6.1: Placenta

Question 1

How long can the embryo grow in vitro without a placenta?

Answer 1

13-14 days

Question 2

What are the two main functions of the placenta?

Answer 2

1. Implantation: so the pregnancy proceeds

2. Maintaining the pregnancy: (transport, endocrine, metabolism)

Question 3

What is the role of the following extra-embryonic structures and from which cell mass are they all derived? 
A) yolk sac 
B) amnion
C) allantois
D) chorion

Answer 3

Derived from the trophoblast cells:
A) Provides nutrients until the placenta is ready to take over (degenerates after a few weeks)

B) membrane surrounding the amniotic fluid

C) waste disposal system, future umbilical cord

D) forms the placenta

Question 4

When does the development of the placenta start?

Answer 4

Embryo reaches blastocyst stage and a maternal-fetal connection must be formed to support the pregnancy

Question 5

What are the two distinct cellular layers arising from the trophectoderm that form the placenta and what does this inner cell mass cells form?

Answer 5

1. Syncytiotrophoblast
2. Cytotrophoblast
   —> these two layers become the placenta

Inner cell mass cells become the bilaminar disc: Epiblast (future ectoderm) and hypoblast (future endoderm)

Question 6

What are the major differences between the syncytiotrophoblast cells and the cytotrophoblast cells?

Include how one of the cell types nourishes the embryo prior to when links with maternal capillaries are made

Answer 6

Cytotrophoblast: layer of cells

Syncytiotrophoblast: derived from cytotrophoblasts, mesh of cell materials and one big nuclei: invades uterine epithelia and reaches uterine glands; the engulfed and degraded contents of uterine cells feed the embryo until links with maternal capillaries made

Question 7

When should implantation be complete?

How should the embryo looks at this point? (3 things)

Answer 7

By end of the second week:

1. Two cavities: An amniotic cavity and a yolk sac
2. Suspended by a connecting stalk; precursor of the umbilical cord, formed by mesenchymal cells and connects the amnion cavity and extracoelomic cavity
3. Be within the supporting sac/chorionic cavity

Question 8

What happens to the three embryonic spaces as development progresses?

Answer 8

1. Yolk sac disappears
2. Amniotic sac enlargers (fluid filled)
3. Chorionic sac is occupied by the expanding amniotic sac

Question 9

Name three ways monozygotic twins can form

Answer 9

A blastocyst can separate in many ways to produce different numbers of chorion and amnions

1. One zygote can split into two; resulting in two amnions, two chorion and separate implantation sites
2. One morula can have two inner cell masses and bilaminar discs; so there are two amnions but one shared chorion
3. One blastocyst has one bilaminar disc but TWO primitive streaks form, so there is a shared amnion and chorion between two fetuses

Question 10

Why is implantation called ‘interstitial’?

Answer 10

Since the embryo goes from the ovaries, down Fallopian tubes to the uterine cavity and then through the uterine lining and into the interstitial space

Question 11

Why is the human placenta haemomonochorial?

Answer 11

This means there’s only one layer of trophoblast cells that ultimately separates the maternal blood from the fetal capillary wall. This is important since as the placenta develops the membrane becomes progressively thinner as the needs of the fetus increase (easier diffusion between maternal-fetal blood)

Question 12

What are the three major aims of implantation?

Answer 12

1. Establishing a basic unit for exchange
2. Anchoring the placenta
3. Establishing maternal blood flow within the placenta

Question 13

Explain what is occurring at each of these three stages:

1. Primary villi
2. Secondary villi
3. Tertiary villi

Answer 13

1. Early finger-like projections of the syncytiotrophoblast
2. Invasion of mesenchyme into the core of the villus (between the epiblast and hypoblast is the mesoderm layer which eventually produces the mesenchyme)
3. Fetal vessels invade the mesenchyme core (now there is a proper blood supply to the middle of the villus)

Question 14

How would the endometrium appear histologically when it is preparing for implantation (3 things)? Which hormone is responsible for this?

