Development of the Trilaminar Embryo

11-10-09

1. Advanced placenta formationa. The umbilical cord connects the embryo fetus to the placenta

i. It contains fetal blood vessels which come down to the placenta and then branch out into large branches with subdivide even further

b. Villii. Ultimately smaller vessels will go into the villi (fingerlike projections) within the

forming placentaii. Within the villi are many vessels and capillary beds

iii. The villi hang into the intervillus space1. The intervillus space used to be the lacuna (which were membrane

bound spaces that developed within the synctiotrophoblast (syn) and enlarge and coealesce becoming the larger intervillus spaces

c. We mentioned that as the syn invades the endometrium it taps into the endometrial (maternal) blood vessels

i. Hopefully small ones first and then gradually larger onesii. Maternal blood flow is created through the lacunae, which, as mentioned

above, soon becomes intervillus spaceiii. The blood flows into the extracorpal space (the intervillus space—specifically

outside the mothers circulation) then will drain out into maternal veins

1

iv. Since the invading syn- taps into rather large maternal arteries, the blood derived from these arteries are richly oxygenated and contains other nutrients

1. The oxygen concentration seems to trigger further villus proliferationv. With villus proliferation there is more tissue there (i.e. more mass), thus taking

up more room1. Thus in these areas the placenta grows thicker

d. In areas where the syn- does not invade into maternal arteries and thus there is no villus formation the placenta is not as thick therby creating infoldings within the placenta known as septa

e. Cotelydens

i. Are the bulges created by the invading villi into the intervillus spaces where maternal arteries are tapped into too

ii. These bulges are surrounded by septa, created by areas where villus formation does not occur

1. These two things form cotelydensiii. In general, the syn- will tap into about 30-35 maternal arteriesiv. Thus, how many cotelydens do you think there should be in a normal placenta

1. Follow the logic outlined above:a. If the cotelydens are caused to develop by villus formation, and

villus formation is triggered by arterial blood coming from maternal arteries directly flowing into the tissues, and there are approximately 30-35 arteries being tapped into there should be…30-35 cotelydens

b. A special note is that if there a couple of endometrial maternal arteries close together their flow may come together and form one large cotelydon with more villi hanging into it

f. Looking at yet another image of the umbilical cord we have fetal vessels that brance and go into the villi and maternal blood coming in and shooting all the way into the fetal

2

plate and percolating down through the villi (inside) and which will ultimately drain out through maternal veins

g. What should the yellow tissue shown above be?i. Is it fetal in origin?, Outer cell mass in origin?

1. It is nucleated with no cell boundaries thus it is syn-2. Again, the above picture illustrates that the only tissue of fetal origin

that ill come in contact with maternal blood is syn-h. Now as the placenta (and entire conceptus) enlarges, grows, ages, and matures, the

cytotrophoblast (cyto) cells will migrate down through the villi that run through the lacunar spaces (later the intervillus space)

i. These villi are called anchoring villi because they will hold the fetal plate and maternal plate together and kind of anchor things (example shown above)

ii. The cyto cells will migrate down through the anchoring villi and come to the ourside and spread along the interface between thes syn- and the altered endometrium known as the disidua (shown on picture right below discussion of cotelydons)

iii. The cyto cells in this area are the the cytotrophoblastic shell1. Why is the cyto- shell necessary?

a. One can hypothesize that it is because the placenta is going to have to increase in size and surface area

b. Cyto cells are necessary for this because they are the source for new syn-

c. Cyto cells are syn-cell stem cells as cyto daughter cells become syn-

d. Thus the cyto-shall can add to the syn- around the outer periphery of the developing placenta

2. Otherwise cyto cells in this area and perhaps within the villi would be the only source for syn- and thereby the cytoplasm, etc, would have to flow all the way dpwn through the anchoring villi to come as far out as

3

the maternal plate in order to allow the intervillus spaces to enlarge (which is not very efficient)

i. Syntiotrophoblast basicsi. Differeniated tissue that will form or secrete the human chorianic gonadotropin

(hCG), the placental estrogens and progesterone and prolactinii. It also participates in steroid metabolism with the fetus, etc

iii. Among its differentiated properties is that it does not elaborate many surface antigens.

