Folding of the Embryo, Basic Body Form
11-13-09
1. Review
a. Where does the notochord or process come from?
i. From the precursor notochordal process, which is embryoblastic in
origin, specifically epiblastic
ii. Specifically, where in the epiblast is it from?
1. Presumptively it is from the caudal regions of the
embryonic disk, in the area at the cranial end of the
primitive streak known as the primitive node
iii. Epiblastic cells migrate through and join the hypoblast as other
epiblastic cells move right up the midline in that hypoblastic layer.
iv. Your book says that the notochordal process perforates (opens up)
inferiorly, and as it does so it becomes intercolated into the
developing endodermal layer.
v. However, as a contrasting theory, if you believe in the migration of
cells, just kind of as a sheet, you would not have a tube and would
not have to go through the opening, and the cells would already be
intercolated into the developing endodermal layer (which was
originally hypoblast but is now turning into endoderm)
vi. For a short period of time, because of the way the cells are
migrating in the primitive node area there will be a communication
between the primitive pit (which is caused by the migration of
cells) and the yolk sac (which is called the neuroenteric canal)
1. Neuro in that it is close to where the spinal cord will
develop and enteric because it is related to the gut that is
going to be derived from the yolk sac
vii. In the vast majority of individuals, the neuroenteric canal will
close up
viii. This phenoma is the way cells migrate through the area.
ix. We have then, irregardless of which way the precursor moves, a
prechordal plate in the mid line running from the primitive node
area up toward the cranial end of the embryonic disc
1. Also, as these cells migrate they kind of pushed
endodermal cells ahead of them and this is where we
develop the oropharyngeal membrane where ectoderm and
endoderm are fused together.
x. And in between that area of fusion and what becomes the
notochord proper will be called the prechordal plate
xi. The notochordal plate is ultimately going to close up starting
crainally and working caudally because even as the cranial end is
closing up, the caudal end is still adding tissue to it (cells to it)
1. This is due to the fact that during this time the embryonic
disc is elongating as more and more cells migrate in,
proliferate, changing from a disc shaped to one that is
oblong (as seen above on the right)
xii. As the definitive notochord forms up, it forms up first crainally
than kind of zippers together caudally
xiii. Ultimately we will end up with a solid rod of cells that separates
from its closely adjacent endodermal layer and becomes embedded
(encirceled by) the mesodermal layer that is forming concomitant
with the formation of the notochord
xiv. Looking at various illustrations (as are present in your book) there
seems to be a hole in the middle of the notochord. This is not the
case. Rather the hole disappears and becomes a solid rod of cells.
2. The Mesoderm
a. As you can see above from the cranial end of the primitive groove, cells
migrating what will be the cranial end of the embryonic disc.
b. As the migration is completed, the oropharygeal membrane is created, the
prechordal plate is compelted, and the precursors of the notochord are
created, as discussed above
c. Else along the primitive groove, which was the primitive streak before
cells migrated, cells of trophoblastic origin are moving through this
region, losing their cell to cell connections moving down a little bit so that
they are lying in between the overlying epiblast and underlying endoderm
and are creating this 3rd layer of cells called the mesoderm
d. That migration, as we said before, is not willy nilly.
e. If you study the epiblast and treat the cells appropriately, you can see that
there is a pattern of migration (of epiblast cells)
i. Cells that move through the primitive node area go right up the
midline and become the notochord
ii. Cells of epiblastic origin proliferating and migrating back through
the primitive groove near the midline will go through the cranial
end of the primitive groove and move up under the area in which
they originated and become known as paraxial mesoderm, AKA
somatic mesoderm
iii. If you go our further laterally, proliferation of cells can be tracked
to the primitive groove
1. These cells will go through the groove and migrate back
under the area in which they originated and be known as
intermediate mesoderm
iv. Cells even more lateral will migrate down through the groove and
then migrate back under the area in which they originated
f. Thus, there is not a whole bunch of random movement, but rather
programmed movement
g. Also developing as these cells are migrating, etc, will be a so called
coalacial membrane, which will contain endoderm and mesoderm fused
together, same as the oropharyngeal membrane.
