Placenta and Implantation Flashcards
Describe the main stages in the development of the physical link between mother and fetus.
• In the earliest stages of pregnancy the anatomical link between mother and foetus develops through a series of phases. The sequence of events is:
1) Invasion of conceptus to endometrium
2) Decidualisation ie endometrial remodeling including secretory transformation of the uterine glands, influx of specialised uterine natural killer cells, and vascular remodeling.
3) Placentation ie placenta formation
Describe the placental blood supply.
• Maternal blood
– Enters placental sinuses/pools via uterine artery.
– Flows through sinuses.
– Exits via uterine veins,
• Foetus blood
– Flows into capillaries of choronic villi via umbilical afteries
– And back to foetus via umbilical vein
(no mixing of the two blood supplies in
health)
• Umbilical cord connects foetus to placenta.
State each of the following occurs, following ovulation.
- Fertilisation occurs : 24 hours post ovulation
- Transport to uterus: 3 – 4 days
- Formation of blastocyst: 4.5 days
- Attachment: 7-9 days
Describe the events occurring to the blastocyst following its formation.
END OF WEEK 1 (ATTACHMENT AND IMPLANTATION)
• Day 6/7 the blastocyst leaves the zona pelucida and is bathed by uterine secretions for 2 days:
– progesterone prepares supportive uterine environment increasing glandular tissue
– oestrodiol is required to release the glandular secretion
• Attachment and Implantation:
– very limited time window (has to occur before next menstrual cycle happens)
– complex interactions between trophoblast and maternal epithelial tissue:
a) causes syncytiotrophoblast cells ‘flow’ into the endometrium
b) causing oedema, glycogen synthesis and increased
vascularisation (decidualisation) (all to provide nutrients and oxygen and support to fertilised blastocyst). The pregnant endometrium is now termed the decidua.
DAY 13 (FURTHER IMPLANTATION/TIME THE WOMAN EXPECTS HER NEXT PERIOD)
- Syncytiotrophoblast cells erode through the walls of large maternal capillaries which then bleed into the spaces - primitive placental circulation (Breakthrough bleeding may occur)
- Growth in the embryonic disk is slow and it remains very small (0.1 - 0.2 mm)
PLACENTAL DEVELOPMENT
As the Syncytiotrophoblast cells erode through the walls of large maternal capillaries, what does nutrition depend on ?
Nutrition still depends on uterine secretion and tissues.
Describe the development of the placenta.
- 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
- Maternal and fetal circulations are separated by the placental membrane. There is no mixing of maternal and fetal blood
- Syncytiotrophoblast is bathed in maternal blood
Describe the end result of placental development.
- Embryonic portion of placenta supplied from outermost layers of trophoblast cells (ie the chorion).
- Maternal portion by endometrium underlying the chorion.
- Choronic villi * extend from chorion to endometrium.
- Villi have network of capillaries – part of embryo’s circulatory system.
- Endometrium around villi is changed by enzymes and paracrine agents so each villi is surrounded by a pool/sinus of maternal blood.
Describe how the embryo derives nutrients and O2 in the first semester.
Limited embryonic growth (complex differentiation) - nutrition of the embryo is largely based on uterine secretion and tissues.
Identify the main contents of chorionic vili.
- Branches of the umbilical artery and umbilical vein grow into the inner core of extraembryonic mesoderm
- Middle layer of cytotrophoblast
- Outer layer of syncytiotrophoblast
How thick should the endometrium be for successful implantation ?
Endometrium should be at least 8mm thick for successful implantation.
What factor accounts for early pregnancy losses ?
Lack of appropriate hormonal support (luteal phase defect) may account for early pregnancy losses.
Describe the evolution in levels of estrogen and progesterone in the follicular phase, and luteal phase.
Steady, low levels of estrogen and progesterone in follicular phase. When one follicle chosen to mature fully then have increase in estrogen levels (then LH surge occurring and then ovulation occurring a little time after that).
If fertilisation occurs, takes few days to be transported into uterus then needs to be attached into maternal tissue before estrogen and progesterone levels fall too much and the next menstrual cycle starts.
Describe the evolution of LH levels in the luteal phase. What is the implication of this on attachment ?
LH supports coprus luteum for about 10 to 12 days and then regresses after that and new menstrual cycle if no fertilisation.
Attachment needs to happen before LH stops supporting CL.
Explain the function of the human chorionic gonadrotropin.
- In a non-fertile cycle the CL will fail after 10 days and menstruation will occur
- An implanting embryo must prevent menstruation, through production of hCG.
- hCG mimics the action of LH and maintains progesterone secretion from the corpus luteum (and therefore prevents both menstruation and any further follicular development) until the placenta can synthesise its own progesterone.
- hCG stimulates the Leydig cells of male fetuses to produce testosterone - important for development of the male duct system
Where and when is hCG produced ?
Syncytiotrophoblasts secrete hCG soon after implantation (peaks ~8-10 weeks of gestation)
