03: General Embryology III

5/28/2018 03: General Embryology III

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Transcribed by Albert Cheng 3/12/14

Craniofaial Lecture 3Embryology III

Slide 62: Intermediate Mesoderm

So lets pick up where we left. We were talking about the different3 germ layers andsome of the derivatives of each one of those germ layers. Now we are still in the

mesoderm talking about caudamesoderm. We talked abouta little bit of ouraxial

mesoderm, were gonna talk more about it. But now we are into intermediate mesodermwhich is the region of the mesoderm that contribute the urogenital system. So its located

between the paraxial mesoderm and the lateral plate mesoderm. And thats the region

where the primordial germ cells will migrate into eventually.

Slide 63:

The development of the urinary system and genital system are interconnected

anatomically. The mesonephros and the mesonephric duct which are the embryonic

kidney, which are viewed here, is closely associated with the developing gonad. Thisembryonic kidney will eventually degenerate in the adult. It will be replaced by the

metaphrenic kidney and metaphrenic duct, which are present here. So both of the those

system are very closely associated during the development. Part of the mesonephros, theembryonic kidney and some of the ducts that are associated with itthe ki dney will

degenerate but some of the ducts will be reused as part of the urogenital system

differently in males and females. And you will hear more about that in the context oforgans system when you talk about reproductive system.

Slide 64: Somite development

So the somite is one of the earlyso yea I started a little earlyso the somite is the

major derivative of the paraxial mesoderm and those somites are essentially anaggregation of cells that you find on each of the neural tube that will appear as those

strucuture here that is more caudal which is known as the presomitic mesoderm. So thepresmomitic mesoderm is unsegmented and those aggregration of cells that form the

somite will appear progressively as the embryo ages.

Slide 65: Somite periodicity

So the somite, there is a periodicity in appearance along in the anterior-posterior axis

which is a process thats very highly regulated and its species specific. So each species

will have a number of somite that are very unique to the species. This process by whichthe somite are sequentially appearing is regulate by two families of molecules Notch and

Wnt signaling pathway. And the number of somite are usually a good indication toestimate the age of an embryo. As you can see here, for example, 28 somites embryo is

approximately 28 days of age for this developming embryo

Slide 66: Somite Differentiation

So the somite will differentiate into a number of components (4): Sclerotome, Myotome,Dermatome, and Syndetome. When the somites are formed, when this aggregation of cell

separate from the prisomitic mesoderm, those cells are not committed to a specific


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ineage. As the somite mature, in the most anterior part of the embryo where the first

somites tend to appear. Those cells will get progressing acquire specific fate as the

embryo gets older.

Slide 67:

So this is the appearance of somites at different time point. It appear as an aggregation ofcells without any specific commitment to any specific lineage and then those cellsthisstructure will be remodeled very extensively to give rise to different domain

(Dermamyotome and Sclerotome). The Dermamyotome is usually more dorsal than the

Sclerotome. A litte bit later, the dermamyotome will be separated into dermatome andmyotome. The dermatone being more localized in that central aspect of the somite. And

then evemtually the dermamyotome and myotome start to be visible more clearly here

rather than if the dermatone and here the more ?? ? region of the somite will differentiate

to be the Sclerotome.

Slide 68:

So to summarize the most ventral aspect of somite, once it starts to differentiate, is theSclerotome which will migrate away from this region of somite and go around the

notochord to form the cartilage vertebrae. This region here of the Dermamyotome is call

primaxial dermamyotome. Its closer to the neural tube, will give rise to the muscles of

the back. The abaxial dermamyotome away from the axis of neural tube. The abaxialdermamyotome will give rise to the muscle of the body wall, limbs, and tongue. Finally

this region here in the middle, Dermatome will give rise to the dermis itself.

Slide 69:

So the 4th

segment that I mentioned which is the Syndetome which is the portion of the

somites that will give rise to tendon form at the junction here between the Myotome and

the Sclerotome. So this is the Syndetome which is the compartment of the somite to giverise to tendon progenitors.

Slide 70So what determines cell fate within the somite. Its all about the location and where the

cell type is located with regards to surrounding tissue. So essentially an inductive

process. You have these group of cells that are not committed to anything and based ontheir location with regards to the neural tube, overlying ectoderm, lateral plate mesoderm,

the notochordthey will receive different sets of signal that will dictate their fate in the

adult.

