REPRO: Implantation, Placentation and Hormone Changes in Pregnancy Flashcards
what is Implantation?
Requirements for implantation
blastocyst attaches to the uterine wall
Fully Developed Blastocyst
- 5th/6th day of development
- hatched out from zona pellucida
Receptive Endometrium
- thickened endometrium during the proliferative phase
- expression of embryo receptivity markers to communicate with blastocyst
Blastocyst structure
Embryoblast (inner cell mass)
-forms foetus
Trophoblast (outer cells)
-forms placenta
Blastocoel (fluid-filled cavity)
What are trophoblasts?
They are cells of the blastocyte that invade the endometrium and myometrium (day 5-6).
They secrete βhCG (human chorionic gonadotrophin).
where are the embryoblast and trophoblast concentrated?
The embryoblast is concentrated at the ‘embryonic pole’
The trophoblast cells are concentrated at the ‘abembryonic pole’
What are the chorion and the amnion?
The chorion is that which becomes the placenta.
The amnion is the layer that becomes the amniotic sac.
When and how does the blastocyst hatch from the zona pellucida?
Day 5:
- enzymes dissolve zona pellucida at aembryonic pole
- rhythmic contraction enable blastocyst to herniate and bulge out of the zona pellucida
Why is hatching essential for implantation?
because the zona coat prevents the blastocyst from communicating with the endometrium
Stages of Implantation
APPOSITION
-close positioning of the blastocyst to the endometrium (decidua basalis)
ATTACHMENT
-trophoblast cells attach to the endometrium
INVASION
-trophoblast cells multiply and invade into the endometrium, implanting embryo in endometrium
Implantation Timeline: Days 7-8
Blastocyst attaches to surface of the endometrial wall
Trophoblast cells assemble to form Syncytiotrophoblast in order to facilitate invasion of endometrium
Implantation Timeline: Days 9-11
Syncytiotrophoblast further invades endometrial wall
By day 11, the blastocyst is almost completely buried in the endometrium
Implantation Timeline: Day 12
Decidual Reaction
-high progesterone levels enlarge and coat decidual cells in glycogen and lipid-rich fluid
-fluid taken up by syncytiotrophoblast to sustain the blastocyst before the placenta is formed
Implantation Timeline: Day 14
Syncytiotrophoblast cells protrude out to form tree-like structures “Primary Villi” which are then formed all around the blastocyst
Decidual cells between primary villi clear out, leaving behind empty spaces called Lacunae
Maternal arteries and veins start to grow into the decidua basali and merge with Lacunae; arteries fill them with oxygenated blood and veins return deoxygenated blood to maternal circulation
Blood filled lacunae merge into one large one pool of blood connected to multiple arteries and veins known as the Junctional Zone (circulatory foundation for placenta formation)
Around what day after fertilisation does the placenta begin to form?
Day 17
-foetal mesoderm cells form a network of arteries, veins & capillaries around primary villi
Week 5
- capillaries formed eventually connect with blood vessels in the umbilical cord
- primary villi grow and develop into Chorionic Frondosum
- Outer decidual lining and Syncytiotrophoblast lining separate maternal and foetal red blood cells and hence the circulation
What is decidualisation?
process that results in significant changes to cells of the endometrium in preparation for, and during, pregnancy.
In humans, it happens before fertilisation even occurs.
Decidualisation happens under progesterone, so it is vital that it keeps getting released until placental steroidogenesis is established.
What are some functions of the placenta?
- Steroidogenesis - oestrogens, progesterone, HPL, cortisol
- Provision of maternal O2, CHO, fats, amino acids, vitamins, minerals, antibodies, etc.
- Removal of CO2, urea, NH4, minerals
- Acts as a barrier against, for example, bacteria, viruses, drugs, etc.
What sequentially happens during the early stages of pregnancy?
- there is differentiation of the trophoblast
- trophoblastic invasion occurs, of the decidua and the myometrium
- remodelling of the maternal vasculature in the uteroplacental circulation
- development of the vasculature within the trophoblast
When is the umbilical cord formed?
5th week
-then connected to capillaries formed by the foetal mesoderm cells which then feed into the junctional zone and supplied by maternal circulation
What happens in 4th/5th months of pregnancy to placenta?
decidual septa forms, dividing the placenta into 15-20 regions known as Cotyledons
this provides efficiency maternal-foetal exchange of nutrients due to an increase in surface area
Pre-eclampsia
Eclampsia
maternal spiral arteries are fibrous and narrow, limiting blood supply to the placenta (placental insufficiency)
usually occurs after around 20 weeks gestation
new onset maternal hypertension and proteinuria.
preeclampsia + seizures
Placental Abruption
maternal spiral arteries supplying blood to placenta degenerate and rupture, causing haemorrhage and premature separation of all or part of the placenta from the endometrium
Placenta Previa
placenta implants in lower uterus, either fully or partially covering the internal cervical os
Around week 7, hCG production is by
placenta
*also decrease in hCG due to degeneration of corpus luteum
Hormones the placenta synthesizes and releases into maternal circulation
Oestrogens from foetal androgens from foetal cortex
Progesterone from maternal cholesterol
human Placental Lactogen (hPL), making mother more insulin resistant so glucose available for both mother and foetus