1. Therby maternal lymphocytes that are circulating in the blood do not have any antigens to come in contact with to trigger an immune response

2. Also fibrin, a product within the blood, is deposited on the surface of the intervillus space

a. Fibrin and other chemicals tend to mask the antigens that may be present

3. Thus circulating lymphocytes do not have much of a chance of coming in contact with fetal tissue

j. Cytotrophoblastic basicsi. Undifferentiated tissue that can have surface antigens on it

ii. How can cyto cells that are in direct contact with maternal endometrium survive?

1. In short due to the disidual reaction (picture shown above of different areas of the disidua)

2. Review of the disidual reaction is that the reaction occurs within the endometrium to form the disidua (altered endometrium)

a. Within the disidua, the maternal lymphocytes that are within the intercellular spaces have a decreased ability to detect foreign material

b. So even though these lymphocytes are out there (T-lymphocytes, etc) their ability to function has been greatly down regulated

c. Thus the lymphocytes could come in contact with the cyto- and never would react to the tissue thus preventing an immune response

k. Review of placental basicsi. Review of how all this came about

4

1. Above is a photograph of the endometrial surface (12 day old embryo that has just completed implantation)

2. This would just about be visible to the unaided eye3. To give some size perspective, the oocyte is just about the size of the

period at the end of this sentence.a. A pretty large cell for all intents and purposes

4. At this point, the structures have not grown very much but the implanted embryo is multicellular

5. If we were to cut through the embryo in the picture shown above, the result would be the picture shown below

6. Present are the bilaminar disc (the inner cell mass), cytotrophoblastic cells (some which have yet to come into contact with maternal tissue

7. Above also we see large cell masses which is the syn- that is actively invading into the underlying endometrial tissue

8. There would be a layer of cyto-cell lining the blastocystic cavity as well

5

9. The structure syn grows into (the endometrium) contain endometrial glands which are producing a glycogen-mucous rich secretion that can diffuse into the syn- and be broken down for nutrients

a. Tissue fluid from endometrium is diffusing into the conceptusb. Gasses are diffusing into the conceptus, etc all through the

tissue fluid10. It is within the syn invaded endometrium that the intervillus spaces will

formii. Continual review of placenta

1. If we peel the conceptus out and kind of leave some tissue left with disidua, as shown in the picture above, we see all the villi sticking out, which as can be seen above, increases the surface area significantly

2. But it we realize that even though we have syn- all the way around initially, we have intervillus space also around initially, as the conceptus grows, it is not going to grow much deeper into the endometrium (disuda) so it is going to bulge out the lumen

3. As it bulges out the lumen, the tissue above on the luminal aspect of the conceptus is going to get stretched as the balloon like structure enlarges

4. Thus as this syn- and related tissues out here stretch, the lacunae get smaller and smaller and blood flow drops down tremendously

a. If there is no blood flow there is not maintenance of villi and the villi disappear

iii. So below we begin to see an area in the conceptus that has been removed from the uterus, where we don’t have as many villi.

6

1. The trophoblast here (chorion) is becoming smooth2. Thus ultimately the outer cell mass (the chorion) is going to be

subdivided into a villous-chorion (with villi) and smooth villus (smooth chorion) with no villi because of the growth process

3. The villus chorion further matures into a larger and larger placentaiv. Shown below on the left is the maternal aspect of the placenta, this would still

have disidua attached to it because when the placenta is delievered some 30 mins after the baby, as the uterus continues to contract, the disidua shears in such a way that the disidual (or endometrial tissue) immediately adjacent to the fetal tissue comes out with the fetal tissue

1. This is why I said that a placenta when it is delivered should be examined carefully to see that there are no areas where intervillus space has been opened up (torn into)

a. This would indictate that disidual tissue was left behind in the uterus (which is ok) but it would also mean conceptus tissue was left in the uterus which can become a bad thing.

v. Shown above on the fetal aspect is the umbilical cord coming into the center of it and vessels branching out

