Functions and Roles of the Placenta

7/30/2019 Functions and Roles of the Placenta

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Functions and Roles Of The Placenta

For nine months the placenta feeds and nourishes the fetus while also disposing of toxic waste. Without

it the baby could not survive. After your baby is born, the placenta no longer serves a function. Among

organs, it is unique. It is the only organ in the human body that serves a vital function and then becomes

obsolete.

What does the Placenta do?

The placentas primary role is to ensure that oxygen is moved into your babys blood stream and carbon

dioxide is carried away from your baby however the waste is not limited to oxygen and also includes

cleaning out other waste which is produced by your baby. In the same way that it ensures oxygen

reaches your baby, it also plays a role in ensuring that some nutrients are received.

The placenta is an extremely complex piece of biological equipment. It is a little bit like an artificialkidney,it allows your blood and the baby's to come into very close contact - but without ever mixing.

This enablesyour blood to pass across nutrients and oxygen to the baby, and waste products like carbon

dioxide to goback from baby to mother. It acts as the lung, kidney and digestive system for the baby.

The placenta also plays an important role in hormone production. Human chronic gonadotropin,

orhCG is produced by the placenta. This hormone can be found in your babys blood stream as early as

10 days into your pregnancy. This is of course not the only hormone which the placenta produces as it is

also responsible for the production of estrogen and progesterone .

The placenta also performs the important function of protecting your baby for possible infection

however, it is not always able to distinguish between what is a good substance and what isnt and this

is why pregnant women are asked to avoid substances which may cause harm, such as caffeine, alcohol,

herbal substances and drugs.


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How big does the Placenta get?

During the course of your pregnancy we follow the growth and development of your baby closely, but

we never look at how the placenta is growing.

By week 10 of your pregnancy the placenta would already weigh in at ounce (20g) and by week 20

itwill already be weighing in at 6 ounces (170g). At 30 weeks it's weighing 15 ounces (430g) and by

thetime your pregnancy is full term the placenta would weigh 1.5 pounds (650g)!

The Placenta & Possible Complications

Unfortunately as important as the placenta is, it is also possible of causing complications, and the two

most common complications are placenta previa and placenta abruption.

What Is Placenta Previa?

With placenta previa the placenta lies close to or covers the cervix.

Although it is not that common, it can be very serious because of the

chance of heavy bleeding during pregnancy or even during labor. It is

not entirely understood what may cause placenta previa, although

there are risk factors which may increase the chance of it, such as

having previously had a cesarean section.

What are the symptoms of placenta previa?

With placenta previa, there is really only one major symptom and that is

painless bleeding without contractions, but this usually doesn't occur

until close to the end of the second trimester, once the cervix thins out.

Placenta previa may cause heavy bleeding, without warning.

Placenta previa occurs when the cervix starts to thin out, which causes the placenta to tear and stretch

loose, which would then result in the flow of blood.


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How is placenta previa diagnosed and what are the consequences?

Doctors usually useultras ound to determine and confirm placenta previa, as a pelvic exam may cause

heavier bleeding.

It is also more common for the baby to be in a breech position when placenta previa is present which

could cause you to give birth via cesarean section rather than naturally, which may cause heavier

bleeding.

The advantages of having a cesarean section is that the baby can be delivered and then the placenta

may be removed, minimizing blood loss and allowing the uterus to contract.

Placenta Abruption During Pregnancy

Placental abruption is when theplac enta separates from the site which it is implanted in the uterus.

If the placenta begins to detach during the course of your pregnancy there will be somebleeding as

theblood vessels which attach to your placenta will begin to detach. The larger the area which detaches,

thegreater the amount of bleeding.

What may cause placenta abruption?

Previous pregnancies with placenta abruption Hypertension (high blood pressure) Cigarettesmoking Multiple/Twin pregnancy

Is placenta abruption dangerous?