Slide 71And thats what I was trying to show on this diagram here. The spinal cord, the notochord

hereand here you have the somite which has not yet differentiated with the different

region here closer to the axis of the neural tube, theres the primaxial muscle and abaxialmuscle. Here the dermis in the middle and the Sclerotome here. So it all makes sense

when you look at the position of those different tissues with regards to the inducer. So the

notochord for example which is located closer to the ventral aspect of the somite willsecrete a factor sonic hedgehog whichwill dictate the formationturn on a number of


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transcription factor such as Pax1here those transcription factor are not for you to

remember but location mostly. Will turn on Pax1 and turn this group of cell into

Sclerotome. So cartilage that will surround the notochord. Another set of signal is comingfrom the lateral plate mesoderm which is close proximity to the abaxial muscle and that

will turn on genes like Pax3 and MyoD and convert those cells to abaxial muscles.

Together, there is also signals coming from the epidermis, Wnt specifically which incombination with those two factors will be essential for that process. Regarding theprimaxial muscle and the dermis, signals coming from the dorsal aspect of the neural tube

are essential (NT3, Wnt1, Wnt3a). The general organization of neural tube, location of

notochord, lateral plate mesoderm, ectodermit all makes sense in that respect

Slide 72: Derivatives of the Endoderm

So thats the deeper of the 3 layer. It will form the digestive tract and respiratory tract,

tonsils, thyroid, parathyroid, thymus, liver, pancreas and gallbladder. All the glands thatare organs associated with digestive tract are also derived in part the endoderm

Slide 73:So the general concept about the development of the organs associated with the gut,

endoderm will form the lining or epithelium and the secondary element of the digestive

tubes and glands. The mesoderm which is the mesenchyme that surround the

tubesurround the tubethink about this gut tube that is surrounded by this mesodermwill be forming the connective tissue and all the smooth muscle essential for peristalsis.

The gut associated organs are generated for reciprocal interaction between the endoderm

and the surrounding mesoderm and mesenchyme. Well show you some examplesregarding liver and pancreas

Slide 74: Formation of the gut tube

Two process that takes place around this time where thegut is being formed, there is aprocess of lateral flexionwe already talked about that which is essentially the curvature

of the amniotic cavity that will come towards the ventral aspects of the embryo to

separate the gut tube from the yolk sac. Concomittant to that there is also longitudinalflexion, which is a flexion of the embryo in the anterior and posterior end that come close

together and define more specifically the digestive tract. So the two process are important

for the formation of the embryo. The lateral flexion to separate the gut from yolk sac. Thelongitudinal flexion to bring back together the anterior and posterior ends of the embryo.

Slide 75: Subdivisions of the gut tube

There are essentially 4 subdivisions (pharyngeal gut, foregut, midgut, hindgut). They all

give rise to different structure

Slide 76: Regionalization of the gut tube

So the pharyngeal gut will give rise to the lung bud which is not part of the digestivesystem but the respiratory system. The respiratory system is a derivative of the

pharyngeal gut. So that will give rise to the lung buds which are the precursor for the

lungs and give rise to the pharyngeal pouches which will be essential for forming thepharyngeal glands which is the thyroid, parathyroid and the thymus. Then we have the


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foregut that will give rise to the stomach and muscle/glands that are associated with

digestive tract, pancreas, liver and gallbladder. The midgut will give rise to the primary

intestinal loop. Finally the hindgut will give rise to the cloaca and urinary bladder


Regionalizationso how is this gut regionalize in such a way. There is believe there area number of transcription factors that are regionally expressed in the endoderm that willspecify those different regions. And those different set of transcription factors that are

shown here in color coded. Do not remember those transcription factorthats not the

purpose here. But know that this is response to gradient of retinoic acid that is comingfrom the posterior end to anterior end of the embryo that will essentially the intiate the

expression of the different transcription factors at different level and determine the fate of

the different region of the gut


In the more posterior region of the embryo, this is an exampleyouve heard of Hox

genes which provide the position the entity along the anterior posterior axiswe knowthat in the most posterior region of the gutsonic hedgehog (SHH) which is expressed

in the endoderm will provide signals in the surrounding mesenchymewhich I was

talking about this reciprocal interaction between endoderm and surrounding

mesochyme/mesoderm. SHH is expressed throughout endoderm will differentiallyactivate different sets of Hox genes in different regions of the posterior part of the

digestive tract. And in such a way allow for the initiation of different segment of the

digestive tract. So the number of the Hox genes are not important but you heard aboutHox genes and they will always do essentially the same functionestablish identity

along the AP axis. So the name is Hox is not familiar, its important in that respect

