Implantation and the Placenta Flashcards
1
Q
Summarise the events of ovulation - fertilisation - implantation.
State where they occur.
A
2
Q
What happens to the conceptus after it is formed?
A
- Cell division to ~32 cells.
- Conceptus ‘held’ in fallopian tube as oestrogen maintains contraction of smooth muscle near where fallopian tube enters the wall of the uterus.
- Conceptus undergoes a number of mitotic cell divisions (cleavage) and a morula is formed.
- Divisions are unusual as no cell growth occurs before each division, therefore the conceptus reaching uterus is the same size as original fertilised egg.
- Cells are totipotent.
3
Q
What are totipotent cells?
A
- Totipotent cells can form all the cell types in a body, plus the extraembryonic, or placental, cells.
- Embryonic cells within the first couple of cell divisions after fertilisation are the only cells that are totipotent.
4
Q
What are pluripotent cells?
A
- Pluripotent cells can give rise to all of the cell types that make up the body; embryonic stem cells are considered pluripotent.
5
Q
What are multipotent cells?
A
- Multipotent cells can develop into more than one cell type, but are more limited than pluripotent cells.
- Adult stem cells and cord blood stem cells are considered multipotent.
6
Q
Which kind of cell is used for genetic screening and why?
A
- Blastocyst is pluripotent, so a cell can be removed for testing without damage to the embryo.
7
Q
What happens when the blastocyst enters the uterine cavity?
A
- Plasma progesterone levels rise 3-4 days after fertilisation, smooth muscle relaxes and conceptus passes into uterus.
- Approximately 4-5 days after fertilisation, cavities develop between the cells.
- For ~3 days, blastocyst lies free in the uterine cavity supported by uterine secretions, receiving nutrients from it.
8
Q
What gives rise to the placenta?
A
Trophoblast (part of blastocyst)
9
Q
What gives rise to the embryo?
A
Inner cell mass (part of the blastocyst)
10
Q
What happens to the blastocyst on day 6?
A
- Day 6 - 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.
11
Q
Summarise the events of migration of the conceptus from fertilisation to implantation.
A
12
Q
What is the sequence of events in the mother-fetus link development?
A
- Invasion of conceptus to endometrium
- Decidualisation - endometrial remodeling including secretory transformation of the uterine glands, influx of specialised uterine natural killer cells, and vascular remodeling.
- Placentation - placenta formation.
13
Q
Describe the attachment and implantation of the blastocyst.
State when this happens.
A
- Day 6/7 - the blastocyst leaves the zona pellucida 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
- Complex interactions between trophoblast and maternal epithelial tissue.
- Causes syncytiotrophoblast cells to ‘flow’ into the endometrium, causing oedema, glycogen synthesis and increased vascularisation (decidualisation). The pregnant endometrium is now termed the decidua.
14
Q
Describe the events of day 13 (implantation).
A
- This is when the woman would expect her next period.
- Syncytiotrophoblast cells erode through the walls of large maternal capillaries which then bleed into the spaces - primitive placental circulation.
- Nutrition still depends on uterine secretion and tissues.
- Breakthrough bleeding may occur.
- Growth in the embryonic disc is slow and it remains very small (0.1-0.2mm).
15
Q
Describe the development of the placenta.
A
- 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 become 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.
16
Q
What is the syncytiotrophoblast bathed in?
A
Maternal blood
17
Q
Summarise the formation of the placenta.
A
- Embryonic portion of placenta supplied from outermost layers of trophoblast cells (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.
18
Q
Describe the placental blood supply.
A
-
Maternal blood
- Enters placental sinuses / pools via uterine artery
- Flows through sinuses
- Exits via uterine veins
-
Fetal blood
- Flows into capillaries of chorionic villi via umbilical arteries
- And back to fetus via umbilical vein
- Umbilical cord connects fetus to placenta
19
Q
What is the role of hCG?
A
- Maintains progesterone secretion from the corpus luteum until the placenta can synthesise its own progesterone.
- Syncytiotrophoblasts secrete hCG soon after implantation (peaks ~8-10 weeks of gestation).
- Measurable by day 7-8 post-conception.
- In a non-fertile cycle, the CL will fail after 10 days and menstruation will occur.
- An implanting embryo must prevent menstruation. The syncytiotrophoblast secretes Human Chorionic Gonadotropin (hCG).
- From day 6-7 after fertilisation, hCG can be detected in maternal blood by immunoassay. Commercial kits pick it up in urine.
- hCG mimics the action of LH and supports the steroid synthesis of the corpus luteum, and therefore 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.
20
Q
What are the functions of the placenta with respect to the fetal organs?
A
- Fetal gut - supplying nutrients.
- Fetal lung - exchanging O2 and CO2.
- Fetal kidney - regulating fluid volumes and disposing of waste metabolites.
- Endocrine gland - synthesis of steroids and proteins which affect both maternal and fetal metabolism.
21
Q
What are the functions of the placenta in the first 3 months?
A
- 1st month - villus formation.
- 2nd month - increasing surface area and circulation.
- 3rd month - growing, becoming increasingly efficient.
- Surface area of the diffusion membrane is huge. Most molecules can pass through the membrane - after 20 weeks, placental membrane thins even more with the loss of cytotrophoblast.
- 3rd trimester syncytiotrophoblastic cells may be lost into the maternal blood.
22
Q
Describe nutrient exchange across the placenta throughout pregnancy.
A
- Nutrient exchange is rapid and increases as pregnancy advances.
- Water and electrolytes diffuse freely.
- Glucose passes via facilitated diffusion. Fetus 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.
23
Q
Describe the gas exchange which occurs across the placenta.
A
- Simple diffusion of gases across the membrane is close to the efficiency of the lungs.
- Concentration gradients are influenced by blood flow rates.
- Quantity of O2 reaching the fetus is flow limited.
- Fetal Hb has a greater affinity for O2 than adult Hb.
- Towards the end of pregnancy, exchange capacity decreases and placenta is less able to meet the demands of the fetus.
