Fertilisation and placental development Flashcards
Outlien the stages involved in maturation from a zygote to a blastomere
- zygote
- 2 cell stage
- 4 cell stage
- 8 cell stage
- morulla (72 hours)
- blastocyst (4 days)
What happens during day 4/5?
- The morula develops a cavity and becomes known as a blastocyst
- Blastocyst things out and becomes the trophoblast (the start of the placenta)
- The rest of the cells move (pushed up) to form the inner cell mass. This creates the embryonic pole
- The blastocyst has now reaches the uterine lumen and is ready for implantation
What happens during day 6/7?
- Inner cell mass differentiates into two layers: epiblast and hypoblast
- These two layers are in contact
- Hypoblast forms extraembryonic membranes and primary yolk sac
- Epiblast forms the embryo
- Amniotic cavity develops within the epiblast mass
How does the adult pattern and germ layers arise?
- The hypoblast degenerates. The epiblast gives rise to all three germ layers
- The embryo folds to form the adult pattern
What happens from day 16?
Bilaminar disc develops further by forming 3 distinct layers (gastrulation)
- Initiated by primitive streak
- The epiblast becomes known as ectoderm
- The hypoblast is replaced by the cells from the epiblast and becomes the endoderm
- The epiblast gives rise the third layer (mesoderm)
Describe the appearance of the cell mass from day 16
Syncytiotrophoblast surrounds cell mass
Amniotic cavity surrounded by three germ layers
Bilaminar embryonic disc
Outline the formation of the placenta
- Syncytiotrophoblast invade decidua (endometrium)
- Cytotrophoblast cells erodes maternal spinal arteries and veins
- Spaces (lacunae) between them fill up with maternal blood
- Followed by mesoderm that develops into fetal vessels
- Aiding the transfer of nutrients, O2, across a simple cellular barrier
Describe the differences between synchytiotrophoblast cells (STBs) and Cytotrophoblast cells (CTBs)
CTBs
- undifferentiated stem cells
- invade the maternal blood vessels and destroy the epithelium
- give rise to the syncytiotrophoblast cells
- reduce in number as pregnancy advances
STBs
- fully differentiated cells
- have direct contact with maternal blood
- produce placental hormones
Describe the placenta as an endocrine organ
Human chorionic gonadotrophin (HCG)
- maintenance of corpus luteum or pregnancy
- progesterone and oestrogen
Human placental lactogen (HPL)
- growth, lactation
- carbohydrate and lipid
Describe the placental barrier and how it changes during pregnancy
What substances need to cross it?
Maternal blood in the lacunae in direct contact with STBs
Monolayer of STB/CTB/ fetal capillary epithelium is all that seperates the fetal and maternal blood
- CTB decreases as pregnancy advances
Therefore the barrier thins leading to a greater surface fir exchange
Gases (oxygen and CO2 by simple diffusion), water and electrolytes, steroid hormones, proteins (poorly, by pinocytosis), maternal antibodies (IgG at 12 weeks, but mainly after 34 weeks, premature babies lack protection)
Name the parts of the decidua
Capsularis - overlying embryo and chorionic cavity
Parietalis - side uterus not occupied by embryo
Basalis- between uterine wall and chorionic villlae
Consider the clinical aspects of the placenta
What positions can it be in and why is this relevant?
- Position of the placenta within the uterus
- mainly fundal (top)
- anterior or posterior
- “low lying” or placenta praevia (near to cervical os)
Implications of placenta praevia
- Massive, painless bleeding in pregnancy
- Fetal death
- Maternal death
Consider the clinical aspects of the placenta
What happens if the trophoblasts fail to invade the maternal circulation at 12 and 18 weeks?
- Poor maternal and fetal blood mixing
- Lack of oxygen and nutrients to the fetus
- Leads to fetal growth restriction
- Preeclampsia
What is the vasa praevia
Velomentous cord insertion that runs across the cervical os
What is placental abruption and what does it cause?
Placental abruption occurs when the placenta separates from the inner wall of the uterus before birth.
- Massive bleeding in pregnancy (often concealed)
- Extremely painful
- Fetal death
- Maternal death