Slide 79: Development of the LiverSo a few words about the development of some of the organs associated with the

digestive tract. The liver develop from a diverticulum of the foregut which is known as

the hepatic diverticulum and as this diverticulum extends out from the foregut into thesurrounding tissue which will be mesoderm/mesenchyme that is surrounding them from

the lateral plate mesoderm. This mesenchyme that is surrounding this diverticulum will

initiate the proliferation of the endoderm so that this duct will branch out and eventuallydevelop the glandular epithelium of the liver. So this is the diverticulum here and this is a

little time where the liver expands and very rapidly the liver will occupy a large portion

of the body cavity.

Slide 80: Molecular regulation of liver inductionMolecularly, we know how the liver is specified. This is the schematic representation

here of an embryo that went through this longitudinal flexingthe head is here and

posterior end hereand here were looking at the heart region. Were looking here at theectodermwere looking here at the notochord and in yellow here is the endoderm in the

anterior region. This is the heart region which is derived from the mesoderm. So whats

happening and how do we know how this liver is forming only in that location inresponse to an inhibitory and stimulatory signals coming from the ??? region of the


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embryo. So the endodermtypically if you remove a piece of endoderm from the

embryo and put it in culture in vitro, it will spontaneously form the liver diverticulum. So

the entire endoderm has the ability to form a liver however, there are signals coming fromthe ectoderm and notochord that block this ability of the endoderm to form liver in any of

those regions. The liver will form only in this region in the hepatic field and that is

happening in response to stimulatory signals that are coming from the cardiac mesodermthat is right underneath this endoderm region here and signals like FGF2 and BMP willbe essential in promoting formation of liver in this region and only in this region. As you

know, liver is one of the rare organs that have the ability to regenerate spontaneously

when you cut it. To regenerate from other region of the endodermthats anotherstory.you have this strong inhibition coming from those tissuesin that region not

only are signals stimulating the formation of hepatic tissue but they have to also remove

this repression coming from this endoderm which is probably coming from the SHH

signal

Slide 81: Development of the Pancreas

The pancreas is an outgrowth of the endoderm muscle that is immediately past thestomach and start to be visible around the 4th

week. There is two domains of the pancreas

(dorsal and ventral) that will eventually come together and typically the secretion of

insulin will began around the 5th

month of gestation. In contrast to the liver, the

notochord promotes pancreatic development by repressing SHH expression in theendoderm. So its a little bit the opposite from what we discuss regarding the liver. Here

the notochord repress ability of endoderm to form liver. Here the notochord has a positive

signals on this specific region of this endoderm to generate the pancreas

Slide 82: Physiological Herniation

An important process that takes place during embryogenesis that is associated with

development of the gut is what is referred to as physiological herniation. Essentially, themidgut and the loop of the gut (intestinal loop) will enter through the umbilical cord

around the 6th

week of gestation. So there is a lot going on in the development of the

organs in the body cavity around that time. The embryo cannot make up for this increasein size of all these organs so one way the embryo compromises for that is to push the

intestinal loop into the umbilical cord to make more space in the body cavity to allow the

organs to grow. So this is a temporary thingthe intestinal loop moves into the umbilicalcord and around 10

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thweek of gestationprogressively this loop will retracted back

into the body cavity.

Slide 83: Development of the Respiratory System

So the respiratory system is developing as part of the respiratory diverticulum which willeventually branch out to form the lung bud and that will start to form around 4th

week of

gestation. Im not gonna go into much detail about that because Im gonna give you a

lecture in OS about the respiratory system and I will go more in detail about this process.

Slide 84:

But know the respiratory diverticulum is an outpouching of the foregut that will branchout to give the lung buds.