3. The Notochord being the master director
a. One of the things the notochord will do is it will be a special organizer or
controller of tissue around it
b. The notochord will help the mesodermal cells (by way of chemicals that
the notochord releases) know where they are if you will, and cause
formation of somites from the paraxial mesoderm (structures known as
somites)
c. A little bit further later the chemicals are not as concentrated so we
develop intermediate mesoderm
d. Even further laterally, the chemical gradient drops off a little more and the
mesoderm develops into lateral plate mesoderm
e. Looking at cross sections, as shown directly above, the notochord is
shown in the mesodermal layer
f. Somites
i. Developing along side the notochord and also helping to be
controlled formation wise is the neural groove, structures that
become part of the CNS, which also are working to organize the
mesodermal cells
ii. There will become a block of cells that develop next to the
notochord known as somites
iii. Somites are ultimately going to give rise to cells that will move
down around the notochord and encircle what will become the
iv.
v.
vi.
vii.
neural tube (which becomes the CNS) and basically becomes the
bony elements of the vertebral column
This portion of the somite is known as the sclerotome
1. A tome is a section, sclerous means hard, so a sclerotome
portion of a smoite gives rise to hard tissue (cartilage and
bone)
Other cells within the somite will ultimately give rise to skeletal
muscle (myo)
1. The skeletal muscle will help form the deep back muscles
and the other muscles predominantly of the trunk and some
muscles within the limbs
The third portion of the somite will give rise to cells that become
the dermatome
1. The dermatome contributes to the dermis of the skin (the
CT layer of the skin—integument)
Somites are paired bilaterally and will give rise to skeletal
elements, muscle elements, and CT elements of the skin
g. These somites we see developing as little block arrangements on either
side of the notochord and developing neural tube will have a pair of
somites for each body segment
h. Somite formation is the first indications of sementation of the body
i. If you do not know what segmentation of the body is, realize we
have a vertebral column that is segmented
ii. A good representation of the segmentation of the body are the
spinal nerves that go out to innervate musculature
iii. Each segment of the body has a set, left and right, of spinal nerves
that go out to innervate musculature derived from the dermatome
of that segment
iv. So we are a stack of coins, if you will, each segment is not
necessarily equivalent, but all the segments do have basic
similarities. One of these similarities that we have are the somites
i. As the embryo gets older, the somites increase in number
i. The first somite retains all of its properties and basically ends up in
the occipital region (base of the skull) and also contributes to the
first cervical vertebrae
ii. There will be some somite like structures that develop cranial to
that but they do not have all the components
4. Mesodermal clefts
a. One of the things that the mesoderm likes to do is the cleft, or break apart
i. We saw that in the extraembryonic mesoderm as the clefts were
created between the cells ultimately allowing the formation of the
extraembryonic mesoderm on the trophoblast to help form the
ii.
iii.
iv.
v.
vi.
vii.
viii.
ix.
x.
xi.
