Implantation, placentas and hormonal changes in pregnancy Flashcards
1
Q
What is the trophoblast?
A
- Cells of the blastocyst that invade the endo and myometrium (days 5-6).
- Also secrete hCG
2
Q
What is the chorion?
A
The layer that becomes the placenta
3
Q
What is the amnion?
A
The layer that becomes the amniotic sac
4
Q
Give a brief process of implantation
A
- Start with fertilised oocyte
- Divides into 2,4, then 8 cells
- Then becomes a morula, which will undergo a lot of reorganisation
- A blastocoele cavity will form and the inner cell mass orientates itself in the blastocyst
- These are very tightly regulated events with critical timings - if something goes wrong it can lead to misccarriage
- Blastocyst then hatches as it enters the uterus and implants - it loses the ZP and it will begin the next stage of implantation
5
Q
How and why does the menstrual cycle change the endometrium?
A
- In order to implant you have to have a receptive endometrium so that the blastocyst can implant
- During the follicular phase, the oestrogen levels gradually rise, causing proliferation of the endometrium
- After ovulation (caused by the LH surge), the Luteal phase starts
- The CL releases lots of progesterone, which causes the endometrium to differentiate into a secretory lining, ready for implantation of the foetus.
- If there is no fertilisation, the CL will die after 14 days, meaning that there is no P produced, causing the endometrium to shed as mensies.
- If there is fertilisation, the foetus will produce hCG, which will rescue the CL, and continue the Progesterone production.
6
Q
What are the main stages of implantation and placentation?
A
- Differentiation of the trophoblast
- Trophoblastic invasion of the decidua and myometrium
- Remodelling of the maternal vasculature in the utero-placental circulation - If not pregnant, you will have spiral arteries in the endometrium (dont have huge vessels otherwise would bleed to death when menstruate). If pregnant, there will be remodelling of vasculature to allow oxygen/nutrients etc
- development of vasculature within trophoblast - foetus has to develop its own vasculature
7
Q
When does implantation occur?
A
Day 5-6
- A very tight window of implantation - 24-36hrs
8
Q
What changes are there when implantation occurs?
A
- trophoblast produces hCG = maternal recognition of pregnancy
- Allows maintenance of the CL - P production
- Without hCG from the viable embryo, you will shed the endometrium
- Decidualisation under P
- Vital until placental steroidgenesis is established
9
Q
What is beta hCG?
A
- Basis of urinary pregnancy tests - detects beta sub-unit of hormone
- Uses a MoAb that will bind and produce a colour change
- Maximal levels by 9-11 weeks (high for remainder)
10
Q
How can b-hCG be used for monitoring?
A
- Serum b-hCG useful for monitoring early pregnancy complications such as ectopic pregnancy
- If nothing shows up on US scan, but there is hCG, you know that there is a trophoblast somewhere, just where it shouldn’t be.
- If trophoblast tissue there, but no rise in hCG -> miscarriage
11
Q
What are the functions of the placenta?
A
- Steroidogenesis - oestrogens, P, HPL, cortisol
- Provision of maternal oxygen, CHO, Fats, AAs, vitamins, minerals, Abs
- Removal of CO2, urea, NH4, minerals
- Barrier against bacteria, viruses, drugs etc
12
Q
Why is the placenta good at its job?
A
- Huge maternal uterine blood supply - low pressure (about 750ml/min)
- Huge reserve in function
- Huge SA in contact with maternal blood
- Highly adapted ad efficient transfer system
13
Q
What happens at 14-15 days?
A
- Connective stalk will become umbilical cord
- Little ball in the middle is the ICM - will grow to become the foetus
- Outside layer has cytotrophoblastic cells (base layer of placenta), these columns will grow into the maternal decidua
14
Q
How does the implantation develop over time?
A
- Most successful pregnancies will implant in the upper part of the uterus around the fundus
- It completely envelops, burrowing in and invades deeply into the maternal decidua
- As it grows it will fill the uterine cavity
- At term, you get a disc like structure of placenta
- The chorion leavae will regress and become the amniotic sac with thin fibrous layer of regressed cells on top.
Chorion frondosum will proliferate and grow into the folded structure
15
Q
How do the villi develop?
A
- Primary stem villi - folded structure of cells, layer of cytotrophoblasts will fuse to form syncytium - makes the bulk of the villi
- Secondary stem villi - get a core network of foetal capillaries in amongst this
- Tertiary stem villi - have a little arteriole feeding in, a network of capillaries, and a vein feeding out of the villus. Also have a layer of cytotrophoblast, and a thin layer of syncytiotrophoblast - this villus then sits in a pool of maternal blood.
- By the 3rd trimester you get a lot more complex folding and an increase in SA to get as much nutrient transfer as possible.