Answer 14

Signalled by progesterone release from the ovary (i.e corpus luteum)

1. Thicker
2. Many glands (releasing nutrients for the fetus) and elaboration of spiral arterial blood supply
3. “Pre-decidual” cells

Question 15

What is decidualisation? What is the role of endometrial stromal cells and uterine NKC in this process?

Answer 15

The process of changing the endometrium to promote implantation of the embryo

1. Endometrial stromal cells; secrete GFs and signalling molecules
2. Uterine NKCs; Regulate the immune response against the embryo (since it has chromosomes from dad it will express foreign proteins)

Question 16

What are two consequences of an incorrect balance between the invasive force of the trophoblast and the maternal structures?

Answer 16

1. Ectopic pregnancy

2. Conditions characterized by excessive invasion

Question 17

Why is the remodelling of the spiral arteries important and how does it happen?

Answer 17

Normal smooth muscle cells and endothelial cells that line blood vessels in the uterus are replaced by the trophoblast (they’re signalled to undergo apoptosis)

This broadens the vessels forming a low resistance vascular bed that maintains the high flow required to meet fetal demands (fast diffusion rates)

Question 18

What is the most common implantation site in an ectopic pregnancy and why are they so dangerous generally?

Answer 18

In the Fallopian tubes, but can be anywhere (peritoneal, ovarian, etc)

If the embryo implants anywhere but through the uterine lining there will not be the correct spiral arteries and structures that form an appropriate placenta

Question 19

Define placenta previa, why is it less dangerous than an ectopic pregnancy but still problematic?

Answer 19

Implantation at the bottom of the uterus; not an initial problem since the correct structures for implantation are still present but as the baby grows pressure in this area can cause

1. Haemorrhaging
2. Blockage of the route out, if the whole passageway is blocked a C section may be required

Question 20

Name two ‘implantation defects’

Answer 20

1. Placental insufficiency

2. Pre-eclampsia; no meeting between the invading trophoblast cells and the spiralling arteries (maternal vessels)

Question 21

Describe the placenta in the first trimester. How does it change as the pregnancy progresses to e.g; 9 months?

Answer 21

1st trimester: placenta has been established but the “barrier” is still relatively thick and there is still a complete cytotrophoblast layer that can produce more syncytiotrophoblasts

As the pregnancy progresses the surface area increases, the placental barrier thins (ultimately to a single layer, for fast easy diffusion) and the cytotrophoblast layer is lost (very few remnants throughout the whole placenta)

Question 22

What is the role of the umbilical cord? How many arteries and veins does it hold and how do they work? Do you need all of them?

Answer 22

Acts as the transport link between the developing fetus and placental blood vessels

The umbilical arteries and veins are longer than the umbilical cord and twist around it to add strength and protection against compression and tension.

1. Two umbilical arteries: carrying deoxygenated blood from the fetus -> placenta *not impossible to survive with one. The contents are then carried to maternal lungs (gas exchange), kidneys (waste), etc to go back to the uterus and umbilical cord to the…
2. One umbilical vein carrying oxygenated blood from the placenta -> fetus

Question 23

Define cotyledons

Answer 23

Structures that transmit fetal blood and allow exchange of oxygen and nutrients with the maternal blood

Question 24

Name three metabolic things the placenta creates

Answer 24

1. Glycogen; stores maternal glucose for transfer to fetus later if necessary
2. Cholesterol; precursor of progesterone and estrogen
3. Fatty acids; synthesized from the breakdown of various fats in maternal circulation

Question 25

Where and when is hCG produced and what does it do? What can it be used for?

Answer 25

Produced during first 2 months of pregnancy by syncytiotrophoblasts and supports the secretory function of the corpus luteum so the endometrium doesn’t degrade

It’s excreted in urine and is used as a basis of pregnancy testing

Question 26

Name two conditions where hCG would be present in a non-pregnant individual

Answer 26

In cases where the individual has a trophoblast disease;
1. Molar pregnancy; a non-viable fertilized egg implants in the uterus but will fail to come to term; the trophoblast develops and invades

2. Choriocarcinoma; cancer of the trophoblast cells

Question 27

What produces relaxin? List three of its functions

Answer 27

Produced by the placenta, it’s a hormone that:

1. increases the flexibility of the pubic symphysis so the pelvis can increase in size
2. Suppresses oxytocin release thereby preventing premature labour
3. Causes dilation of the cervix before delivery

Question 28

What do progesterone and estrogen work together to do? What produces these hormones?

