How does once cell become a multicellular body?
Growth, Morphogenesis, Differentiation
What does morphogenesis involve?
Creating lots of cells in the right place at the right time
What is the result of morphogenesis?
The development of form and structure
Why is differentiation needed?
For specialisation for function
What does differentiation allow?
Certain groups of cells to specialise for a common function
What are the 3 stages from fertilisation to birth?
When is the pre-embryonic period?
When is the embryonic period?
Why is the embryonic period such a critical period?
All structures and systems of the body are made
When is the foetal period?
What happens in the foetal period?
The focus is on growth and physical maturation of systems
When are pregnancy weeks calculated from?
LMP, i.e. conception weeks +2
How long is a term pregnancy?
What happens in the pre-embryonic period?
- Implantation begins
What is happening in cleavage?
Why is cleavage significant?
Because have established a new individual
What is formed from cleavage?
What is the morula?
A clump of cells
What happens in compaction?
The formation of a blastocyst
What is implantation?
The process by which the blastocyst makes contact with the endometrium of the uterus
What does implantation begin?
The establishment of the pregnancy
Where is the oocyte released from?
What happens to the oocyte once it’s been released?
It travels along the Fallopian tubes
Where is the oocyte fertilised?
What fertilises the oocyte?
What is the ampulla of the fallopian tubes?
The fan bit just above the ovary
What is the fertilised oocyte called?
Where is the ideal site for implantation?
The posterior uterine wall
What does the zygote need to do before it can implant?
Continue to travel alone the fallopian tube to get where it needs to be to establish the pregnancy
How long is an oocyte viable for?
How long is sperm viable for?
What happens immediately after fertilisation?
Sperm is excluded due to a complex chemical reaction
When does cleavage begin?
30 hours after fertilisation
What does cleavage result in?
2 blastomeres of equal size
What is the zona pellucida?
A glycoprotein ‘shell’
What is the result of the presence of the zona pellucida?
The blastomeres are about half the size of the oocyte, as the cell prevents it from expanding
What has happened by day 3 post fertilisation?
The morula has formed
What feature does each cell have at the morula formation stage?
They are totipotent
What is meant by the cells being totipotent?
They have the capacity to become any cell type
What happens in assisted reproductive techniques?
The oocytes are fertilised in vitro, and allowed to divide to the 4- or 8- cell stage. The morula is then transferred into the uterus
What is PGD?
When a cell is removed from the morula and tested for serious heritable conditions prior to the transfer of the embryo into the mother
What is compaction?
Formation of the first cavity
What is the first cavity formed?
How is the blastocyst formed?
The cells of the morula are secreting tiny amounts of tissue fluid. Together, collect a sufficient amount that it collects, and one clump of cells pushes to one side, giving a space
What does the formation of the blastocyst give?
2 groups of cells
What is the inner group of cells called?
What is the outer group of cels called?
What happens to the inner group of cells of the blastocyst?
It goes on to give the embryo
What happens to the outer cells of the blastocyst?
They go on to produce the supporting tissues
What tissues support the embryo during pregnancy?
Why can cells produced by the first divisions after fertilisation become inner or outer cell mass?
Because they are all totipotent, so they have the capacity to become any cell type
What are cells after compaction?
What is meant by pluripotent?
Cells have the capacity to become one of many cell types
What potential are pluripotent cells said to have?
What do the inner cell mass cells have the capacity to do?
Become any cell in the human body
What happens at day 5 post fertilisation?
What is meant by hatching?
The blastocyst hatches from the zona pellucida
Why must the blastocyst hatch?
Up until now, the zona pellucida remains in tact, meaning it becomes quite restrictive, so we must get rid of it
What is the result of hatching?
The blastocyst is no longer constrained, and so is free to enlarge
What can the blastocyst do once it’s hatched?
Interact with the uterine surface to implant
When does implantation begin?
How long does implantation run for?
A very long time- not complete until the end of the first trimester (~3 months)
What is the conceptus?
All the products of conception- the embryo and all its tissues
How many cells does the conceptus have at implantation?
How many cells of the conceptus at implantation will make the embryo?
How many cells of the conceptus at implantation will contribute to the development of foetal membrane?
Why is it important that foetal membranes are established?
It ensures that we can establish and maintain the pregnancy
Where does the conceptus implant?
Onto the uterine epithelium, which lines the uterine stroma
What development has priority at the very earliest stages?
What important process occurs in week 2?
Where do two distinct cell layers emerge from?
The outer cell mass
What distinct cell layers emerge from the outer cell mass?