chorion, as the extraembryonic mesoderm created a layer of CT
cells that strengthen the amnion and the yolk sac or umbilical
vesicle
The area where the extraembryonic mesoderm did not cleft
remained as the connecting stalk (mesenchyme or CT of the
umbilical cord)
Well, intraembryonic mesoderm has a tendency to cleft, especially
in the lateral plate region
Clefts will develop in between mesodermal cells in the lateral
plate region
1. Remember the lateral plate is lateral within the mesoderm
of the developing embryo
2. From the midline we have paraxial mesoderm (somite
mesoderm), intermediate mesoderm, and finally lateral
plate mesoderm
In the lateral plate mesoderm the clefts will become the
intraembryonic coelom
Now as we will see in the lateral regions or in the middle of the
embryo this cleft will go all the way to the edge of the embryonic
disc so that the intraembryonic coelom will be continuous with the
extraembryonic coelom
Thereby as we will see, the embilical vesicle (secondary yolk sac)
is going to be free from other structures within the embryo except
in the midline
In the more cranial region of the embryonic disk, clefts will
develop but those clefts will not go all the way to the edge of the
disc so well just have a tunnel running through the mesoderm that
will become coelom
Intraembryonic coelom will become the body cavity or serous
bound cavity of the body
The lateral plate mesoderm that we have been talking about with
the coelom formation can be divided into what is known as
somatic mesoderm and visceral splanchnic mesoderm
1. Visceral and splanchnic are equivalent terms
2. Both these terms refer to cental organs (heart, digestive
tract, etc)
The lateral plate mesoderm is split by the formation of clefts that
become the coelom and the mesoderm or visceral and splanchnic
mesoderm
1. If we combine somato (body) and pleur (membrane),
somatopleur is the term used to describe the combination of
somatic mesoderm and overlying ectoderm
2. So somatopleur is the shorthand for splanchnic or somatic
mesoderm and overlying ectoderm, basically the body wall
if you think about it
3. Splanchnopleur is visceral mesoderm and endoderm
4. So the central viscera that are endodermally lined like the
digestive tract and its related CT and smooth muscle, the
tissue that gives rise to those structures (visceral mesoderm
and endoderm) is the splanchnopleur
xii. Above you can see lateral plate mesoderm, somatic mesoderm
related to the overlying ectoderm (remember overlying ectoderm
are epiblastic cells that did not migrate), this combination is the
somatopleur
xiii. The secondary yolk sac lined with the endoderm largely becomes
the digestive tract
xiv. Lateral plate mesoderm associated with that is splanchnic or
visceral mesoderm. Combination of the 2 tissues will be
splanchnopleur
5. Notochord and CNS development
a. Another thing that the notochord will do is help regulate the development
of the central portion of the nervous system (CNS- or more specifically
what we know as the brainstem and spinal cord)
i. The exact way it performs this duty we will take up next time
b. But because of the presence of the notochord, deep to the overlying
ectoderm, that ectoderm transforms into neural tissue (primitive neural
tissue). Cells are columnar in this region of the ectoderm
c. Elsewhere, away from the notochord, the cells become cuboidal and
ultimately squamous and become epidermis.
d. But close to the notochord, the ectodermal cells are transformed into the
precursors of nervous tissue and will form initially a neural plate, kind of
reminiscent of the notochordal plate, a group of cells with further
development
e. That neural plate begins to fold upon itself creating a longitudinal groove
running the length of the neural plate
f. This groove is now known as the neural groove and where we have
junction between the neural tissue and what will become regular ectoderm
for epidermis, there will be a special class of cells (tissue) formed that will
be called the neural crest
g. The neural crest has formed around the periphery of the neural plate again
at the transition, becomes more evident with the formation of the neural
groove, so right at the crest of the groove
h. As the neural groove continues to close, to become a neural plate, the
neural crest breaks free, migrates away from the region and the surface
ectoderm will zipper across and heal if you will
i. So what we are doing is forming neural tissues, first in the plate
confirguration (with neural crest around periphery of it) that plate folds,
becomes a neural groove that runs longitudinally in the embryonic disc
region, that will ultimately close, and with closure of the dorsal aspect of
the neural groove to become the neural tube.
j. The surface ectoderm is brought together, that will fuse across the midline
and the specialized tissue known as the neural crest will then separate and
many of them migrate away from the area
k. In the picture above you can see the nearly formed neural tube. As that
becomes a tube the surface ectoderm, which are now squamous cells, are
covering everything. Neural crest cells between the neural tube and the
surface ectoderm begin to migrate away and here is the somite passing
from the side going over overlying ectoderm.
i. There are essentially no CT fibers at this time-rather all cellular
with cell to cell adhesions holding these things together
6. Clinical correlation-Problems with development
a. Sometimes things go wrong in development-such as the primitive groove
doesn’t know when to stop. You will continue to grow relative to it and it
will give rise to what is a sacral coccygeal teratoma (an oma is a tumor
and a terate is a monster-thus monster tumor,
i. You never know what you are going to find in these tumors-maybe
teeth, hair, bone, muscle
b. Since this is in the sacral region it tells you where the groove is going to
end up-in the coccyx region
c. The body has gotten longer, the primitive groove area has gotten relatively
smaller ending up at the caudal end of the axial structure, which is the
vertebral column
d. If you look at a newborn and some adults you will see a dimple at that
point which kind of indicates where that primitive groove is
e. You can have cysts growing in this region as well
7. Body folding
a. Review
i. From the epiblast we have spun off the notochord giving rise to the
underlying endoderm giving rise to the underlying endoderm
giving rise to the intermediate mesoderm and helped to form the
oralpharyngeal membrane and the clocal membrane, and midline
where the notochord is (you will have it around it-not within it
though).