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Slide 85: Stages of Pulmonary Development

The lung buds will eventually differentiate into a differentphasepseudoglandularcanalicularsaccularalveolar which are referring to the

shape of the branching of the respiratory bronchioles and bronchi and the whole process

takes a long timeas you can see 40 weeks. So its one of the organ system that ismatures later in the embryo along with the nervous system

Slide 86: 4 week human embryo

So this is human embryo in the 4th

week. Looking at the head region, you see thebranchial arches here in which the neural crest will migrate. See the heart occupy a very

important position herevery predominantyou start to see the beginning of the

eyeyoull see all the somites here that are forming along this region. Counting the

number of somite you can approximately determine the age of this embryo


So the embryo already went through this flexion. You have development of the limb thatstarts to be clearly visible. The heart and the liver again occupy a very high, important

predominant position in the body cavity.

Slide 88: 6 week human embryoThis is when the intestinal loop starts to move into the umbilical cord which appears a

little larger as a result. The limb continue to grow


Few weeks later, we see the predominance of the size of head compared to the rest of the

body which is a very important characteristic of the developing embryo during those 8

weeks where the head tend to take a very high volume as compared to the rest of the body

General Embryology III

Slide 2:Growth of the organ is faster than growth of embryo itself so once there is a reasonable

ratio between organ and size of embryo then the loop can come back into the body cavity.

So here were gonnatalk about what happens in the next 8weeks after that. Very briefly,talk about the fetal membrane and placenta. Talked about birth defects and prenatal

diagnosis

Slide 3: Timeline of Human Development

Were gonna cover essentially whats referred to as the fetal period. So youve seen mostof the organs are put in place already during this 8 week period. After that, its just a

matter of expanding the different organ systems.

Slide 4: Development of the Fetus

As I was mentioning in the last slide is that this fetal period at the end of the embryonic

period, the embryo has decide essentially where the size of the head is half the size of theentire embryo itself. So the fetal period will be characterized by the maturation of the


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tissues and the organs that have already been put in place during embryonic period. There

will be a rapid growth in the body length and weight. The growth of the head will start to

slow down in such a way that progressively after 5 monthsbirth, the ratio of headversus the rest of the body is 1:3 going to 1:4 essentially.

Slide 5: 11 week human fetusSome of the most remarkable process that take place during that fetal period, in which theembryo is 5-7 cm in length. There is a swelling of the umbilical cord that is still visible

here in this case. Remember this loop that is emanating through the umbilical cord. The

digits are not fully formed but already well developed in the forelimb and hindlimb. Theskull is relatively irregular so there is still a lot of remodeling and growth taking place. So

you dont want the suture of the skull to be established too early so that there is time for

growth and expansion. Interestingly, the eyes at this stage here close to the

midlineventrally located but they are really close to the midline and they will movemore laterally later

Slide 6: 12 week human fetusThe embryo is 8cm roughly. Start to see the primary ossification center that are

established in the long bones of the limb. The external genitalia are visible. The loop has

been completely retracted from the umbilical cord and thats when we first start to see

signs of muscular activity.

Slide 7: 18 week human fetus

The outer ear are in their final position which are really lateral as they should be. Thelimbs continue to grow with the digits more and more formed. You start to see the first

hair that start to appear. This is a very smooth appearance of the top of the head as

compared to what weve seen earlier where it was more irregular in shape.

Slide 8: 28 weeks human fetus

Well rounded contour. Most organ systems are functional except the respiratory and

nervous system that are the last ones to fully mature. And birth around the 7th

month,90% of survival chancepretty high. Most of the major systems are in place at that time

Slide 9: Fetal Membrane & PlacentaSo the placenta is the primary site for exchange of nutrients and gas between the mother

and the fetus. Also the role of protection, nutrition, respiration, excretion and hormone

production. Two components to the placenta which is referred to as fetomaternal

organfetal component and maternal component. Fetal component known as the chorion

is derived from the trophoblast and the extraembryonic mesoderm. Trophoblasts are thecells that segregate from the inner cell mass that are invading the uterine wall during

implantation. The maternal component is derived from the uterine endometrium which is

the location where the embryo implants. As the fetus grows, there is an increase of needsin nutrients for the fetus and this will be facilitated by increasing the exchange surface

between the trophoblast from the fetus and the endometrium from the mother. There will

be formation of villi that increase the surface contact between the two.