Placenta abruption is dangerous mainly because of the risk of uncontrolled bleeding or haemorrhaging

which may occur as a result. It may also lead to other possible complications, such as:

Haemorrhage and shock Disseminated vascular coagulation (DIC) - a serious blood clotting complication Poor blood flow and damage to kidneys or brain Stillbirth Postpartum haemorrhage

What are the symptoms of placenta abruption?

Vaginal bleeding Abdominal pain Uterine contractions that do not relax Blood in amniotic fluid Nausea

Thirst Feeling faint Decreased fetal movements


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Summary ofthe events in the development of the placenta.

30 hours: Beginning of mitotic divisions (blastomeres).

3 days: Morula (12-16 cells) enters uterus.

4 days: Spaces appear in central blastomeres. Fluid from uterine cavity enters. Spaces unite toform one

cavity. Inner cells pushed together. Formation ofb lastocyst with inner cell mass

(embryoblast cells) and outer ring oftrop h ob last cells (~ 50-60 cells). Zona pellucida starts to

degenerate.

5 days: Zona pellucida has degenerated and now blastocyst can increase in size (late blastocyst).

6 days: Blastocyst attaches to uterine epithelium, usually adjacent to inner cell mass. Area whereinner

cell mass is located is called the embryonic pole.

7 days: Trophoblast proliferates and differentiates into innercytotrop h ob last and outer

syncytiotrophoblast. Nutrition is by diffusion and erosion of maternal tissues.Syncytiotrophoblast

secretes enzymes to invade maternal tissues. (~45% of embryos undergo

early spontaneous abortion). Hypoblast forms; believed to occur via delamination from the inner

cell mass.

8 days: Finger-like processes of syncytiotrophoblast continue to invade. Cytotrophoblast cellsundergo

mitosis and enter syncytiotrophoblast. Secretion of hCG begins. Uterine stroma cells

become decidua cells (glycogen/lipid-laden) around blastocyst. Inner cell mass forms bilaminar

disc (upper layer called epiblast, lower called hypoblast).

Amniotic cavity appears at embryonic pole:a layer of epiblast cells is displaced toward theembryonic

pole by fluid that has begun to collect between epiblast cells. These cells, now calledamnioblasts,

differentiate into a thin membrane that separates the new cavity (amnion) from

thecytotrophoblast.Hypoblast cells give rise to layer of cells (exocoelomic membrane) lining lowercavity,

called exocoelomic cavity (to be primary yolk sac).

9 days: Cells from hypoblast give rise to loosely arranged tissue, calledext rae mb ryon icmesoderm that

surrounds amnion and primary yolk sac. Lacunae appear in syncytiotrophoblast.Endometrial capillaries

rupture, glands erode. Syncytiotrophoblast has spread to totally surroundblastocytst.

10 days: Lacunae in syncytiotrophoblast fill with maternal blood and glandular secretions.Maternalsinusoids (capillaries) anastomose with lacunae = start ofuteroplacental circulation.

Lacunar networks begin to form = future intervillous spaces. Embedding of embryo is

completed.


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11-12 days: Directional blood flow from maternal arteries to maternal veins through lacunae.Isolated

spaces appear in enlarging extraembryonic mesoderm. Epithelium of endometriumcompletely

regenerated.

13 days: Spaces in extraembryonic mesoderm fuse to form extraembryonic coelome, amnionremains

attached to chorion at connecting stalk. Extraembryonic coelome separates somatic(lines trophoblast and amnion) from splanchnic (around primary yolk sac) extraembryonic

mesoderm. Primary yolk sac decreases in size. (Secondary) yolk sac forms from hypoblast. Early

start of primary villi.Chorion: 2 layers of trophoblast and extraembryonic somatic mesoderm.

14 days: Primary yolk sac pinches off. Cytotrophoblast sends cords of cells intosyncytiotrophoblast

= primary chorionic villus. Prochordal plate forms at rostral end ofbilaminar disc (thickening of

hypoblast, site of future mouth).