ii. The notochord is epiblastic in origin but the notochordal cells have
special properties-they control tissue around them.
iii. Remember that everything we have talked about—the definitive
endoderm, mesoderm, and the ectoderm is all of epiblastic origin.
Hypoblast is a transient tissue
iv. The embryoblast contains epiblast which turns into the trilaminar
embryonic disk
v. The endoderm gives rise to the lining of the respiratory system
which itself is a outgrowth of the digestive system (digestive tract
is from pharynx to anus)
vi. On the outside of the embryo, surface ectoderm gives rise to
epidermis and the anterior portion of the pituitary as well as teeth
enamel and the lens of the eye
vii. Neuroectoderm gives rise to the CNS
viii. The neural crest is almost a 4th germ layer- we usually say there are
only 3 levels, but it is like another. Its history goes back to the
ectoderm
1. It gives rise to sensory neurons throughout the body (i.e.
those of the skins, etc), pigment cells (melanocytes that
migrate into the epidermis), cartilage within the head, CT
within the head, CT within the great vessels and around the
heart, medulla of the super renal gland, post ganglionic
cells- it is kind of a jack of all trades
2. It gives rise to everything from CT (especially in the head)
to things like Schwann cells
ix. The endoderm lining the yolk sac gives rise to the digestive tract
x. The ectoderm (strengthened by somatic mesoderm) gives rise to
tissue on the outside of the body and nervous system components.
xi. Mesoderm lies in between and gives rise to everything else
1. Intermediate mesoderm gives rise to reproductive and
urinary systems
2. Lateral plate mesoderm gives rise to the CT of the body
wall and other structures
3. Visceral mesoderm gives rise to CT and musculature of the
digestive tract
4. Extraembryonic mesoderm gives rise to CT associated with
trophoblast mostly, CT on the outside of the amnion and
yolk/connecting stalk, and BV.
5. BV in general arise from mesoderm
b. Yolk sac
i. Serve as a temporary haven for the primordial germ cells
ii. Within the embryonic disc, the primordial germ cells tend to be in
the caudal end of the disc around the primitive groove
iii. They have retained their identity as the various layers develop and
differentiate
iv. You do not want the primordial germ cells to get pushed along—
you want them to stay put
v. They are within the secondary yolk sac
vi. When the migrations are done, the primordial germ cells go back
to the embryo proper and back to the gonads
vii. Teratomas usually have primordial germ cells in them
c. Embryo folding
i. We are not disc shaped. Fact. We are cylinder shaped.
ii. The disc has to fold and it will fold at the same time as the lateral
folds and longitudinal folds
iii. Remember: the edge of the embryonic disc is going to grow slower
than the inside of the disc
1. This is because the nervous system has rapid cell
proliferation as well as with somites
2. Thus a lot of cells are moving in the center and on the edge
not so much
iv. One also key thing to remember is the edge of the disc becomes
the umbilicus
v. As we add tissue in the midline (center of the embryo) the embryo
rises up from its margin like a cupcake and overflows the margin
vi. Doing this, part of the yolk sac is incorporated and goes up with
the other embryonic tissues. Part of the secondary yolk sac gets
incorporated in the cylinder in the longitudinal body folds
vii. As the embryo grows the margin of the disc does not grow as
fast—thus the flow over grows larger and larger
viii. As the embryo increases in size, the end result is now that we have
taken up the yolk sac is a cylindrical embryo
1. That is the endoderm of the digestive system
ix. As the folding process takes place, the amnion does not break free,
rather it stays connected to the edge of the disc
x. When we fold it around the amnion grows down and around the
connecting stalk forming the umbilical cord. And thus part of the
stalk is incorporated into the body during this process
d. We have an embryo, embryonic disk, ectoderm and yolk sac…where
would the amnion be?
i. Around this way…a connecting stalk, the sock represents the
diverticulum of the yolk sac as the allantois, which is a tubular
diverticulum (term for an outpouching of a hollow (or fluid filled)
structure in the body) from the yolk sac growing into the
connecting stalk
ii. What is the importance of the allantosis? Does it have any
importance to us?
