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) amnionC) allantoisD) 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. Syncytiotrophoblast2. 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 disappears2. 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

Embryo goes through the uterine lining and into the ‘interstitial space’

Question 11

Why is the human placenta haemomonochorial? Why is this an important aspect?

Answer 11

This means there’s only one layer of trophoblast cells separating maternal blood from the fetal capillary wall.

As the placenta develops the membrane becomes thinner for easier diffusion as needs of the fetus increase

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 villus core
3. Fetal vessels invade the mesenchyme core (now the middle of the villus has proper blood supply)

Question 14

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

Answer 14

Progesterone via corpus luteum

1. Thicker
2. Many glands (releasing nutrients for 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

Process of changing the endometrium to promote implantation

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

Question 16

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

Answer 16

Normal smooth muscle cells and endothelial lining uterine blood vessels are signalled to undergo apoptosis and replaced by the trophoblast -> broadens the vessels -> forms
low resistance vascular bed that maintains the high flow/fast diffusion rates required to meet fetal demands

Question 17

What is the most common implantation site in an ectopic pregnancy? Why is an ectopic pregnancy unlikely to make it to term?

Answer 17

Fallopian tubes.

Absence of correct spiral arteries and structures to form a placenta (if embryo implants elsewhere than the uterine lining)

Question 18

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

Answer 18

Implantation at the bottom of the uterus; not initially problematic since there are still correct structures for implantation but as the baby grows pressure in this area can cause

1. Haemorrhaging
2. Blockage of the route out, baby may require a C section

Question 19

Name two ‘implantation defects’

Answer 19

1. Placental insufficiency

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

Question 20

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

Answer 20

1st trimester: placenta “barrier” is still thick and has a complete cytotrophoblast layer producing more syncytiotrophoblasts

As pregnancy progresses -> SA increases -> barrier thins to single layer for fast easy diffusion + cytotrophoblast layer is lost

Question 21

What is the role of the umbilical cord? Which structures are within it and what do they do? Do you need all of them?

Answer 21

Transport link between developing fetus and placental blood vessels. Has umbilical arteries and veins longer than the umbilical cord and twist around it to add strength and protection

Two umbilical arteries carry deoxygenated blood from the fetus -> placenta *can survive with one. -> contents carried to maternal lungs (gas exchange) + kidneys (waste), etc -> go back to umbilical cord -> umbilical vein carries oxygenated blood from the placenta -> fetus

Question 22

Define cotyledons

Answer 22

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

Question 23

Briefly describe three metabolic things the placenta creates

Answer 23

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

Question 24

When is hCG produced?

Answer 24

First 2 months of pregnancy

Question 25

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

Answer 25

Trophoblast diseases

1. Molar pregnancy; a non-viable fertilized egg implants in the uterus but fails to come to term; the trophoblast develops and invades
2. Choriocarcinoma; cancer of the trophoblast cells

Question 26

What produces relaxin? List three of its functions

Answer 26

Placenta

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

Question 27

When is the placenta established?

Answer 27

11th week

Question 28

Describe which two hormones influence the maternal metabolism and how

Answer 28

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

Question 29

Describe the methods (3) in which structures can pass through the placenta

Answer 29

1. Simple diffusion (high concentration on maternal side -> low concentration on fetal side); water, electrolytes, urea and Uric acid, gases O2 and CO2
2. Facilitated diffusion: glucose (GLUT1 transporter)
3. Active Transport: transporters expressed by syncytiotrophoblasts for amino acids, iron and vitamins

Question 30

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

Answer 30

The uteroplacental circulation/amount of blood flow

Question 31

How does the fetus develop passive immunity?

Answer 31

IgG molecules passed across the placenta for first few months (passing on mother’s immunity) so IgG concentrations in fetal plasma exceed those in maternal circulation (Receptor-mediated process)

Question 32

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

Answer 32

Teratogens; thalidomide, alcohol, drugs, smoking

Question 33

What is hemolytic disease of the newborn? Is this common?

Answer 33

If mother is rhesus (-) but fetus is (+) from father the mother will make IgG molecules against the fetal antigens

Not common due to prophylactic treatment

Question 34

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

Answer 34

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

Question 35

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

Answer 35

Maternal surface facing out:
-decidua/endometrial uterine lining (give the placenta its dark red appearance)

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

Underneath are 15-20 lobules/cotyledons divided by deep channels called sulci
-each lobule divides into smaller sections with one villus (emerging from the fetal side’s chorion) with fetal capillaries that bathe in the intervillous space.

Fetal surface 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 36

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

Answer 36

Secretes amniotic fluid: which

1. provides a protective cushion for the fetus against uterus walls
2. Maintains pressures and temperatures
3. space for fetal growth
4. protects against infection

Question 37

What lies directly underneath the placenta’s amnion?

Answer 37

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

Question 38

What emerges from the chorion?

Answer 38

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

Question 39

Where is the mesenchyme embryologically derived?

Answer 39

From the mesoderm (between epiblast and hypoblast)

Question 40

Why is maintenance of adequate flow from placenta -> fetus especially essential?

Answer 40

Small fetal O2 stores