7

2. Further discussion of the disidua

a. Disidua basalis is between the implantation site (conceptus) and the myometriumb. The disidua capsularis makes a temporary capsule of disidual tissue in between the

conceptus and the uterine cavity and the disiduum paratalis, which is everything elsec. As the conceptus enlarges, not only does the outer cell mass get stretched and become

smooth, but also the disdual capsularis becomes stretched and loses its vascular supply.i. No vascular supply, no perfusion, and necrosis occurs

d. Ultimately the disidual capsularis is going to degenerate.i. But by the time that happens, the conceptus is large enough that it is actually

apposed to the other surfaces of the uterine cavity.1. Thus there is no danger of the conceptus breaking free and floating

away or falling out3. Ectopic Pregnancies

a. Normally we want implantation to occur within the endometriumb. However, sometimes it does not.c. Below we see example of an implantation that was within the uterine tube that back-

flushed out (middle right)

8

d. Implantation shown above did not back flush out but rather ruptured the tube (bottom left)

e. Below is an implantation in the intrauterine portion of the uterine tube (aka intramural portion) where the uterine tube goes through the myometrium

i. Intrauterine implantations that go to full term get our through an insertion in the abdominal wall (requires c-section delivery)

f. Shown below is a full term pregnancy within the abdominal cavity

9

i. Specifically the intestinal mucosa (serosa) and the liver1. The conceptus is just trying to find a blood supply and nutrional

elements when the endometrium is not present.2. It choose the intestinal mucosa in this instance

ii. There have been a few cases where an ectopic abdominal pregnancy has gone to term, the problem is not getting the little one out (that is done by simple surgery)

1. Issue is removing the placenta2. Placenta has tapped into all the blood vessels in the intestinal mucosa

and when you remove the placenta this vessels are open3. When you pull the placenta out, the placenta is going to bleed out4. Thus you have to get the “gel” and cauterize quickly to close the vessels

iii. This is not as big an issue in the uterus as the uterus has a mechanism to close the vessels

1. The vessels that go into the endometrium must pass through the myometrium

2. One of the thing that the uterus does during birth is that it is contracting down, squeezing out the little one

3. After the little one comes out, hopefully the uterus continues to contract and the nurses will massage the abdominal wall of the mother, helping to stimulate uterine contraction (oxytocin can also be given)

4. The contraction of the myometrium clamps down and closes the blood vessels heading to the endometrium thus preventing bleeding out

a. This serves as an automatic means of closing off the circulation that had just been opened by the loss of the placenta

5. There are cases, however, where the uterus fails to contract. In those cases more oxytocin is given to enhance contraction.

10

a. Interventional radiologists are called in who will try to embolize uterine arteries.

g. The covering of the organs and the vasculature of the uterine tube seems to be enough to maintain an implantation in some cases.

i. The problem with an implantation within the uterine tube is that as the conceptus enlarges the uterine tube cannot enlarge in the same way the uterus does

1. Thus the uterine tube ruptures or will squeeze the conceptus out2. However, the implantation within the uterine tube does cause blood

vessel development and enlargementa. So whether you squeeze it out or rupture the tube you are

going to have a hemorrhage, causing potential uterine tube rupture and maternal death

4. Position of the implanting placentaa. Again, the smooth chorion does not have villus.b. The villus chorion becomes the chorionc. What shape is the placenta in?

i. A circle (disk like)d. Why is the placenta usually disk shaped?

i. A section of a spehere topographically is a circlee. What would happen is during the intitial contact between the conceptus and the

endometrium, the non-embryonic (aembryonic) pole of the conceptus made contact and fused with the endometrium and invaded in?

i. Where would the placenta develop in this case?1. The placenta is always going to be on the deep aspect2. And the umbilical cord always runs the shortest distance between the

inner cell mass the outer cell mass3. That means if we have a reverse implantation, the umbilical cord will be

attached into the trophoblast and the umbilical vessels would have to run all the distance to where the placenta was.