1. The allantosis itself will help become part of the urinary
system and very top of the bladder.
2. Most of it obliterates, but the blood vessels that supply the
allantosis become the umbilical vessels connecting the
intraembryonic blood vessels that are developing within the
mesoderm to other extraembryonic BV that are developing
in the extraembryonic mesoderm related to the placenta
3. So, it is the intermediate group of blood vessels connecting
the intraembryonic and placental BVs
iii. There are 3 areas of interest here, one is the oropharyngeal
membrane, the other is the clocal membrane. What happens there?
Is there any mesoderm between the two?
1. No! The oropharyngeal membrane ends up in the back of
the mouth.
a. After it breaks down the oral cavity and pharynx
communicate so the oropharyngeal membrane
becomes the dividing place between the ectodermal
portion of the digestive tract and the endodermal
b. This transition takes place about where the palatine
tonsils are (the orocavity and pharynx)
2. The clocal membrane..again no mesoderm intervenes there
a. That is going to be the end of the digestive tract and
output area that’s basically an output area for both
the digestive tract and urinary systems
e. The development of the heart
i. We see that developing, the cardiogenic area in which type of
mesoderm?
1. Lateral plate
ii. Is it somatic or visceral?
1. It is the heart, which is a central viscous, what do you think
that is going to develop in? It develops is visceral
mesoderm
2. That means the heart is going to be developing between the
coelom in the area and the yolk sac which we will see in a
little bit
3. The heart is developing relationships with part of the
coelom
4. The heart is between the cranial (leading) edge of the
embryonic disk and the oropharyngeal membrane
iii. What did we say the yolk sac was going to develop into?
1. The digestive tract, or at least part of it
iv. What did we say happens to the edge of the embryonic disk?
Where does that ultimately end up?
1. The umbilicus
2. Thus in the book the talk about an umbilical ring, or
primitive umbilicus (thus the edge of the embryonic disk)
