Mechanisms of disease during embryogenesis Flashcards
Name the 2 periods in human development - when does the second one start?
Embryonic period
- Up to the end of week 8
- Most of the organogenesis occurs in these first 8 weeks
Fetal period
- The remaining time in utero
- Involves growth and modelling
What (briefly) happens in each of the two stages of in utero development?
in the embryonic period (up to 8 weeks) it is mostly the formation of organs and a body plan and in the fetal period it is mostly growth and modelling
defects in embryogenesis cause congenital malformations
Name all of the main steps in the embryonic period of development
fertilisation → cleavage → gastrulation → neurulation and somitogenesis → organogenesis
What is a morula?
When the embryo is 12-16 cells (around day 3)
What is compaction?
This is when the morula cells compact around one of the poles, this is bc the blastocoel (produced by trophoblast) pushes the cells the outer cells will become trophoblast as they have an apical and a basolateral surface and so they are bipolar
the cells inside are unipolar
Explain what the hypoblast and the epiblast is
When the blastocyst implants, the inner cell mass cells that are in contact with the endometrium become the epiblast and the outer (more towards the blastocoel) layer becomes the hypoblast
this forms the bilaminar germ disc and the amniotic cavity
Describe what gastrulation is
“Gastrulation is defined as an early developmental process in which an embryo transforms from a one-dimensional layer of epithelial cells (blastula) and reorganizes into a multilayered and multidimensional structure called the gastrula”
What layers does gastrulation begin with - what does it end with?
2 layers to 3 layers
The 2 layers in the blastocyst bilaminar germ disc (epiblast and hypoblast) are the starting point
At the end of gastrulation, there is endoderm (was hypoblast)+ mesoderm + ectoderm (was epiblast?)
Explain what the primitive groove is, what is its function?
At the start of gastrulation, on the superior border of the epiblast (so look at it down from through the amniotic cavity) a groove forms
Cells from the epiblast move through the primitive groove to displace the endoderm (which was called the hypoblast)
What is one of the first developmental process that comes from the ectoderm?
Neurulation = the specification of the central nervous system
What is the first evidence of neurulation in the ectoderm?
The neural plate
Describe how the primitive streak changes
Ingression of the epiblast into the primitive streak to form the mesoderm (gastrulation) is sequential - starting at one end of the blastocyst and moving along towards the oropharyngeal membrane so elongates
After the formation of the neural plate (so after day 20) what happens?
The neural plate gives rise to a neural tube
So what happens when the primitive streak regresses?
Once it reaches the end, it regresses and as it regresses it lays down a rod-like structure called the notochord underneath the ectoderm
Name the function of the notochord
The notochord is below the ectoderm and it releases extracellular molecules to cause the ectoderm to become neural tissue - forming the neural plate
Fully explain the process of how folding of the neural plate happens
The neural plate forms neural folds that rise up and meet to fuse together and form a tube - this tube is then embedded in ectoderm
remember that the neural plate (and so also the neural tube) is just ectoderm that has been specialised by the signals released from the notochord
that transverse view is the most useful to understand this
What is the anterior and posterior neuropore?
The fusing of the neural folds to form the neural tube starts in the middle and progresses outwards like a zip to each pole - the points that are not yet ‘zipped’ where the folds have not fused to form the tube are called neuropores
What are somites?
While the neural tube is forming, the mesoderm underneath is also reorganising - and it forms tissue blocks that we call somites. Shown below with the beige ‘bubbles’ and the two circles lateral to the neural tube in the upper image of the transverse view
Name what somites are a precursor to
Bones, muscles and tendons
While neurulation is happening, what other things are happening?
Gut formation and body folding in the ‘silk purse model’
What is body folding?
The embryo needs to be enclosed and so it folds over itself so that the mesoderm and ectoderm can form organs
Why is this called the silk/cloth purse model?
We can imagine that a string is being tied around a purse so that the purse moves up and folds over itself
What structures are moved and what structures are formed in this folding (during embryogenesis)?
- this allows the heart and other structures to move to the centre of the embryo and also allows for formation of the umbilical cord and gut
In organogenesis (the last part of this differentiation in the 8 weeks in embryonic stage of development) name some of the structures that are formed
Differentiation of Somatic derivatives (bones, muscles, tendons) from somites
Development of Sensory organs (ears, eyes, olfactory pits)
Limbs formation (forelimbs first then hindlimbs - various assymetries are evident now such as the anterior-posterior and dorsal-ventral)
Face structures (jaws, nose, tongue, palate)
Genital structures