4. If the umbilical cord courses superiorly (opposite the cervix side of the uterus) then there is no major problem

5. However if the cord passes inferiorly between the fetus and the cervix then it could very well get in the way of the fetus as it tries to move down the cervical canal at the time of birth

5. Extraembryonic Tissues derived from the inner cell massa. Extraembryonic coelom is the same thing as the chorionic cavity.b. Coelom is another word for cavityc. The extraembryonic coelom is the space within the trophoblastic outercell mass

structures we call choriond. The amnion and amniotic cavity

11

i. Derived from epiblastic tissueii. The amnion will split from the epiblast and thus will develop a cavait and that

cavity will become filled with fluid called amniotic fluidiii. Amniotic fluid is largely derived from maternal tissue fluid that diffuses through

the conceptus early oniv. At later stages as the embryo matures the embryo itself will also contribute to

the amniotic fluidv. Amniotic fluid has no nutrient value

1. However, the older fetuses can practice swallowing by swallowing amniotic fluid

2. The fetus will urinate the amniotic fluida. Technically not urine, rather is tissue fluid because the kidney is

not functioning to concentrate things, so it is just recycling fluid and putting it back into the amniotic fluid

vi. The amniotic membrane and cavity is going to grow tremendously ultimately filling the chorionic cavity so that the embryo fetus is going to be hanging its umbilical cord into its own little “hot tub”

vii. There the fetus can move easily because we have to realize that the embryo and early fetuses have no skeleton, rather their skeletons are soft and gelatinous

1. The fetuses cannot take the impact of squeezing down the uterine wall (myometrium) so they have to be in a fluid filled environment that is

a. protective, from uterine contractions and outside blows that may impinge upon the mother’s body

b. keeps them at the same temperature,c. keeps them well hydrated (as the fetus does not have another

hydration option)d. allows them to move easily

12

i. If the little one is restrained it will end up with limb abnormalities

e. However, it should be noted that as the embryo and the fetus grows and develops the beginnings of a skeleton, etc, and the uterus stops growing, the embryo or fetus will come in contact through the extraembryonic membranes with the myometrium

i. But by the time this happens, the fetus is strong enough to stand myometrial pressures

viii. Toward the end of pregnancy, if you push on the abdominal wall sometimes things push back at you. Or you can see the little bulge go from one side to the other side as the fetus is turning over

1. But at this point the fetus is big enough to withstand the pressurese. Umbilical vesicle (yolk sac)

i. This structure has no nutrient value thus the name change from yolk to umbilical vesicle

1. Structure does not contain any yolkii. There are two stages of the yolk sac

1. Primary yolk sac2. Secondary, or definitive yolk sac (umbilical vesicle)

f. Connecting stalk is going to be comprised of extraembyonic mesoderm and some intraembryonic mesoderm

i. Mesoderm is going to give rise to connective tissues amoung other things1. Functional cells of the mesoderm is sometimes called the mesenchyme

(mesenchymal cells)ii. Extraembryonic mesoderm will be cells that create CT and blood vessels outside

of the embryo properiii. Intraembryonic mesoderm is going to give rise to CT and blood vessels within

the embryo properiv. The connecting stalk will mostly be formed with extraembryonic mesoderm, but

as the intraembryonic mesoderm forms, it will go beyond the margins of the embryo and merge with the extraemryonic mesoderm

g. Mesenchymal tissue (wharton’s jelly)

13

i. Name given to the tissue of the umbilical cordii. Another term is called allantois its going to be a little divirticulum of the yolk sac

that grows into the connecting stalkiii. The allantois will become part of the urinary system (the urinary bladder)

1. In humans, the vessels that supply the allantois are going to become the umbilical vessels

iv. So we are going to develop blood vessels within the placenta (fetal blood vessels), develop vessels within the embryo proper, and we will develp vessels within the connecting stalk

v. The vessels within the connecting stalk are initially developed in other animals to vascularize the allantois

1. Acts as a kind of a waste reserve because animals that develop within shells (like birds), their waste products cannot diffuse away so they are stored in the allantois and for that process we need blood vessels to maintain the tissue