v. We said this embryo is going to grow more rapidly in the midline
because of nervous system development, somite development, etc
1. Thus the midline will develop more rapidly than in the
periphery
2. Thus the midline is going to have to bulge and buckle
someway to have the growth continue
3. What we will find is that the embryo will basically rise up
& as it does so does part of the yolk sac
4. Now as I use the term “rise up” & in the illustrations you
may think the sides of the embryonic disc are coming down
a. This is not the case as the edge of the embryonic
disk is the “stable” area and the center is moing
5. As the midline rises up it will create lateral folds, and it
also pushes forward with the head fold process developing
6. As that happens some of the yolk sac will be taken up into
the area
7. A similar event is going to occur at the tail end of the
embryo & it will extend beyond the edge of the embryonic
disc
8. One thing to remember in embryonic development is that
we may stretch and bend things, but we very rarely break
things
9. Lateral and longitudinal folding is taking place
simultaneously
10. The best way to think about this is taking some modeling
clay and folding it over your fingers
11. All cells aren’t growing at same rate within the embryonic
disc
12. We are stretching and bending structures, but we are not
breaking structures
13. After the longitudinal process has occurred, the digestive
tract is taken into the fold behind the oropharyngeal
membrane
14. As a review, where is the oropharyngeal membrane?
a. In the mouth
vi. Through this folding process, we have changed the orientation but
not necessarily the sequence of the heart, oropharyngeal
membrane, and embryonic disk
1. Initially the embryonic disc was the leading structure, and
then came the heart, and then came the oropharyngeal
membrane, and then came the developing brain
2. After the longitudinal folding process, the developing brain
is most cranial and then the oropharyngeal membrane, and
then the heart, and then what was the edge of the
embryonic disc
3. These structures have moved from the dorsal part of the
embryo/embryonic disc if you will to the ventral aspect of
the cylinder
4. At the caudal end of the embryonic disk, we’ve grown out
the tail fold and the clocal membrane and ectodermal
membrane is now seen on the anterior aspect of this
modified cylinder
5. And between the cloacal membrane (which is ultimately
the uro-genital region) & the edge of the embryonic disk is
where the anterior abdominal wall will form
6. The connecting stalk was at the caudal end of the
embryonic disk and after lateral and longitudinal folding
process in the caudal region of the disc the connecting stalk
is still related to the edge of the embryonic disk but its on
the ventral aspect, extending through the umbilical ring,
into the inside of the embryo’s body
7. The allantois has been taken into the embryos body…BV’s
have been taken into the embryos body
8. Now as we change from a disc to a cylinder by the
longitudinal foldings, and the lateral folds
8. A new model: The developing gut
a. The top part of the yolk sac related to the disc has been taken into the
cylinder & as a result of the lateral folds, that space within itself has
become cylinidrical ( a tube if you will) starting from the oropharyngeal
and going to the clocal membrane
i. Remember the coelom that formed and mesoderm, it is within the
mid region of the embryo seen caudally
b. And now the yolk sac becomes separate from the developing body wall
except in the midline, and even their tissue can stretch
c. Why is it important that the yolk sac can develop separately from the body
wall?
i. Well my body wall is here in the abdominal region a foot to a foot
and a half in length & the intestinal tract is developing from the
yolk sac & it stretches the length of a table, so if the yolk sac didn’t
develop separately from body wall either we would have a very
long body wall (like a snake) or a very short digestive system
9. Continued folding
a. What’s going to happen as the amniotic cavity continues to enlarge?
Where was the amnion initially?
i. The edge of the disk, going up and around
b. As the amniotic cavity continues to enlarge & the amniotic membrane
shrinks wraps down along the connecting stalk, & at that same time the
connecting stalk, the yolk sac within it, the blood vessels, & allantosis
within become known as the umbilical cord
c. The amnion wraps down all the way to the placenta
i. And when this occurs the chorionic cavity is obliterated
d. In lateral plate mesoderm, there will be local proliferations of CT and
muscles will move in, nerves will move in, & you get little limb buds
formed
e. Let’s think about what is happening in the embryo
i. When I say coelom and equate it to a cavity, that is a correct
statement, however there is a stipulation:
1. It is a cavity lined with mesothelium
2. It is a cavity with a serous membrane as its boundary
3. The reason I say this is if we were taking out all the central
viscera, take the heart, lungs, esophagus out, etc… you
would end up with a big cavity (or 2 cavities depending on
what you do with the diaphragm)
4. Those cavities are the abdominal/pelvic cavity and the
thoracic cavity
5. These cavities are NOT the coelom
6. These are the cavities of the trunk, they are musculoskeletal
cavities
7. The coelom will be within those cavities related to the
organs within those cavities
ii. Now when we say the word mesothelium, what do you think mesorefers to?
