6.1 The Placenta

Question 1

In the blastocyst during the very early stages we begin with an inner and outer cell mass, what do the components of each go on to become?

Answer 1

1) Inner cells mass ➞ becomes the foetus
2) Outer cell mass ➞ becomes the extra-embryonic components

Question 2

What are the extra-embryonic components that are derived from the outer cell mass?

List their functions

Answer 2

1) Yolk sac - provides nutrients until placenta ready to take over
2) Amnion - membrane surrounding amniotic fluid
3) Allantois - waste disposal system i.e. umbilical cord
4) Chorion - forms the placenta

Question 3

What occurs during the “week of two’s”?

Once differentiation occurs what is it known as?

Answer 3

Differentiation - two distinct cellular layers emerge from:

1) Trophectoderm divides into the syncytiotrophoblast and cytotrophoblast
2) Inner cell mass divides into the epiblast and the hypoblast (becomes the ectoderm and endoderm)

Known as the bilaminar disc

Question 4

Label the following image

Answer 4

Question 5

At around day 7 “hatching occurs” what is this?

What happens after hatching?

Answer 5

Blastocyst sheds its zona pelluida (enzymes released to do this)

Once hatched the cells of the cytotrophoblast are able to make syncytiotrophoblasts and it is these cells which can then make contact and invade the uterine lining

Question 6

During day 9 what happens to the degraded uterine cells when invasion into the uterine wall occurs?

Answer 6

Degraded contents of uterine cells are engulfed by the trophoblasts and are used to feed the embryo until links with maternal capillaries are made

Question 7

List 4 things that have occured by the END of the 2nd week

Answer 7

1) conceptus has implanted
2) two cavities formed (amniotic cavity & yolk sac)
3) suspended by a connecting stalk (becomes the umbilical chord)
4) lies the chorionic cavity (a supporting sac)

Question 8

What are the 3 embryonic spaces and what happens to each throughout development?

Answer 8

1) The yolk sac disappears
2) The amniotic sac enlarges
3) The chorionic sac is occupied by the expanding amniotic sac

Question 9

List 2 ways in which twins can form

Answer 9

1) multiple eggs being fertilised
2) individual blastocyst that separates to produce a different number of chorions and amnions

Hence, the degree to which membranes are shared in monozygotic twins can vary

Question 10

Implantation of embryo in humans is described as what?

Answer 10

Implantation is interstitial - the uterine epithelium is breached and the conceptus implants within the stroma

Question 11

As foetal needs increase what can be said about the placental membrane

Answer 11

it becomes progressivly thinner

Question 12

The human placenta is haemomonochoria, what does this mean? What is the benefit of this?

Answer 12

Only one layer of trophoblast ultimately separates maternal blood from foetal capillary wall

It is as the placenta develops that the two bloods supplies get closer until we are only left with one layer. This purpose of this is to aid diffusion of materials and nutirents from maternal circulation to foetus

Question 13

What are the 3 aims of implantation and briefly explain how is each established?

Answer 13

1) establish basic unit of exchange through 3 developmental processes (primary, secondary and tertiary villi)
2) anchor placenta to uterine lining through establishment of outermost cytotrophoblast shell
3) establish maternal blood flow within the placenta

Question 14

What are the 3 specific stages of implantation and what does each achieve?

Answer 14

1) primary villi: syncytotrophoblast invades uterine lining by villi ➞ cytotrophoblast + covering of syncytotrophoblast
2) secondary villi: mesenchyme invades core of villus ➞ outer syncytotrophoblast + intermediate cytotrophoblast + inner extra-embryonic mesoderm
3) tertiary villi: foetal blood vessels invade mesenchyme core (establish BF) ➞ like secondary + blood in mesodermal capillaries

Question 15

During the second half of the menstrual cycle (after ovulation) list 3 changes that occur in the endometrium

What signals these changes to occur?

Answer 15

Endometrium prepared for implantation (regardless of fertilisation)

1) Endometrial lining thickens (increae in BVs and glands)
2) “pre-decidual” cells form
3) elaboration of spiral arterial (impt for implantation and establishment of maternal-foetal connection)

Progesterone released from the ovary signals these changes

Question 16

What is Decidualisation? List 2 signalling molecules released during this process

Answer 16

Changes to the endometrium that promote implantation of the embryo

1) Endometrial stromal cells secrete growth factors and signalling molecules
2) Uterine NK cells regulate the immune response against the non-self embryo

Question 17

What is meant by “the balancing force against the invasive force of the trophoblast”

What can errors in the balance cause

Answer 17

We must have a maternal immune response that ensures that not just anything can invade the uterine wall BUT this must be suppressed to an appropriate level to ensure we do not reject the implantation of the embryo

foetal invasion has a stronger “pushing forward” than the maternal immune reponse against

Errors can results in:

• ectopic pregnancy
• conditions characterised by excessive invasion

Question 18

What occurs during “spiral remodelling” and why is this important?

When does this occur

Answer 18

Smooth muscle and endometrial cells of the uterus undergo apoptosis and are replaced by trophoblast cells.

These cells then release their own signalling molecules that direct their function.

Ultimately this allows the formation of a low resistance vascular bed that maintains the high flow required to meet foetal demand

Occurs particularly late in gestation

Question 19

What is an ectopic pregnancy?

Answer 19

Implantation into a site other than the uterine body (commonly the fallopian tube)

can be perinatal or ovarian and is a medical emergency

Question 20

what is placenta praevia?