14

2. In us we do not need the allantois to function that way because we have the placenta, but the allantois’ blood vessels develop and become the umbilical vessels connecting intreamembryonic blood vessels to placental blood vessels

vi. When all of this, (connecting stalk, BVs, CT, etc) are shrink wrapped with the amnion, this becomes known as the umbilical cord

h. Trophoblasti. As stated before, the trophoblast gives rise to the placenta or chorion

ii. The important life giving portion of the chorion is the placentai. The Inner Cell mass

i. The inner cell mass is going to differentiate into an epiblast initially and a layer of cells on the blastocyst cavity of it called the hypoblast

1. Very early the epiblastic cells are going to separate themselves away from the epiblast, remain in contact with it, but lift away creating a cavity, the cells that lift away from the epiblast and form the so called “roof” of the cavity will be known as amnioblasts

2. They’re going to become the epithelium of the amniotic membrane3. The space that was created as amnioblasts lift off from the epiblast will

be the amniotic cavity4. Above, amnioblast still attach to epiblast so they will retain their contact

with it, the amniotic cavity fills with fluid (maternal tissue fluid)5. The hypoblast is on the undersurface of the eoiblst

a. These hypoblastic cells will proliferate and migrate down on the inner wall othe cytotrophoblast

15

b. These cells migrating down and away from the inner cell mass are going to become known as the exocoelomic membrane

c. So hypoblastic cells, part of a two-layered disc, are going to proliferate and migrate down the inner wall of the trophoblast creating an inner-lining to the blastocysts cavity which can now we known as the exocoelomic cavity

6. Almost as soon as the hypoblastic cells have migrated down lining the cyto-, they proliferate

a. The daughter cells come to lie in between the migrated hypoblastic cells or the exocolemic cavity and the cyto-

b. If the above picture were accurate we would see stellate cells in here in between the cyto- and what is now known as the pimary yolk sac and the amnion,

i. This space is filled with cells and tissue fluidc. Those cells now begin to streak apart so that we will have cells

applied to the inner aspect of the cyto-d. There will also be cells present on the outer aspect of the

amnione. There will also be cells on the outer aspect of the primary

umbilical vesiclef. The tissue that was known as extraembryonic mesoderm

divides into one that is lining the cyto- as one population, and another population is covering the outside of the amnioblasts

i. Kind of like the covering outside the bilaminar disc covering the outside of the umbilical vesicle (yolk sac)

g. What can we develop from mesoderm (mesenchyme)?i. CT, BV

16

ii. The extraembyonic mesoderm connects between the inner cell mass and its derivatives and the outer cell mass and its derivatives

1. The stalk of mesoderm that connects between the inner cell mass derivatives and outer cell mass derivatives is called the connective stalk

7. Quick summary of what was just discussed

a. Bilaminar disc consists of epiblast and hypoblastb. Amnioblasts come from inner cell mass (embryoblast)c. Hypoblast is a temporary tissue derived from the inner cell massd. Hypoblastic cells proliferate and migrate down the inside of the

blastocyst cavity (cyto-)e. Those migrated hypoblastic cells proliferate and give rise to

rather large stellate cells (spider-like cells) which says that this area between the extracoelomic membrane and the cyto- is not going to be solid with cells

i. There will be cell processes and fluid in there And the cells that have been created are known as extraembryonic mesoderm

f. Extraembyonic mesoderm will develop all the way around, lining the inside of the trophoblast

i. It will cover the epithelium of the amnion (the amnioblasts)

ii. It will get the edges of the embryonic disciii. It will lie between extracoelomic membrane and

cytotrophoblast

17

iv. In vivo, large spaces are fluid filled with cytoplasmic processes holding things together temporily

v. The extraembryonic mesoderm will break down leaving a layer on the inner aspect of the trophoblast as well as a layer on the outer aspect of what will become the amnionic membrane.

