placental, pre-urinary tract (the placenta later becomes the main
organ for the removal of nitrogenous waste products); allantoic blood
vessels will become part of the umbilical cord after embryonic folding.
After the third week, the three germinal layers begin to differentiate and transform so that
the initially flat embryonic disk develops into a cylindrical structure like a "C". The folding and
genesis of the abdominal wall permits a delimitation of the embryo. At this point, the
extraembryonic tissue folds over into the intraembryonic tissue with no boundaries. The folding
and the resulting formation of the abdominal wall lead to an enclosure of the mesoderm and the
endoderm. They become surrounded by the ectoderm, which later forms the skin.
Two mechanisms lead simultaneously to the formation of the
abdominal wall [see picture to left]:


The cephalo-caudal flexion (in the longitudinal direction) takes
place in the A plane.
The lateral folding (in the transversal direction, rolling up) takes
place in the C plane.
NOTA BENE: For the sake of completeness, a general overview of both the cephalo-caudal
folding and the lateral folding has been prepared. On the following pages, a comparison of the
two folding patterns has been presented side by side. Examine the sequence of images in a
columnar fashion (i.e., read the left-hand column from the top of the page to the bottom of the
page before reading the right-hand column in the same fashion; arrows have been provided).
NOTA BENE: Each set of images has a numerical code at the top left corner. The first
number indicates the Carnegie stage at which the given embryonic structure appears; the
second number indicates the day at which the given embryonic structure appears. Subsequent
images in this section will have a similar numerical tag.
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Block 2 Unit 1
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Block 2 Unit 1
The Lateral Folding (transversal direction)
At the same time as the cephalocaudal flexion, a lateral folding occurs in
the initially still flat embryo. This two-part
folding results in an enclosure of the
endoderm by the ectoderm:
In a first step, the laterally lying
structures, due to the large and rapid
growth of the internal embryonic anlage
(especially due to the disintegration of the
somites), are shoved in a ventral direction.
Some of the structures lying in the middle
are pressed against each other and fuse.
The second step of the lateral folding
has to do with the endoderm, from which
the inner covering of the GI tract arises.
The ectoderm of the caudal and
cephalic ends of the embryo coalesce, due
to the ventral folding along a medial line.
As the amnion is pulled by the ectoderm,
the small, dorsal amniotic cavity enlarges
to surround the whole embryo. Amniotic
cavity also surrounds the body stalk and the
yolk sac—forming the umbilical cord.
The endoderm, which becomes closed
at both ends and along the embryo’s sides,
forms a tube (future for-, mid-, hindgut). In
the beginning the midgut stands in an open
connection to the umbilical vesicle and the
allantois—both of which are later taken up
in the umbilical cord. The connection
between the embryo and extraembryonic
appending organs persists in order to permit
the passage of the vital umbilical vessels
which are located in the umbilical cord.
The intraembryonic coelom, a cavity
between the splanchnopleura mesoderm
(outer covering of the intestines) and the
somatopleura mesoderm (inner covering of
the trunk wall)—which in the beginning is
connected with the extraembryonic coelom
(i.e., chorionic cavity)—becomes separated
from it by the folding and fusion of the
lateral sides of the embryo. Thereby, the
intraembryonic coelom ring is formed.
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The Cephalo-Caudal Folding
Block 2 Unit 1
(longitudinal direction)
In order to understand how this
Folding of Cephalic End: Day 27
Folding of Cephalic End: Day 30
turning takes place, the structures
must first be described that are found
in the cephalic end before the
folding: in the cephalic region,
rostral to the prechordal plate and
the pharyngeal membrane, the
mesenchymal cells form the cardiac
plate (pericardium) and the septum
transversum (which later becomes a
part of the diaphragm and separates
the coelom into thoracic and
abdominal cavities).
6 Pharyngeal membrane
With the 180° degree turn that 1 Future prosencephalon
2 Notochord
7 Extraembryonic
results from the folding, the 3 Neural Tube
mesoderm
following occurs: the pharyngeal 4 Pericardial cavity
8 Throat
9 Septum transversum
membrane extends towards the 5 Cardiac tube
lower front (mouth area) and the
cardiogenic plate (which initially lay most cranially) into the thorax area. Between the cardiac
anlage and the umbilical vesicle a mesenchymal bridge forms, the septum transversum [see
pictures at upper right].
After this movement is completed, the brain (encephalon) lies the most cranially, followed
by the mouth, heart, and diaphragm (septum transversum). During this folding the endoderm
below the pharyngeal membrane becomes surrounded ventrally by the cardiac anlage. From this
region, the throat (pharynx) arises and, subsequently, the thyroid gland, the lungs, and the
esophagus. The pharyngeal membrane which, for now, separates the mouth (ectoderm) from the
throat (endoderm) later atrophies.
Folding of Caudal End: Day 30
Folding of Caudal End: Day 27
The folding of the caudal end
occurs after the cephalic folding and
has the result that the body stalk
comes closer to the umbilical
vesicle.
Due to the large axial growth,
the caudal end of the embryonic disk
(with the cloacal membrane) comes
to lie under the original embryonic
disk and thus shoves the allantois
and the body stalk in the ventral
direction, up to the umbilical vesicle
(yolk sac) and merges with its stalk
[see pictures to right].
The end of the primitive streak,
which initially lies dorsally after the
flexion of the embryo, now lies
ventrally.
1
2
3
4
5
6
7
8
Notochord
Neural fold
Amniotic cavity
Primitive streak
Primary endoderm
Cloacal membrane
Allantois
Body stalk
1
2
3
4
5
6
7
8
9
Notochord
Neural Tube
Amniotic cavity
Primitive streak
Primary endoderm
Cloacal membrane
Allantois
Body stalk
Hind gut
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Development of the Placental Villi
In order to understand the chronological development of the chorionic villi, it is
important to have a comprehensive overview of placental anatomy. In this diagram, the placenta
is roughly four months old and various fundamental structures can be recognized, namely the
umbilical cord, the amnion, the chorionic plate, the already advanced branching of the villi, the
basal plate and the cotyledon [see picture below].
Placenta at around four months
1
2
3
4
Umbilical cord
Amnion
Chorionic plate
Intervillous space (maternal blood)
5 Basal plate
6 Cotyledon
7 Villus
At birth, the placenta consists of two parts: maternal portion & fetal portion [see pictures below].
Fetal Portion / Side
Maternal Portion / Side
1 Cotyledon
2 Cut edge of
amnion
3 Umbilical cord
4 Decidua
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http://embryology.ch/anglais/fplacenta/villosite03.html
Numerous "daughter" villi arise out of the tertiary villi. These remain either free and
project into the intervillous space (free villi), or they anchor themselves to the basal plate
(anchoring villi)
BEGIN AT MODULE 10, CHAPTER 2 (“THE CYTOTROPHOBLAST LATER”)
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Block 2 Unit 1
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