3. Placentation and implantation Flashcards
In the earliest stages of pregnancy the anatomical link between mother and foetus develops through a series of phases. The sequence of events is…
INVASION of conceptus to endometrium
DECIDUALISATION i.e. endometrial remodelling
PLACENTATION i.e. placenta formation
What are the different changes that occur during endometrial remodelling in decidualisation?
Secretory transformation of the uterine glands
Influx of specialised uterine natural killer cells
Vascular remodelling.
Where is the embryonic portion of the placenta supplied from?
Outermost layers of trophoblast cells (i.e. the chorion)
Where is the maternal portion of the placenta supplied from?
Endometrium underlying the chorion.
Chorionic villi extend from…
Chorion to endometrium
How is each chorionic villi surrounded by a pool of maternal blood?
As the endometrium that surrounds the villi is changed by enzymes and paracrine agents.
What is the blood supply to the placenta?
Maternal blood:
- Enters placental sinuses/pools via uterine artery
- Flows through sinuses
- Exits via uterine veins
Foetus blood:
- Flows into capillaries of chorionic villi via umbilical arteries
- and back to foetus via umbilical vein
The umbilical cord connects foetus to placenta
Implantation and placentation timeline post ovulation?
Fertilisation occurs: 24hrs post ovulation
Transport to uterus: 3-4 days
Formation of blastocyst: 4.5 days
Attachement: 7-9 days
Day 6/7 the blastocyst leaves the zona pelucida and is bathed by uterine secretions for 2 days, what is the purpose of this?
- Progesterone prepares supportive uterine environment increasing glandular tissue
- Oestrodiol is required to release the glandular secretion
During attachment and implantation, what interactions occur between the trophoblast and maternal epithelial tissue?
- Syncytiotrophoblast cells flow into the endometrium
- Oedema, glycogen synthesis and increased valcularisation (decidualisation). The pregnant endometrium is now termed the decidua
What is the decidua?
The pregnant endometrium
Day 13 of implantation is also the…
time the woman expects her next period
What are the events at Day 13 of implantation?
- Syncytiotrophoblast cells erode through the walls of large maternal capillaries which then bleed into spaces. Giving a primitive placental circulation.
- Nutrition still depends on uterine secretion and tissues
- Breakthrough bleeding may occur
- Growth in the embryonic disk is slow and it remains very small
Placental development:
Syncytiotrophoblast forms ____ that
project into the ____ filled spaces
(chorionic villi). In the core of the villus is a ____ capillary loop - dilated at the tip (slow flow rate)
Embryonic placental structure develops over several weeks. The villi eventually becoming localised at the embryonic ___ and presenting a huge surface area for exchange of __, nutrients and
waste ______.
Maternal side of the placental circulation is ________ and is not functional until 10 – 12 weeks
First trimester embryo largely dependant on _____ tissues for nutrients and O2.
Placental development:
Syncytiotrophoblast forms villi that
project into the blood filled spaces
(chorionic villi). In the core of the villus is a fetal capillary loop - dilated at the tip (slow flow rate)
Embryonic placental structure develops over several weeks. The villi eventually becoming localised at the embryonic pole and presenting a huge surface area for exchange of O2, nutrients and
waste products
Maternal side of the placental circulation is restricted and is not functional until 10 – 12 weeks
First trimester embryo largely dependant on uterine tissues for nutrients and O2
Why is there limited embryonic growth in the first trimester?
Nutrition of the embryo is largely based on uterine secretion and tissues
In the first trimester, name 2 placental causes for pregnancy loss?
Lack of appropriate hormonal support (i.e. luteal phase defect)
Endometrium should be at least 8mm thick for successful implantation
What forms the secondary villi?
An inner core of extraembryonic mesoderm (enters the primary villi)
A middle cytotrophoblast layer
An outer syncytiotrophoblast layer
What forms a primary villi?
A core of cytotrophoblast covered by multinucleated syncytiotrophoblast
After ovulation, what supports the steroid secretion of the CL for 10-12 days until attachment occurs?
LH
Role of hCG?
Maintains progesterone secretion from the CL until the placenta can synthesise its own progesterone.
What secretes hCG soon after implantation?
Syncytiotrophoblasts secrete hCG soon after implantation (peak at 8-10 weeks gestation)
Role of hCG?
Mimics the action of LH and supports the steroid synthesis of the CL.
Prevents both menstruation and further follicular development.
In males: Stimulates the Leydig cells of male fetuses to produce testosterone. Important for development of the male duct system
What happens to hCG level in a non-fertile cycle?
Low so CL will fail after 10 days and menstruation will occur
Detecting of hCG?
From day 6-7 post-fertilisation, hCG can be detected in maternal blood by immunoassay
Commercial kits are sensitive enough to detect hCG in urine after 14days.
Evolution of placenta from 1st to 3rd month?
1st month: Villus formation
2nd month: Increasing SA and circulation
3rd month: Growing, becoming increasingly efficient
Loss of cytotrophoblast from placenta?
Most molecules can pass through the membrane, but after 20 weeks the placenta membrane thins even more with the loss of cytotrophoblast.
___ trimester syncytiotrophoblastic cells may be lost into the maternal blood
3rd trimester syncytiotrophoblastic cells may be lost into the maternal blood
Placental function
Combination of interlocking foetal and maternal tissue
Serves as the organ of exchange between moth and foetus for remainder of pregnancy
Acts as foetal gut, lung, kidney and endocrine gland?
How does the placenta act as the foetal: Gut? Lung? Kidney? Endocrine gland?
Gut: Supplying nutrients
Lung: Exchanging O2 and CO2
Kidney: Regulates fluid volumes and disposing of waste metabolites
Endocrine gland: Synthesises steroids and proteins that effect maternal and foetal metabolism
Why is progesterone key for maintaining pregnancy?
- Suppression of follicular growth and ovulation
- Suppression of the immune response
- Maintenance of endometrium
At which point is the CL not requires anymore to maintain pregnancy?
Week 5. Placenta is secreting all steroid hormones required for pregnancy
What is the substrate from the maternal circulation for progesterone production?
Cholesterol
What is the main oestrogen in pregnancy?
Oestrodiol
What is responsible for oestrodiol secretion?
Feta-placental unit
Functions if oestrogen in pregnancy?
- Stimulate continuous growth of uterine myometrium
- Stimulates growth (with progesterone) of ductal tissue of breast
- (Along with relaxin) relaxes and softens maternal pelvic ligaments and symphysis pubis of pelvic bones. Allows expansion of uterus
- Stimulate LDL cholesterol uptake and activity of P450 enzymes. Contributes to progesterone synthesis
What nutrients are exchanged across the placenta?
Water and electrolytes diffuse freely
Glucose passes via facilitated diffusion. Foetus has little capacity for gluconeogenesis. (Diabetic mothers have heavy babies, babies store glycogen in liver for postnatal requirements)
Amino acids are actively transported for foetal growth
Lipids cross as free fatty acids
Vitamins
Fetal ______ has a greater affinity for O2 than adult ______
Fetal haemoglobin has a greater affinity for O2 than adult haemoglobin.
Gas exchange ability towards end of pregnancy?
Decreases and placenta is less able to meet demands of the foetus
Cause of ectopic pregnancy?
As a result of adhesions caused by pelvic inflammatory disease (PID) or endometriosis
Where do ectopic pregnancies occur?
Ampulla or isthmus