vi. The extraembryonic mesoderm will also cover the outer aspect of yolk sac structures

g. One area where the extraembryonic mesoderm does not break down is going to connect between the inner cell mass derivatives and the outer cell mass derivatives and will become the connecting stalk

h. What is the importance of the extraembryonic mesoderm?i. It is going to give rise to CT and BV

i. Why do we need CT?i. Simply, to hold things together

j. Does the inner cell mass have any inherent strength of its own?i. No because it is all cellular

ii. But the syn and cyto are all cellular as well you say…1. Thus extraembryonic mesoderm is needed to

give rise to CT that will now give support to epilelial like tissue (cellular tissue)

2. Adding strength to the components, BV will develop so when we talked villus formation the other day we said we had primary villi with cyto- and syn-

3. Seconday villus have extraembryonic mesoderm grown in

4. Tertiary villus have BV in there from the extraembryonic mesoderm

iii. Fetal blood vessels develop into the connecting stalk to vascularize the allantois so they become the umbilical vessels, and blood vessels also develop within the embryonic disc itself from the intraembryonic mesoderm

k. So extraembryonic mesoderm covering the outside of the structures derived from the inner cell mass, the bilaminar disc, amnion, yolk sac, and lining the inside of the trophoblast

i. We combine all that above and call it chorion8. As the amnion enlarges, it begins to reflect from the embryo fetus down

along the connecting stalk and changes that connecting stalk into an umbilical cord

18

9. So as the amniotic membrane enlarges, it grows away from the embryo fetus as it continues to enlarge, it shrinks wrap down around the connecting stalk and when the amnion has encircled the connecting stalk.

10. Ultimately the extraembryonic mesoderm of the amnion and the extraembryonic mesoderm of the chorion is going to fuse when they come in contact with each other and when well have a combined amnio-choronic membrane

a. It is this membrane (the amnio-chorionic) that will rupture when someone says there water broke

b. These two membranes function as one membrane11. The primary umbilical vesicle lined with hypoblastic cells that migrated

away from the inner cell massa. A little bit later the epiblastic cells are going to proliferate and

some of those epiblastic cells will drop down into the hypoblastic layer.

b. As these epiblastic cells invade the hypoblastic cell layer, they push the hypoblastic cells away

c. So we are forming a new layer derived from a new generation of cells on the under surface of the epiblast

i. These are cells dropping down from the epiblast co-mingling with hypoblastic cells

ii. As these co-mingling cells proliferate and are augmented with more cells from the epiblast, they push the hypoblast cells away from the embryonic disc into the primary umbilical vesicle

iii. As these new generation of cells now known as endoderm continues to proliferate they continue to push the hypoblastic cells down but they will also begin to cause a pinching in of what was the primary umbilical vesicle (primary yolk sac)

iv. As they are migrating down, pushing the hypoblastic cells ahead of them, moving down on the extraembryonic mesoderm, growth forces will cause a “necking-in” of the primary umbilical vesicle and ultimately a pinching off into a secondary yolk sac (definitive yolk sac) that is now lined with endoderm (new population of cells) and an exocoelmic cyst

d. The extraembryonic mesoderm from the chorionic cavity or extraembryonic coelom

e. We’ve replaced the original hypoblastic cells with a new generation of cells of epiblastic origin called endoderm

19

f. These endodermal cells have proliferated, migrated down pushing hypoblastic cells ahead of them which caused a pinching in (constriction) of the primary umbilical vesicle with ultimately “pops” (pinches off) leaving a secondary umbilical vesicle (yolk sac) attached to the embryonic disc and a remnant of the primary umbilical vesicle known as the extracoelomic cyst

i. The remnant may degenerate or it may stick around but it is of no use anymore

ii. A portion of the secondary or definitive yolk sac will become apart of the new individual

g. Hypoblastic tissue is a TEMPORARY tissuei. Once its job is done it is disposed of

ii. The amnion fuses with the chorion making a combined amniochorion membrane, as mentioned above

1. Hopefully smooth chorion participated at this level because sometimes implantation occurs at a certain level where placenta develops across the cervical canal

a. This is not good if you want outb. The amniotic fluid and the amniotic sac that the little one is

floating and has been surrounded in and when the uterus contracts, it creates a hydrostatic wedge which will help dilate the cervix

