The placenta

Question 1

What is the difference between placenta accreta, increta and percreta

Answer 1

These are all pathological conditions due to the blastocyst implanting too deep into the uterus

placenta accreta is where invades into decidua basalis of the endometrium

placenta increta is where it invades into the myometrium

placenta precreta is where it invades through all the walls of the uterus and in some cases can extend into the bladder

Question 2

Describe the stages of cell division of zygote to form morula

Answer 2

2 stage–> 4 stage–> 8 stage–> 16 stage (16+ known as the morula)

This occurs through successive cleavage of cells with no increase in cytoplasmic mass

Question 3

How does a morula become a blastocyst

Answer 3

Water enters the morula by osmosis/diffusion and this creates cavity known as the blastocoele as it pushes the cells in morula to a pole of the cell.

This will therefore generate a inner cell mass and an outer cell mass

Question 4

Describe the three stages of implantation

Answer 4

Apposition- loose associations are formed between the blastocyst, through microvilli on blastocyst to the pinopodes in endometrium

Attachment: firm adhesion, selectins/collagen on endometrium and integrins on the blastocyst outer cell mass

Invasion: Chemokines released which stabilise the firm adhesion in the attachment phase. Invades the glycogen rich endometrial stroma by production and activation of proteolytic enzymes, can be produced by the syncitiotrophoblasts.

Question 5

Describe the formation of the sinusoids

Answer 5

The lacunae are spaces formed within the syncitiotrophoblasts (STBs)

As the STBs invade they release proteolytic enzymes that invade and erode maternal capillaries within the uterine stroma

The now erroded capillary anastamoses with the trophoblastic lacunae

these form projections of blood vessels known as sinusoids from the STBs

Question 6

Describe the formation of the pimary villi

Answer 6

Cytotrophoblats begin to invade into the STBs between the sinusoids

this creates finger-like projections between the sinusoids known as the primary villi

these primary villi are found throughout the surface of the blastocyst and so give the appearence of a ‘hairy ball’ under the microscope

Question 7

Describe the transition from primary villi to secondary villi

Answer 7

Extra-embryonic mesoderm together with the trophoblastic cells forms the chorion which is the lining of the chorionic sac/cavity

The primay villi are outpouchings of syncitiotrophoblasts

So when the cohorionic cavity is formed the extra-embryonic mesoderm will then invade the primary villi to form the core within the primary villi

This then forms what is known as the secondary villi which continues to pentetrate into the uterine stroma between the sinusoids

Question 8

Describe the transition from secondary to tertiary villi and what week are blood vessels all linked

Answer 8

Extra-embryonic mesoderm differentiate into endothelial cells

these endothelial cells form blood vessels which link up together to form aterio-capillary network

These vessels link with other developing vessels in the stalk and by the end of week 3 they are mature

Question 9

Define the chorionic plate, umbilical stalk and umbilical cord and its relation to the primary villi

Answer 9

The chorionic plate is the extra-embryonic mesodermal lining of the trophoblasts

the umbilical stalk is the only place where extra-embryonic mesoderm travserses through the chorionic cavity to connect to the embryonic disk

The umbilical cord is where blood vessels begin to develop within the umbilical cord

the umbilical cord is located inferiorly to the villi and is all connected to the primary villi through the core of the extra-embryonic mesoderm

Question 10

Describe the structural parts of the tertiary villi and the changes to the cytotrophoblasts

Answer 10

a) stem villi which is portion connecting to the chorionic plate
c) basal villi which are projections from the sides of the stem villi
d) terminal villi which are swellings at the tips of the stem villi containing terminal vessels that form the arteriovenous capillary networks and so is the area where the greatest amount of exchange occurs with the maternal vessels

the cytotrophoblasts become very thin but remain mostly intact covering about 80% of the placental surface

Question 11

Describe the specialisation of chorionic villi and what days does this occur from

Answer 11

At 10 weeks got thick STB layer and blood vessels central in fibroblastic core in cross-section so large diffusion distance

however at 3rd trimester to term the STB thins and the blood vessels become more eccentric (nearer to the edges) to lower the diffusion distance

Question 12

Describe the placental membrane

Answer 12

it is combination of the endothelium of capillaries, cytotrophoblasts and syncitiotrophoblasts in the third trimester to term of tertiary villi

in 10 weeks it is just Cytotrophoblasts and syncitiotrophoblasts mostly as the blood vessels not eccentric yet and is more thicker

Question 13

describe formation of intervillous space and the cortyden lobes

Answer 13

by about third trimester to pregnancy when got chorionic villi

the sinusoids will all connect to each other and surround each of the chorionic villi

however they are incompletely separated from one another via the placental septum and covered by the cytotrophoblastic shell

this and the underlying chorionic plate form cotylden lobes

Question 14

state the function of remodelling the spiral arteries and main benefit and number of arteries affected

Answer 14

Essential to establish a low resistance high flow blood
supply to the intervillous space
Critical for normal pregnancy
• 100 - 150 arteries are transformed
• Diameter is increased 10-fold
(200 m to 2 mm )

Question 15

what are the spiral arteries, where are they located

Answer 15

• Resistance vessels supplying the
endometrium/decidua
• Coiled appearance in the inner
myometrium and decidua

Question 16

Describe the Extravillous trophoblast outgrowth and when does this occur

Answer 16

First trimester:
• Cytotrophoblast columns form at the tips of anchoring villi
• EVT invade the decidua and occlude the spiral arteries and differentiate so those EVT that are within lumen of spiral arteries called extravascular and those outside interstitial
• Replace the endothelium and smooth muscle cells forming the
endovascular trophoblast layer
• Establish normal utero-placental dynamicsEVT plugs breakdown initiating blood flow to intervillous space >14
weeks

Question 17

state functions of spiral arteries remodelling

Answer 17

EVT plug the spiral
arteries – reduce blood
flow to the developing
placenta
Creates a low oxygen
environment – Protects
fetus from oxidative
stress
when EVT plug breaks down at more than 14 weeks converts spiral arteries from high resistance low flow to low resistance high flow

Question 18

describe process of spiral artery re-modelling, its regulation and how corrected after pregnancy

Answer 18

endovascular VT cause apoptosis of endothelial cells and will replace them taking on endothelial phenotype

Smooth muscle either migrates away in deciuda or is apoptosed and is replaced by fibrinoid material (laid down ECM)

this means got rigid vessel with large lumen and no vasoregulation so low constant resistance so low resistance high flow

regulated by the EV interstitial cells and its communication to umbilical NK cells, also regulate immune tolerance

after pregnancy it is replaced by endothelial cells so goes back to normal