1. It related to mesoderm. Epithelium= a covering above.
Mesothelium is an epithelium created from mesoderm that
lines the coeloms
2. Have you head the word mesothelioma?
a. Mesothelium involved in mesothelioma
b. The coelom or the serrous bound/lined membranes
in the body will be divided into 3 major regions:
i. The pericardial coelom (relating to the heart)
ii. Two pleural coeloms (relating to the lungs)
iii. One peritoneal coelom lined with a serous
membrane & mesothelium being epithelium
in origin
iii. So we project onto the thoracic wall the lungs, the diaphragm, and
the heart
1. take the thoracic wall away & we open up the pleural
cavities, see the lungs within it
2. Each pleural cavity should be separate from everything else
3. At the midline within the fibrous pericardium, we’ll find a
pericardial cavity
4. In between the serous membrane known as the pleural
cavity, & tissue around the heart we will find a phrenic
nerve coming down out of the neck to innervate the
diaphragm
iv. Take a cross section through and you the heart with a space around
it majorly encircling it—this is the pericardial coelom or cavity
1. The lungs as they grow out laterally from the midline of the
medistinal region, ultimately will be surrounded by a cavity
known as the pleural cavity; in both of these or all three of
these cavities there will be serous membrane, mesothelium,
and CT or structures that protect the lungs or body wall
v. The serous membrane on the organ will be called visceral serous
membrane or more specific, visceral serous pericardium, visceral
pleura on the lungs
1. the continuation of the serous membrane that lines the body
cavity or tissue around the heart, not on the surface of the
organ will be called parietal (meaning wall)
2. So thus the serous membrane that helps make the wall of
the cavity will be called parietal; in the case of the lungs
and pleural cavity it will be parietal pleural and case of the
pericardial cavity it will be parietal serous pericardiam
f. At the bottom of the thorax separating the thoracic organs from abdominal
organs is the diaphragm; passing through it will be the inferiot vena cava
(IVC), the esophagus, the aorta, and some other structures
i. The diaphragm has a central area that is tendonous and muscle
fibers radiating from the central to the ribs and to the lumbar
vertebrae
ii. Passing down out of the neck between tissue of the heart and the
parietal pleura is the phrenic nerve down to innervate muscles of
the diaphragm and sensory surfaces of the diaphragm
iii. In the abdominal region under the diaphragm is the peritoneal
portion of the coelom and relates to the abdominal pelvic structures
1. There will be a parietal peritoneal ling the body wall and a
visceral peritoneal, serous covering the organs, protruding
into that space
2. All of these cavities have a small amount of serous fluid in
them to allow the epithelium of the serous membranes to
rub on each other and move easily; allowing easy
movement of organs in the CT surrounding it; all of this is
intraembryonic coelom
g. In the lateral mesoderm, in the midline the intraembryonic coelom will
form and will go all the way to the outside of the embryonic disk, thus
allowing the yolk sac to be separate from the body wall except in the
midline
i. In the cranial end of the embryonic disc, the coelom does not go all
the way to the outside and is sealed off to form a tubular structure
that goes up past the oral-pharygeal membrane into the heart
region, crosses over and comes back;
ii. Thus if we were a microorganism in amniotic fluid, we could get
into one of the openings where extraembryonic coelom
communicates with intraembryonic coelom and turns toward the
head, walk into cylindrical tunnel, continute to walk up toward the
heart developing region, cross the midline and walk back and leave
out the other side
iii. So we have an open area which is part of the coelom, and then a ushaped tubular coelom extending cranially toward the feet
h. With lateral body folds, we will ultimately close off after several steps
what was the wide open area of the coelom related to the mid area, and
ultimately will form the peritoneal cavity;
i. with the coelom being sort of U-shaped, coming up alongside the
neural tube in the mesoderm into the developing heart area and
back to the other side
ii. With the folding process this portion of the coelom gets bent a
little bit and we can the developing heart and the pericardial
coelom that crosses the midline
iii. Here, below, in sagital sections, you see a portion of the coelom
crossing the midline-ventral and anterior to the developing heart
1. If the disk were laid out, the heart would be in splanchnic
mesoderm between the coelom and the yolk sac: with the
head fold
2. The heart is still between the coelom and yolk sac
structures but they’ve changed their orientation
iv. After folding, the heart is deeper than coelom; u-shaped tube runs
parallel on either side of the esophagus
i. Septum transversum
i. Translates into transverse septum, or transverse block of tissue
ii. It was the original leading edge of the embryonic disk that broke
apart to go down into the yolk sac
iii. Part of the intraembryonic mesoderm is going to become the
septum transversum