2. Hopefully the membrane doesn’t rupture too early so the amniochorionic membrane and the hydrostatic pressure have compressed amniotic fluid to dilate cervical canal because otherwise if membrane ruptures early, then it would be the babys head or whatever presents first that dilates the canal

a. It is a little easier on the baby’s head if the amniochorionic membrane and contained amniotic fluid under some pressure dilate the cervic

b. If we look at the allantois, it’s a diverticulum that grows out of secondary yolk sac or secondary umbilical vesicle into the connecting stalk

i. If the animal had a head around it then the waste products would collect within the allantosis.

ii. And as has been stated a billion times before we have a placenta to get rid of waste products

1. These fetal waste products diffuse back into maternal circulation (through the placenta) where it can be eliminated via gaseous exchange in lungs or liquids via the kidneys

20

2. As also mentioned previously, the allantosis will become part of the urinary system or precursor urinary system

6. Genetic problems with development and testing

a. Things obviously can go wrong with developmentb. It is possible to pick up genetic material from 2 major sources within the pregnant

uterusi. From the amniotic cavity & amniotic fluid because epidermal cells/ectodermal

cells slough off1. Other cells slough off from internal aspect of embryo and collect within

embryonic fluid2. You can collect this fluid, spin it down, do a karyotype, amplify the DNA,

and then you can tell whats going on3. Notice above they’ve upgraded the image here with an ultrasound

transducer so you get a picture and you know where the needle isii. From the placenta (specifically the villus of the placenta)

1. This can be obtained transabdominally or transvaginally.2. You simply go in and just collect the sample through biopsy procdures3. Villious sampling is more to the little one than the amniocentesis is, but

it is sometimes possible to get tissue samples at an earlier date from villous sampling than from amniotic fluid sampling

7. The development of the Inner cell mass and the embryo

21

a. There are two layers initiallyi. epiblast and hypoblast (at the second week of development)

b. The epiblastic cells proliferate and as they do some will drop down and help form intraembryonic endoderm

i. As these epiblastic cells move down they will supplant the hypoblast

c. Other epiblastic cells will collect in a line

22

i. The density become opaque and is known as the primitive streak1. At the end of the primitive streak away from the connecting stalk is an

accumulation of cells that in continuity with primitive streak known as the primitive node.

2. In order too see this you must remove the amnion which makes a dome extending into the room over the image above

ii. So epiblastic cells are proliferating and we see them accumulating in primitive streak and node area

1. Seen from the side again, as shown directly above, we see the embryonic disk, 2 layers, epiblast, hypoblast that is in the process of being moved out

2. We can also see something else forming at the end of disk away from the connecting stalk.

a. It’s the buccopharyngeal (oropharyngeal) membranei. Oro relates to oral cavity and pharyngeal relating to

pharynx and throatii. At the oropharyngeal membrane, epiblastic cells and

endodermal cells are joined together

23

iii. So at the oropharyngeal membrane it remains a 2 layered structure

3. A cloacal membrane forms at the caudal end of embryonic disks slightly later

a. Here also, epiblastic cells will become ectoderm and underlying endoderm cells are fused together and it remains 2 layers there.

b. So at the cloacal membrane and oropharyngeal membrane, the 2 initial cell layers remain fused together.

c. A third layer is going to develop elsewhere throughout the embryo in these two areas

d. Moving through the primitive streak area cells that have proliferated there will begin to lose their cell-cell adhesions, becoming freely movable, migrate away from the streak between the overlying epiblast and underlying endoderm

i. Some primitive streak cells add to the endoderm layer, but most move in between the two preexisting layers to create a 3rd layer of cells, which is depicted here as cells migrating from the surface through the primitive streak and making a primitive groove and migrating away from the area

e. Cells moving through the primitive node area will migrate up the midline and as they do so are referred to as prenotochordal cells

i. If we cut across the primitive streak, as we see on the image shown above on the right, we see epiblastic cells proliferating, cells migrating through the groove and creating the groove, some of them mixing early on mixing with hypoblast and along with other cells that have dropped down without going through the groove

ii. Another population staying between the epiblast and this combo of hypoblast endodermal cells creating a third layer of cells which we call mesoderm

iii. Epiblastic cells are proliferating and very early on some epiblastic cells drop down without paying attention to the primitive streak and node area