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Slide 10:

So this is some examples going back to a little bit earlier this morning. We are looking at

the embryo at 13 days after implantation. I indicated the trophoblasts make the tworegions (syncytiotrophoblast and cytotrophoblast) that start to make those villi into the

endometrium from the uterine wall. And thats where here you find the extraembryonic

mesoderm here in pink that will follow those villi. You go from primary villi where it isessentially syncytiotrophoblast and cytotrophoblast to a secondary villi where there is inposition of mesoderm from the extraembryonic mesoderm between the

syncytiotrophoblast and cytotrophoblasts. So if you make a section at this level, you see

mesoderm, cytotrophoblast, and syncytiotrophoblast. The tertiary villi which is anexample hereyou start to see those capillary that form in those villi that will essential

for establishing contact between the two

Slide 11: Beginning 2nd

monthThis is an embryo around 2 months of gestation. You see here the artery from the mother

and this is all the villi that is emitted by syncytiotrophoblast and cytotrophoblast. You can

see initially that those villi appearing throughout the entire circumference of the chorioniccavity. However, they will only be maintained at one pole which is called the embryonic

pole where essentially the embryo is directly attached. This is referred to as the

embryonic pole and this is ab-embryonic pole (opposite to the embryo). But note there is

initially villi that are forming throughout the periphery of the implanted embryo and theywill be preferentially maintained in this region at the embryonic pole

Slide 12:So this is just a diagram here to show the blood coming from the fetus here that makes

those vessels that are actually form in the extraembryonic mesoderm that are establishing

contact with the blood from the mother and there will be a passive transfer of nutrients

between the two in this setting

Slide 13: Relation of fetal membranes to wall of the uterus

So I mentioned that initially the villi are forming until the entire periphery of theimplanted embryo however, there will eventually only maintained in this region where

the embryo is located. So there is 3 components to the uterine wall and endometrium.

There is the deciduas basalis here which will be discarded at birth. There is deciduaparietalis which is the other component here and then the deciduas capsularis which is

this component in which you have initially those villi but eventually those villi at the ab-

embryonic pole are eventually lost. Decidua capsularis will eventually disappear

completely. As the embryo grows, the chorion will come in close contact with the

growing amniotic cavity and the decidua parietalis and the decidua capsularis will becompletely lost. So the placenta is really formation between chorion frondosum, which is

the region at the embryonic pole where you have those extensive villi that are forming

into the decidua basalis. Here those two regions come in close juxtaposition. So theplacenta is really the juxtaposition between chorion frondosum and decidua basilis of the

endometrium


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Slide 14: Birth Defects

Few words about birth defect. They are sometime referred to as congenital malformations

or congenital anomaly. They can be structural, behavioral, functionalthey can bemetabolic disorders that are discovered at birth. Birth defects are the leading cause of

infant mortality. The major anomalies occur in 3% of live-born infants. Major anomalies

referring to anomaly that are impairing major organ system. Mino anomalies occur in15% of newborns. Those are that not life-threatening anomalies (e.g. extra digits).Genetic factors account for approximately 28% of birth defects while environmental

factors create 3-4% of birth defects. And vast majority of birth defects are multifactorial

inheritance that can be genetic or environmental or a combination of both. A vastmajority of birth defects are largely unknown

Slide 15: Risk of Birth being Induced

So when you look at risk of birth defect which is visualized here. The increase of riskover time. The highest risk of birth defect occur during embryonic period, which makes

sensethis is the period where everything is taking placeall the organogenesis taking

place.

Slide 16: Types of Anomalities

Types of anomalities can be disruptions, deformations, and syndrome which is I show

you some syndrome alreadylike DiGeorge syndrome which is a combination ofanomalies that are occurring together and that have a common cause.

Slide 17: Environmental FactorsThe role of environmental factors has been really considered as an important factor in

birth defects only relative recently. May seem like a long time 1941, but 1941 and 1961

where the first true report showing that there was a correlation between birth defect and

the use of some agent. In the case of thalidomide, which was a sedative that was given tomother during pregnancy in the 1950sthat had very dramatic effect on the development

of the limbs. Took a long time to establish this link between the sedative and the

formation of those defect limbs which was very dramatic most of the time and essentiallythe attachment of hand at the level of the shoulder without any other component of the

limb. The key report that really established a link between environmental factor and birth

defect.