16 days: Allantois forms = diverticulum from caudal yolk sac into connecting stalk.Secondary villi form

when mesoderm pushes into primary villi. Secondary villi = mesoderm +cytotrophoblast +

syncytiotrophoblast. Secondary villi cover all of chorionic sac. Tertiary villiform when mesenchymal cells

in villi differentiate into blood vessels. Vessels also form in themesenchyme of the chorion, connecting

stalk and embryo.

21 days: Embryonic blood begins to flow through capillaries of chorionic villi. Diffusion of

nutrients/wastes between maternal and embryonic circulations through walls of villi.

Cytotrophoblastic shell forms when cytotrophoblast cells proliferate and push through

syncytiotrophoblast to attach chorionic sac to endometrium. Stem villi (anchoring villi): attached

to maternal tissue via cytotroph shell. Branch villi: grow from sides of stem villi, are main site of

nutrient/O2 exchanges.

Maternal part of the placenta

There is a narrow window between days 20-24 in the ''ideal'' menstrual cycle when implantationcan

occur.The hormonally-conditioned endometrial epithelial cells contain adhesion molecules(integrin

units) that allow implantation. Correspondingly, trophoblast cells also express integrinson their

surface.Possibly, bridging ligands connect the integrin molecules of the embryo andendometrium.The

surface of the trophoblast is probably not uniform because both in vivo andin vitro, the trophoblast

attaches at the area of the inner cell mass.

The syncytiotrophoblast invades between uterine epithelial cells.This invasion in enzymatically

mediated, but the biochemical basis is not well understood.

The fibroblast-like stromal cells of the edematous endometrium swell with the accumulation ofglycogen

and lipid droplets.They are now called decidual cells. (This name -''decidua'' is Latinfor ''a falling off'' -

reflects their fate;they will be sloughed offat birth.) The decidual cells forma tightly adherent, massive

cellular matrix that first surrounds the implanting embryo and lateroccupies most of the endometrium.

This development is called the decidua reaction.


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Different terms are used to describe the different locations:

Decidua basalis: deep to the conceptus and forms the maternal part of the placenta.

Decidua capsularis: covers the conceptus.

Decidua parietalis: all the rest of the endometrium.

Many decidual cells degenerate near the embryo in the region of the syncytiotrophoblast.

Together with maternal blood and uterine secretions, they provide a rich source ofnutrition.

Another function of the decidua reaction may be to protect maternal tissue from uncontrolled

invasion of the syncytiotrophoblast.Decidua cells may also be involved in hormone

production.

A primary function of the decidua reaction may be to provide an immunologically privileged site

for the embryo.Concurrent with this reaction, the leucocytes that infiltrated the endometrial

stroma during the late progestational phase secrete interleukin-2, which prevents maternal

recognition of the embryo as a foreign body during the early stages of implantation.Failure of

implantation is a serious problem for in vitro fertilization and embryo transfers.

Circulation in the placenta

Maternal blood is discharged in a pulsatile fashion into the intervillous space by 80 to 100 spiral

arteries in the decidua basalis. It spurts toward the chorionic plate and flows slowly around the

villi, eventually returning to the endometrial veins and the maternal circulation.The maternal

arteries which open into the intervillous spaces are partially occluded by a plug of

cytotrophoblastic cells, presumably to regulate blood flow.There are about 150 ml of maternal

blood in the intervillous spaces, which is exchanged 3 or 4 times a minute.

During the first 12 weeks, the fluid in the intervillous spaces is a filtrate of maternal plasma

without blood cells. During this period, the fetus has embryonic hemoglobin which binds oxygen

under very low tension.After 12 weeks, maternal blood cells appear in the intervillous spaces,

and the fetus produces fetal hemoglobin which requires a higher oxygen tension.

Metabolism:

During early pregnancy, the placenta synthesizes glycogen, cholesterol and fatty acids, which

serve as sources of nutrients and energy for the embryo and fetus.

Transport:

The placenta has a very large surface area, which facilitates the transport of substances in

bothdirections.The surface area at 28 weeks is 5 square metres, and at term it is almost 11

squaremetres.About 5 to 10% of this surface area is extremely thin, measuring only a few microns.