24

1. These cells will then take residency amoung the hypoblastic cells and proliferate

2. Other cells a couple of days later when the primitive streak/groove area has gotten going if you will, cell proliferation will occur within the epiblast

a. Cells will migrate to primitive streak/groove, lose their adhesions to each other, drop down in between the epiblast and the layer shown above as combo of hypoblast and new endodermal cells

3. So we have a population of cells occupying the space between those two preexisting layers

4. With a little further time, the epiblastic cells that have migrated into the original hypoblastic layer will continue to proliferate and continue to be augmented for awhile, shove the hypoblast out of the way, then this layer will become known as the ectoterm

5. Then we have epiblast, endoderm, and this new forming layer of mesoderm

iv. Epiblastic cells, that stay at this layer will become known as ectodermf. So we have had cells proliferating, some of them moving into the primitive streak and

some going out to form the mesodermi. Other cells moving through the cranial end of the primitive groorve (AKA

primitive node) come right up the midline and become the notochord1. Thus the primitive groove, node, and pit will form the mesoderm2. Cells that migrate right up to the midline will become the notochord

a. There is controversy over how the notochord formsi. Book says that it will migrate out of the primitive pit

area, move up as a tube of cells, the tube of cells will then merge with the underlying endoderm, open up to form an intermediate structure, and then close up to form the definitive notochord

ii. Other people say just some cells migrate through the primitive node area, do not form a tubular structure, but just migrate forward occupying space within the hypoblastic endodermal combo layer

iii. The end point is you do end up do either way mentioned above is the formation of a prenotochordal plate—which is a collection of specialized cells that are within the endodermal layer

iv. The endodermal plate will then fold and become definitive notochord

g. The prechordal plate and discrepancy

25

i. Books will refer to the precordal plate group of migratring epiblastic cells that localize immediate caudal to the oropharyngeal membrane

1. Our book says the oropharyngeal membrane and the prechordal plate are the same thing

2. Think, what do these words mean?a. The author from the older book does not have the right

terminologyi. The oropharyngeal membrane in the book are

ectoderm and endoderm fused together, which is referred to as the prechordal plate

b. There is early migration of cells that migrate and localize between the epiblast and endoderm-hypoblast

i. That collection of cells with the newest terminology would be the prechordal plate

26

3. The prechordal plate is called prechordal because when the notochord forms out of the primitive node area it will grow toward this area and be stopped by prechordal plate

a. The prechordal plate is also the organizing center that helps guide and direct development of the head and part of the neck

b. The notochord directs development of the head and the rest of the body

ii. Most recent books say the prechordal plate is early migrating group of cells that migrate before the notochord process, take up position, and help direct development of the head region

1. As mentioned before…the notochord grows up to them and stopsiii. As a review epiblastic cells that migrate away in between the preexisting layers

become mesoderm. Epiblastic cells that did not migrate become ectodermiv. Regardless of how the notochordal plate is formed the important thing is there

is a migration of cells that move forward and form an notochordal plate1. The prechordal plate is actually the roof of the yolk sac amongst the

endodermal cells2. The notochordal plate will ultimately swing together, close along its

length, starting cranially and progress caudally, becoming a solid rod of cells

3. These cells will then have both strength and controlling properties, controlling the development of the tissues around them

v. Going right up the midline of the cells related to the notochord and its development

27

1. Some cells become mesoderm, as we have seen2. The cells seemingly know what type of cells they will end up as3. Those that go right up the midline become the notochord4. Epiblastic cells that come through the anterior portion (or cranial

portion) of the primitive groove and are next to the midline become paraxial mesoderm

a. Which will give rise to vertebrae, muscles of the body, etc5. Intermediate mesoderm moves from epiblast through the primitive

streak, up and becomes the urinary and reproductive system6. Lateral mesoderm is derived from epiblast and moves through primitive

groove and goes back into the connecting stalk

28