Answer 28

Maintain the uterine lining

Originally they are produced by the corpus luteum (part left behind by the ovum), but the placenta takes over by the 11th week (when its established)

Question 29

Describe which two hormones influence the maternal metabolism and how

Answer 29

1. Progesterone; increases maternal appetite (more nutrients)
2. hCS/hPL (human placental lactogen): increases glucose availability to the fetus by switching the mother from glucose to fatty acid metabolism (so mother doesn’t break down and use the glucose in her body), also needed for breast development for lactation

Question 30

Describe the placenta’s role in transport, including which materials are being transported and how

*include why maintenance of adequate flow is especially essential

Answer 30

Simple diffusion (high concentration on maternal side -> low concentration on fetal side); water, electrolytes, urea and Uric acid, gases (O2 and CO2, fetal O2 stores are small so maintenance of adequate flow is essential)

Facilitated diffusion: glucose (GLUT1 transporter)

Active Transport (needing ATP to move across concentration gradient): specific transporters expressed by syncytiotrophoblasts transport amino acids, iron and vitamins

Question 31

What limits the amount of gas exchange that can occur between mother-fetus?

Answer 31

The uteroplacental circulation/ amount of blood flow

Question 32

How does the fetus develop passive immunity?

Answer 32

IgG molecules are passed across the placenta for the first few months - providing immunity for everything that your mother is immune to, so that IgG concentrations in fetal plasma exceed those in maternal circulation.
(Receptor-mediated process)

Question 33

What can access the fetus via the placenta that damage fetal development? Name four examples

Answer 33

Teratogens; thalidomide, alcohol, Drugs (therapeutic and other abused drugs), smoking

Question 34

What is hemolytic disease of the newborn?

Is it common and why?

Answer 34

Since the embryo contains molecules from the father it’s not identical to the mother, e.g; if the mother is rhesus (-) (a blood factor) but the fetus is (+) from the father, so the fetal antigens in the maternal blood will cause the mother to make IgG against them, and these IgG molecules can be transferred to the fetus

Not common due to prophylactic treatment

Question 35

Name 3 infectious agents that can be transferred from maternal to fetal circulation

Answer 35

1. Rubella
2. Zika virus; also known to damage the placenta itself
3. Cytomegalovirus

Question 36

How long and how much does the placenta weigh averagely? Describe its two surfaces and their relation to the umbilical cord

Answer 36

~20cm and averagely 600 grams
It has a maternal surface facing the outside: this is composed of decidua/endometrial uterine lining, (there are different portions to the decidua that have different names according to their location and function, and they give the placenta a dark red appearance.)

Underneath is the intervillous space: a large blood filled space drained by endometrial vessels (where both sides meet!)

Underneath there are 15-20 lobules/cotyledons divided by deep channels called sulci, and each lobule is divided into smaller sections containing one villus (the villi emerging from the fetal side’s chorion) which contains fetal capillaries that bathe in the intervillous space.

The fetal surface is covered by the amniotic membrane and is where the umbilical veins and arteries spreading from the umbilical cord are visualized near the organ’s centre.

Question 37

What does the amniotic membrane do and why is it so important?

Answer 37

Secretes amniotic fluid; which provides a protective cushion for the fetus against the walls of the uterus, helps maintain pressures and temperatures, allows space for fetal growth and protects against infection

Question 38

What lies directly underneath the placenta’s amnion?

Answer 38

The placenta’s chorion, a thicker membrane which is continuous with the uterine lining

Question 39

What emerges from the chorion?

Answer 39

The villi where a system of fetal capillaries exists to allow maximum contact with the maternal blood (also known as the intervillous space)