Slide18 & 19:

So I put out this paper that is from 2010 that look at the higher incidence of birth defects

in the US for a period between 2004 and 2006 that show a list of the birth defect that

have a higher incidence. So those are some of the birth defects listed here. Just wannapoint out a few. The orofacial defect like cleft palate or cleft lip have a relatively high

incidents. Craniofacial defects are a very predominant type of birth defect. Another one is

heart defect. So the incidence of heart defects are relatively high. The prevalence here is16 and when you add up all that, its close to that point also12 or 13 maybe. And those

can beyou heard of Tetralogy of Fallot.ventricular septal defect is another very

important and common birth defect. Another onethe incidenceis not as high but thoseare associated with defects in the digestive system. So the CNS defectone of the most


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common is spina bifida like I indicated which can have different relation that can be life

threatening or barely noticeable. There is not as high of occurrence as the craniofacial or

heart defects. Then there is some chromosome abnormalities for example herehighlighted trisomy 21 or Down syndrome which have relatively high frequency also.

Slide 20:Here are some examples of defects. I mentioned this problem with the sedativethalidomide which was resulting when the mother took this sedative during preganancy

would lead to very high incidence of newborns that were missing essentially the different

components of limb and end up with the most extreme case the hand attached to theshoulder. Cleft lip quite common. Turner syndrome, its a chromosome abnormality

where you lack one X chromosome. Treacher Collins Syndrome which I mentioned is a

neurocristopathy defect in neural crest. Very extreme case of spina bifida where the

spinal cord is dramatically exposed. Tetralogy of fallot which shows the hypertrophy ofright ventricle, ventricular septal defect, overriding aorta, and pulmonary stenosis. For

cleft palate, there is genetic evidence that some genes that are very essential for the palate

to close but there is also if your tongue during development you will see thatif thetongue doesnt come down the palatal shelf that are closing wont be able to close and

that could be an environmental factor such as the tongue doesnt come down or remains

swollen for an extended period of time. So it can be a combination of all those factors.

Some we know, some we dont. Spina bifida can be caused by many different things(genetic factor, environmental factor)

Slide 21: Prenatal DiagnosisTo finish, just a few words about prenatal diagnosis. So there is a number of procedure

that are typically used that you are probably familiar with. So ultrasonogram, which is a

non-invasive technique that use high-frequency sound waves which is reflected on the

tissue to create images. Some of these images are here. So they can be trans-abdominallike in this casetransvaginal which offer higher image resolution typically and they are

usually performed around 18 or 20 weeks. You can assess overgrowth and development

and look for birth defect also. This is a sonogram showing you this feetone handtheheadIm not sure what that is.

Slide 22: Maternal Serum ScreeningThere is a Maternal Serum Screening which is commonly used also to look for

concentration of serum alpha-fetoprotein (AFP) which is an indicator ofso if you have

abnormally elevated level of AFP that can lead to neural tube defect such as spina bifida.

So AFP is normally produced by fetal liver and peaks at 14 weeks, and pass into the

maternal circulation via placenta. The AFP concentration increase in the serum during thesecond trimester and then it goes through a steadily decrease after the 30th

week of

gestation. So if there is a maintained high level of AFP that could be an indication of

potential birth defect. And there is a whole range of serum molecules that can tested inrelation to pregnancy.

Slide 23: Aminocentesis


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Aminocentesis is another one that is commonly used also which consist of removing

amniotic fluid from the amniotic cavity. Typically you need between 20 and 30ml of

fluid to be able to do an appropriate test which means the procedure cannot be performedbefore 15-20 weeks when the amniotic cavity has grown enough that you have enough of

this fluid to collect. In the fluid, you can look for factors such as AFP and fetal cells that

are sometime present there. And you can use those cells to do karyotyping essentially tolook for abnormalities in chromosomes. The way you can find defects such as Trisomy21 or Down syndrome. Also you can perform PCR to look for specific genetic disease.

The problem here is that you start with very small amounts of cells from this amniotic

fluid so that you have to expand those cells in culture for a few weeks. So you wont beable to get a result as fast as other technique such as

Slide 24: Chorionic Villus Sampling

which is essentially very similar to the technique I just described but in this case youdont collect amniotic fluidyou directly collect some of the villus tissue that is found in

the placenta. So you as a result end up with a larger amount of tissue and you can do all

kinds of genetic testing on those cells. So you have large amount of cells you dont needto expand for a period of time and you can have the results very fast in a few days by

extracting the DNA and doing PCR for specific genetic markers that can indicate a

specific disease.

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03: General Embryology III