Implantation and Placentation

Question 1

what is the trophoblast?

Answer 1

cells of the blastocyst which invade the endometrium and myometrium
differentiates into the amnion and chorion
at day 5-6
secretes betaHCG

Question 2

what is the purpose of the amnion?

Answer 2

becomes the amniotic sac:

• • homeostasis of the foetus
• • protection from external infection
• • development of vital organs - e.g. lungs and limbs

Question 3

how does the foetus develop from day 0 to a blastocyst?

Answer 3

day 0 - fertilised ovum
by day 4 it is an 8 cell morula
by day 5 - 100 cell blastocyst with a blastocoele cavity and inner cell mass

by day 5/6 the blastocyst implants into the endometrium - the zona pellucida

Question 4

what is the window of placentation - why is this important?

Answer 4

it is the time which the blastocyst implants into the endometrium which has been prepared by the high progesterone in the luteal phase

it lasts 24-36 hours on day 5/6

allows the zygote to produce betaHCG to maintain the corpus luteum and progesterone production

Question 5

what is the purpose of betaHCG? when are the maximum levels reached - why?

Answer 5

it is a monoclonal antibody which acts on the LH receptors of the corpus luteum causing it to be retained

maximal levels reached by week 9-11, as at week 11-13 there is placental steroidogenisis, and it produced progesterone which is able to maintain the endometrial lining

Question 6

why are beta-hCG levels monitored?

Answer 6

to check for signs of misscarridge or ectopic pregnancy

Question 7

what is the cause of miscarriage at the early stages?

Answer 7

chromosomal abnormalities - trisomy 9, 11 and 16

means the endometrium rejects the embryo

Question 8

what is the chorion?

Answer 8

it is the part of the trophoblast which becomes the placenta - split into two layers:

• cytotrophoblast - inner layer
• syncitiotrophoblast – outer layre

Question 9

what is the development process for the placenta?

Answer 9

first a primary stem villus
processes to a secondary stem villus - where the cytotrophoblast becomes implanted with embryonic stem cells

then becomes the tertiary stem villus - where the chronic arteries and viens push through the mesoderm

by the third trimester- there is an increase in folding and increase in surface area of the placenta - allowing increased transfer of nutrients - this is via the communicating stalk of the umbilical cord.

Question 10

what is placental CRH?

Answer 10

the hormone responsible for the placental biological clock - involved in the initiation of labour

Question 11

how does the concentration of oestrogen change with time?

Answer 11

increases - highest increase in E3, lowest in E1