What is the syncytiotrophoblast good for?
What is purpose of the cytotrophoblast?
Allows the syncytiotrophoblast grow and increase in SA
What does the inner cell mass develop into?
A bilaminar disc
What does the bilaminar disc consist of?
In summary, what has happened by the end of the second week?
What are the two cavities of the embryo by the end of week two?
What is the name of the sac supporting the embryo?
The chorionic cavity
What is the amniotic sac formed from?
Spaces within the epiblast
What can implantation cause?
A small amount of bleeding
Why might implantation cause a small amount of bleeding?
Because it’s in invasive process
Why is implantation an invasive process?
The conceptus has to break through the endothelial lining
What must happen once the conceptus has broken through the endothelial lining?
The lining must be repaired
How is the endothelial lining repaired?
Using a fibrin plug
What is meant my implantation being interstitial?
The two tissues have a very close relationship- they are embedded
Where does the conceptus implant?
Within the uterine stroma
Why does the conceptus implant within the uterine stroma?
Because then it has access to all glands
What does implantation establish?
Maternal blood flow within the placenta- the basic structural unit of materno-fetal exchange
What change does implantation mark?
The support for embryo changes from histiotrophic to haemotrophic
What is meant by histiotrophic?
Support for tissues based on simple diffusion
Why must support for the embryo change from histiotrophic?
Because staying with this would drastically limit the size that it could grow
What is meant by haemotrophic?
Relying on support from maternal circulatory system
Why does the basic structural unit for materno-fetal exchange have to be within the placenta?
To allow for gas exchange, absorption of nutrients and removal of waste products from the foetus
Give 4 conditions linked to implantation defects
- Interuterine growth restriction (IUGR)
- Ectopic pregnancy
- Placenta praevia
What kind of defect causes ectopic pregnancy and placenta praevia?
When the conceptus implants at the wrong site
Where does the conceptus implant in an ectopic pregnancy?
At a site other than the uterine body, most commonly the Fallopian tubes
What are the two types of ectopic pregnancy?
What is the problem with ectopic pregnancy?
It can very quickly become a life threatening emergency, as it can cause haemorrhage
Where does the conceptus implant in placenta praevia?
In the lower uterine segment
What is the problem with placenta praevia?
It can cause haemorrhage in pregnancy
Why can placenta praevia cause haemorrhage?
If implantation occurs very near the point of exit, the placenta can grow across, causing the mother to bleed out
What does placenta praevia necessitate?
What happens on day 9 post fertilisation?
- Embryonic pole develops
- Abembryonic pole develops
What causes the development of the embryonic pole?
The rapid development of the syncytiotrophoblast
What happens once the abembryonic pole has been formed?
The primitive yolk sac is formed
What is the significance of the formation of the primitive yolk sac?
You then have an embryo with two cavities
What is the yolk sac membrane in contact with?
The cytotrophoblast layer
What are present in the syncytiotrophoblast?
What are lacunae?
Spaces opening up
What is the purpose of lacunae?
They allow for rapid increasing SA
What happens on day 11 post fertilisation?
Primitive yolk sac membrane pushed away from the cytotrophoblast layer
What pushes the cytotrophoblast layer away from the primitive yolk sac membrane?
An acellular extraembryonic reticulum
What later happens to the acellular extraembryonic reticulum?
Its converted into extraembryonic mesoderm by cellular migration
What happens on day 12 post fertilisation?
Maternal sinusoids are invaded by syncytiotrophoblast
What happens once the maternal sinusoids have been invaded by the syncytiotrophoblast?
The lacunae become continuous with the sinusoids
What does maternal sinusoids being invaded by syncytiotrophoblast begin?
What is the uterine stroma preparing for when the maternal sinusoids are invaded by syncytiotrophoblast?
Support of the embryo
What happens day 13 post fertilisation?
Formation of the secondary yolk sac
Where does the secondary yolk sac come from?
Pinches off from the primitive yolk sac
What happens day 14 post fertilisation?
Spaces within the extra-embryonic mesoderm merge
What is formed when spaces within the extra-embryonic mesoderm merge?
The chorionic cavity
What suspends the embryo and its cavities?
The connecting stalk
What is the connective stalk made up of?
A column of mesoderm and future umbilical cord
What can bleeding be confused for at day 14?
What % of zygotes are lost within the first 2-3 weeks?
What % of diagnosed pregnancies miscarry?
What % of women suffer from recurrent miscarriages?
What is meant by recurrent miscarriages?
Miscarriages in 3 consecutive pregnancies