10 - First trimester Flashcards
What happens in the pre-implantation stage of human development?
- Up until day 7 cleavage occurs and a blastocyst is formed.
- At day 7 the trophoblast begins to secret hylaluronidase to break down the cells to implant. The embryo rolls along the wall until it becomes in contact with the inner cell mass.
- The lacuna forms from the moms blood vessels.
What is an eptopic pregnancy?
occurs outside uterus
most commonly in Fallopian tubes
Causes abdominal pain, bleeding and slow hCG release.
Treatment if the hCG levels get really high or there is extreme pain.
Use methotrexate.
Normally end naturally.
What are the 2 types of gestational trophoblast is neoplasias?
1) benign- hyatidiform mole
2) malignant- choriocarinoma
Due to high proliferation of the trophoblast, causing high hCG levels.
What happens in embryo implantation?
- The blastocyst makes contact with the endometrium and when the inner cell mass is closest to the uterine wall, the trophoblast layer starts to secrete hyaluronidase, breaks down hyaluronic acid – an extracellular matrix ‘holding’ cells together.
- This allows the trophoblast layer to grow into the endometrium.
- At this point, it is rapidly growing and the cells fuse together to form a syncytium (nuclei lie in a common cytoplasm, no cell division).
- This part of the trophoblast is called the syncytiotrophoblast.
- The layer of intact trophoblast cells closest to the inner cell mass is the cytotrophoblast.
- The syncytial trophoblast grows until it surrounds the developing embryo and implantation is complete (approximately day 9).
Day 9
- Syncytiotrophoblast invades endometrial layer.
- It encounters blood capillaries & grows around them, digesting their walls, forming lacuna (pools of blood).
- Villi also develop, extending further into the endometrium; these later become chorionic villi.
- This enables it to derive nutrients from the mother.
- Cellular trophoblast shields ICM from the syncytiotrophoblast.
- At the time of implantation, the ICM separates from the cytotrophoblast and forms two distinct layers.
- The cavity formed is the amniotic cavity.
- ICM cells line the amniotic cavity to form the amnion.
Day 10
- ICM now has 2 layers: epiblast (upper) and hypoblast (lower).
- Hypoblast cells migrate to form a hollow sphere which is the yolk sac.
What happen in gastrulation of the human embryo?
- Cells from the epiblast layer have the amniotic cavity on the surface & migrate through the primitive streak.
- This forms the mesoderm layer (approximately day 12).
- Mesoderm cells migrate to line outside of amnion and yolk sac and inside of blastocoel.
- These two layers together will fuse later on to form the chorion.
What happens to an embryo 2-4 weeks post-fertilisation?
- Until about day 14, diffusion of nutrients from the syncytial trophoblast is sufficient to support growth but over the next couple of weeks blood vessles develop in the mesoderm of the chorion and these transport nutrients from the maternal blood to the developing embryo.
- These vessels pass through a structure called the allantois (covered in mesoderm surrounding the embryo and the chorion, so linking the two).
- Mixing of maternal and embryonic blood is prevented by a layer of syncytial trophoblast.
What happens to an embryo 4-5 weeks post-fertilisation?
The embryo tail and head start to fold.
• Amniotic cavity still exists & the chorion has fused.
• The implanted embryo is now too big for the endometrial layer and protrudes out into the uterine cavity.
• The chorionic villi on the uterine side regress (decidua capsularis) but on the opposite side they enlarge and continue to develop (decidua basalis).
• These will eventually form the placenta proper.
• At this stage, still have maternal blood flowing into/ through lacunae.
What happens to an embryo 5 weeks post-fertilisation?
- The decidua capsularis no longer takes part in gas exchange and doesn’t form part of the placenta.
- Decidua basalis has chorionic villi of the placenta.
- We’ve got development in that placenta being put down really early on to be able to support that that early growth of of the developing embryo.
What happens to an embryo 10-12 weeks post-fertilisation?
- Centre is basically fully formed.
- By the 12th-week post fertilisation, the placenta has fully formed - now a foetus.
- Developing embryo foetus is now suspended in the amniotic cavity with the fluid.
- You can see the umbilical cord (which contains the allantois, placental blood vessels and yolk stalk remnants).
- There is an onion membrane & chorion.
What are the differences in affinity for oxygen between adult & foetal haemoglobin?
- The exchange of O₂ between maternal and foetal blood is facilitated by foetal Hb having a higher affinity for oxygen than maternal Hb.
- If the oxygen dissociation curves for foetal and adult Hb are compared, at 4kPa pp O₂ adult Hb is just less than 50% saturated but foetal Hb is 80% saturated.
- It’s able to take oxygen away from the mother’s circulation because it’s got this higher affinity, which is how the baby can survive
What happens in placental circulation?
• By the 12th week, placental circulation has now been established.
- Maternal blood flows into pools (not lacunae) into which chorionic villi project.
- Blood flows into these capillaries from the embryonic heart, via the umbilical arteries.
- Although flowing in arteries it is deoxygenated.
- Once in the chorionic capillaries, the foetal blood picks up oxygen and nutrients from the maternal blood.
- Blood then flows back in veins via the foetal liver (site of foetal blood cell production).
- Arteries carry deoxygenated blood away from the foetus to become re-oxygenated in the placenta with gaseous exchange.
4 weeks from fertilisation
Heart beating
trachea, lung, liver, Pancreas there but not functioning. No kidney yet.
5 mm, 20mg
8 weeks after fertilisation
- 28mm
- 2.7g
- nail beds, sweat glands, hair follicles
- cartilage, fingers, toes
- thymus, thyroid, pituitary, adrenal glands
- bronchioles
- embryonic kidney (metanephros)
- 9 weeks: gonads differentiate