iv. Three structures develop from the septum transversum
1. CT of the diaphragm
2. CT of the liver
3. The tissue between the liver and stomach called the ventral
mesenteric
j. In between the heart and the umbilicis should be derivatives of the septum
transversum; most superiorly will be the CT of the diaphragm, then the
liver more inferiorly (the liver cells that grow from the gut grow into the
septum transversum and take up residency there)
i. Then the tissue between the liver and stomach or between the liver
and the anterior body wall will be the ventral mesenterics
k. We develop a coleom that goes through the cardiogenic area; the heart
develops within the splanchnic mesoderm, meaning on the yolk sac side of
the coelom
i. The pericardial coelom crosses the midline
ii. After longitudinal folding, the heart is still in splanchnic mesoderm
next to the developing digestive tract but the coelom is still related
to somatic mesoderm but is now anterior to the heart; running
parallel to the esophagus will be the portion of the coelom that
communicates between the pericardial coelom and the peritoneal
coelom
1. One of the terms that can be used for this is the pericardialperitoneal canals (AKA the primitive pleural cavities)
2. This is where the septum transversum is found before the
folding process-between the developing heart region and
the edge of the disk
3. After the folding process and during it, the septum
transversum is still between the heart and the edge of the
disk
l. The septum transversum is called that because after the folding process,
this is where we find it—a big block of tissue making a transverse septum
across the embryo, but that septum is not complete; there are openings in it
called the pericardial-peritoneal canals or primitive pleural cavities
m. With further development, folds grow in and close off the pericardial
cavity from the primitive pleural cavities;
i. These folds growing in are a result of vessel enlargement and
migration;
ii. Several large veins are developing and moving into the heart
1. They create these folds or membrane between the pleural
and pericardial space or so called pleural pericardial
membranes
2. They will move in and separate the two pleural spaces from
the one pericardial space
3. With time the pleural cavities will enlarge at the expense of
tissue between the developing body wall and the tissue that
remains around the heart
4. Early on the pleural cavities are very small, dorsally
situated, and the heart has some excess tissue around it
5. The pericardial coelom is in contact or held to the
developing thoracic wall
6. The pleural coelom will dissect into that tissue anteriorly
and dissect pretty much the heart and its surrounding tissue
almost completely free from the anterior thoracic wall
7. When we open the thorax, we find pleural cavity and pleura
coming to the midline and maybe even overlapping, not
joining together
a. So there will be a minimal amount of tissue
between the tissue surrounding the heart and the
deep aspect of the sternum;
b. The pleural cavities come in and encircle the heart;
all this space was initially CT but the pleural cavity
dissected into it and the lungs grow up to fill it up
n. The phrenic nerve gets caught between the pleural cavities and the tissue
surrounding the heart
i. When the embryo is growing, the head area is first very large
compared to the rest of the body
ii. When the septum transversum comes back and merges with the
rest of the body, it is at the most cervical level (most precisely at
somite levels 3, 4, and 5)
iii. Subsequently, with differential growth of the embryo, the
diaphragm will descend,
1. However, while it is at cervical level 3, 4, 5 myoblast will
move out of cervical levels 3, 4, and 5 and grow into CT of
the septum transversum
2. Myoblasts will pull or attract neurons related to cervical
somites 3, 4, and 5 to go with them
3. So even though we find muscles of the diaphragm at lower
thoracic levels in adults, it is innervated from cervical
nerves 3, 4, and 5 and thus also keep it alive
a. Phrenic nerve (3, 4, and 5 keep the diaphragm
alive!)
o. At the level of the developing diaphragm, we are going to have tissue
growing in from the dorsal body wall,
i. This tissue will separate pleural and peritoneal cavities
ii. The membrane growing in is called the pleural-periotoneal
membrane
1. This grows in from the side
iii. Tissue running between the body wall and the stomach, esophagus
and through it and then further in will merge with CT of the
diaphragm
iv. This combination of tissues and CT temporarily had holes on either
side which will be closed by the peritoneal-pleural membrane
v. The diaphragm forms between the thoracic and peritoneal cavity
has CT largely from the septum transversum
vi. Muscles cells grow out of cervical somites and grow into the tissue
combination and populate the CT
vii. Additional CT moves in from the pleural-peritoneal folds or
membranes and close the canal
1. The pleural cavity not only dissects around the anterior to
the heart but also dissects down into the developing
diaphragm to make it dome shaped so that the lateral part
of the diaphragm will be derived from the body wall
2. The pleural-peritoneal membranes should be closed but
sometimes they remain open and cause certain pathologies