Answer 20

Implantation into the lower uterine segment

San cause haemorrhage in pregnancy and therfore requires C-section delivery

Question 21

Give 2 causes of implantation insufficiency

Answer 21

1) placental insufficiency
2) pre-eclampsia

Question 22

explain how pre-eclampsia can lead to placental insufficiency

Answer 22

Inadequate invasion of trophoblasts to reach maternal vessels means no materal-foetal circulation can be established

Question 23

Describe the placenta in the first trimester of pregnancy

Answer 23

• placenta established
• placental “barrier” is still relatively thick
• complete cytotrophoblast layer beneath syncytiotrophoblast

Question 24

Describe the placenta at term

Answer 24

• surface area for exchange dramatically increased (increase in villi)
• placental “barrier” is now thin
• cytotrophoblast layer beneath syncytiotrophoblast lost

Question 25

What specific structure links placental blood vessels with developing foetus?

What is contained within this structure?

Answer 25

The umbilical cord, It contains:

• 2 umbilical arteries (deoxygenated blood from foetus to placenta)
• 1 umbilical vein (oxygenated blood from placenta to foetus)

Question 26

What is the “metabolic” function of the placenta? (3)

Answer 26

Synthesis of:

1) Glycogen - storage of maternal glucose for transfer to foetus later as necessary
2) Cholesterol - precursor of progesterone and oestrogen
3) Fatty acids - synthesised from breakdown products of various fats in maternal circulation

Question 27

What is the “endocrine” function of the placenta? (2)

Answer 27

1) Protein hormones - hCG, hCcorticotrophin, hCthyrotropin
2) Steroid hormones - progesterone and oestrogen

Question 28

What is the function of hCG?

When is it produced and by what cell type?

How can this hormone be beneficial clinically

Answer 28

Human chorionic gonadotropin functions to support the secretory function of corpus luteum

It produced during the first 2 months of pregnancy by syncytiotrophoblasts (pregnancy specific)

It is excreted in maternal urine therefore used as the basis for pregnancy testing

Question 29

Give 2 examples where hCG may be raised and detectable that is NOT pregnancy

Answer 29

Raised hCG in the abcence of pregnancy may be indicative of trophoblast disease:

1) Molar pregnancy - trophoblast develops and invades even when inner cell mass doesn’t or develops very abnormally. Means there is either no foetus or life is not viable ➞ known as a hydatidiform mole
2) Choriocarcinoma - cancer of the trophoblast cells

Question 30

List 4 functions of the hormone relaxin

Answer 30

1) Increases flexibility of pubic symphysis
2) Increases size of pelvis
3) Supresses oxytocin release thereby preventing premature labour
4) Causes dilation of cervix before delivery

Question 31

What are the 2 main steroid hormones and what secretes them?

What is their function?

Answer 31

progesterone and oestrogen (early stages produced by the corpus luteum, by 11th week placenta takes over production)

Responsible for maintaining the pregnant state (progesterone is dominant in the beginning to build up the lining and oestrogen is dominant towards the end to help maintain the lining)

Question 32

How may placental hormones influence maternal metabolism?

Answer 32

1|) Progesterone - increased appetite

2) hCS / hPL - increases glucose availability to foetus by switching mother from glucose to fatty acid metabolism

Question 33

List the 2 ways that placental transport may occur

Answer 33

1) Simple diffusion - molecules moving down conc gradient (water, electrolytes, urea, gases)
2) Facilitated diffusion - applies to glucose transport

Question 34

How does gas excahnge occur across the placenta?

Answer 34

simple diffusion - flow limited meaning the rate at which gas can diffuse is dependant ONLY blood flow

foetal O2 stores are small therefore maintenance of adequate is flow essential- must be adequate uteroplacental circulation

Question 35

How are amino acids, Iron and vitamins transported to the foetus

Answer 35

Via active transport - there are specific “transporters” expressed by the syncytiotrophoblast

Question 36

How does the foetus aquire passive immunity and what is meant by this term?

How do the levels of these antibodies compare in a mother and foetus?

Answer 36

Foetal immune system is immature. Immunoglobulin class-specific are transferred across the placenta through a receptor-mediated process

IgG only!!! ➞ IgG conc in foetal plasma exceed those in maternal circulation

Question 37

The placenta is NOT a barrier for everything, some things can pass that have negative effects on the foetus. What are these substances called and give an example of 4

Answer 37

Teratogens can access the foetus via the placenta

Example of teratogenic substances:

1. thalidomide
2. alcohol
3. therapeutic drugs
4. drugs of abuse
5. maternal smoking

Question 38

How could a baby develop a hemolytic disease as a newborn (eg. rhesus disease) in terms of antibodies during pregnancy.

Why is this very uncommon today?

Answer 38

If mother is rhesus negative and baby is rhesus positive (from father), then the foetal antigens in the mothers blood will cause the mother to produce antibodies against the foetal antigens (Rhesus blood group incompatibility of mother and foetus)

For a first baby this will not cause harm as these antibodies take a while to develop… however, if second baby is also rhesus positive then the antibodies in the mother developed previously will cause hemolytic disease to the newborn.

This is NOT common nowadays as we have prophylactic treatment. Testing for rhesus is usually done at around weeks 11-12. If baby is rhesus positive and mother is negative then an anti-D immunoglobulin injection will be given to the mother.

Question 39

List 4 infectious agents (bugs) that can cross the placenta into the foetal circulation?

Why is the one begging with Z of particular concern

Answer 39

1) Rubella
2) cytomegalovirus
3) Varicella zoster
4) Zika virus ➞ this is of particular concern because not only does it cross placenta it also causes damage to the placenta itself