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The bulk of the substances transferred from mother to fetus consists of oxygen and nutrients.

The fetus eliminates carbon dioxide and waste materials (eg., urea and bilirubin)into the

maternal circulation.

The exchange of gases occurs via diffusion.The placenta is also highly permeable to glucose,

but much less permeable to fructose and other common disaccharides.Amino acids aretransported through speciic receptors.Some proteins are transferred slowly through the placenta,

mainly via pinocytosis. The transfer of maternal antibodies (mainly IgG) is important in

providing passive immunity to the newborn.Another maternal protein, transferrin, carries iron

to the placental surface, from there it is actively transported into the fetal tissues.Steroid hormones

easily cross the placental barrier; protein hormones are much more poorly transported

(but maternal thyroid hormone gains slow access to the fetus, and fetal insulin can reduce

symptoms of maternal diabetes).

The placenta is also very permeable to alcohol and other drugs and to some viruses.These

agents can cause birth defects.

Placental hormone synthesis:

The syncytiotrophoblast is an important endocrine organ for much of the pregnancy.It produces both

protein and steroid hormones. The major placental hormones are listed below.

Human chorionic gonadotropin (hCG): Synthesis of hCGbegins before implantation, and isresponsible

for maintaining the maternal corpus luteum that secretes progesterone and estrogens.

It is the basis for early pregnancy tests.Production peaks at eight weeks and then gradually

declines. Structurally, this glycoprotein resembles LH.

Estrogens and progesterone:The placenta can produce progesterone independently fromcholesterol

precursors,and estrogen in concert with the fetal adrenal gland, as it does not containall the necessary

enzymes itself.By the end of the first trimester, the placenta produces enoughof these steroids to

maintain the pregnancy andthe corpus luteum is no longer needed.

Human placental lactogen (hPL) or human chorionic somatomammotropin (hCS): Thishormone is

similar to growth hormone and influences growth, maternal mammary ductproliferation, and lipid and

carbohydrate metabolism.

Human placental growth hormone:This hormone differs from pituitary GH by 13 aminoacids.From 15

weeks until the end of pregnancy, this hormone gradually replaces maternal pituitary GH.Its major

function is the regulation of maternal blood glucose levels so that the fetus is ensured of an adequate

nutrient supply.Its secretion is stimulated by low maternal blood glucose levels; in turn, it stimulates

gluconeogenesis in the maternal liver.

Human chorionic thyrotropin (hCT): Small amounts produced, functions similar to pituitaryhormone.


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Human chorionic adrenocorticotropin (hACTH): Small amounts produced, functions similarto pituitary

hormone.

Insulin-like growth factors: Stimulates proliferation and differentiation of the cytotrophoblast.

Endothelial growth factor:First produced by 4 to 5-week-old placenta; stimulates proliferationof the

trophoblast.

Relaxin: Produced by decidua cells;softens the cervix and pelvic ligaments in preparation forchildbirth.

Formation of blood vessels and blood:

The process:Mesenchymal cells called angioblasts aggregate to form blood islands. Small cavities

appear in the blood islands. Some angioblasts flatten to become endothelial cells and arrange

themselves around the cavities. Blood develops from endothelial cells. Vessels bud and fuse with other

vessels.

ca 15 days: Blood vessel formation begins in extraembryonic mesoderm covering yolk sac, connecting

stalk and chorion.

ca 17 days: Blood vessel formation begins in embryo.

ca 18 days: Heart begins to form (two endothelial tubes).

ca 21 days: Blood forms in vessels of yolk sac and allantois (which appears on day 16)

ca 22 days: Heart muscle begins to beat.

ca 24-25 days: Heart tubes fuse to form single tube (during lateral folding of embryo).

ca 29 days: Blood formation in embryo begins (liver, spleen, bone marrow, lymph nodes).


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Functions and Roles of the Placenta

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