82 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
What does trophoblast and inner cell mass give rise to?
- Trophoblast –> placenta
* Inner cell mass –> Embryo
Day 6 the blastocyst attaches to the endometrium adjacent to the inner cell mass (embryonic pole). Trophoblast differentiates into:
- Inner cytotrophoblast
- outer syncytiotrophoblast (loses cell
boundaries)
• Fingers of syncytiotrophoblast invade the endometrium
Day 6/7 the blastocyst leaves the zona pellucida 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 - 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 (0.1-0.2mm)
Syncytiotrophoblast forms villi that projects into….
Into the blood filler 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 becomes localised where? And presents?
• Villi eventually becoming localised at the embryonic pole
• 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 ________ for nutrients and O2
Uterine tissues
What is the maternal and fetal circulations separated by?
- Separated by placental membrane
* There is no mixing of maternal and fetal blood
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 primary villus?
- A core of cytotrophoblast
* Covered by multinucleated syncytiotrophoblast
What forms the secondary villus?
- An inner core of extraembryonic mesoderm (enters the primary villi)
- A middle cytotrophoblast layer
- An outer syncytiotrophoblast layer
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 corpus luteum until the placenta can synthesise its own progesterone
What secretes hCG soon after implantation?
Syncytiotrophoblasts
peaks ~8-10 weeks of gestation
When is hCG measurable by?
By day 7-8 postconception
Role of hCG?
• An implanting embryo must prevent menstruation
=> syncytiotrophoblast secretes Human Chorionic Gonadotropin (hCG)
• hCG mimics the action of LH and supports the steroid synthesis of CL => prevents both menstruation and any further follicular development
• hCG stimulates the Leydig cells of male fetuses to produce testosterone - important for development of the male duct system
What happens to CL in a non-fertile cycle?
CL will fail after 10 days and menstruation will occur
Detecting of hCG?
- From day 6-7 after fertilisation hCG can be detected in maternal blood by immunoassay
- Commercial kits are sensitive enough to detect hCG in urine after ~14 days
Evolution of placenta functions from 1st to 3rd month?
- 1st month - villus formation
- 2nd month - increasing surface area and circulation
- 3rd month - growing, becoming increasingly efficient
Loss of cytotrophoblast from placenta?
After 20 weeks placental membrane thins even more with 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 functions
- Combination of interlocking foetal and maternal tissue
- Serves as the organ of exchange between mother and
foetus for remainder of pregnancy
How does the placenta act as the foetal: Gut? Lung? Kidney? Endocrine gland?
- Foetal “gut”: supplying nutrients
- Foetal “lung”: exchanging O2 and CO2
- Foetal “kidney”: regulates fluid volumes and disposing of waste metabolites
- Endocrine gland: synthesises steroids and proteins that affect both 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 required anymore to maintain pregnancy? Why?
After 4-5 weeks placenta is secreting all steroid hormones required for pregnancy (CL not required after 5 weeks)
What is the substrate from the maternal circulation for progesterone production?
Cholesterol
What is the main oestrogen in pregnancy?
Oestriol
What is responsible for oestrogen (mainly oestriol) secretion in pregnancy?
Foetus and placenta (feto-placental unit) co-operate
Functions of 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 - Contribute to progesterone synthesis
How can foetal well being and placental function be measured?
By monitoring oestrogen levels
What nutrients are exchanged across the placenta?
• Water and electrolytes diffuse freely
• Glucose passes via facilitated diffusion
- Foetus has little
capacity for gluconeogenesis
- Babies of diabetic mothers are heavier than normal range
- Storage of glycogen in liver for postnatal requirements
• Amino acids are actively transported for fetal 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?
Towards end of pregnancy exchange capacity decreases and placenta is less able to meet the demands of the foetus
Concentration gradients are influenced by ______.
Quantity of O2 reaching the foetus is ______
Concentration gradients are influenced by blood flow rates.
Quantity of O2 reaching the foetus is flow limited
Cause of ectopic pregnancy?
- Adhesions caused by pelvic inflammatory disease (PID)
2. Endometriosis
Where do ectopic pregnancies occur?
95 - 97% are in the ampulla/isthmus of the tube with the vast majority in the ampulla
Where does attachment usually occur?
Posterior wall of the fundus of the uterus
What is the consequence of ectopic pregnancy?
Rupture of the tube causes blood loss that may be life threatening to mother and fatal for the embryo
What can the symptoms of ectopic pregnancy be confused with?
Appendicitis
