Mechanisms of Disease During Embryogenesis? Flashcards
What are the two main periods of human development?
Two main periods: Embryonic period: - Up to the end of week 8 - Most of the organogenesis occurs in these first 8 weeks Foetal period: - The remaining time in utero - Involves growth and modelling Defects during embryogenesis result in congenital malformations
What are the different mechanisms of development an egg undergoes?
From egg:
- Fertilisation - Cleavage - Gastrulation - Neurulation and somitogenesis - Organogenesis
What happens before the embryo proper develops?
Before the development of the embryo proper, the conceptus must first implant, then generate the “germ” disc.
This takes ~10 days.
What happens in the cleavage stage of embryo development?
The fertilised zygote divides into a 2-cell zygote, then 4 etc.
What happens after the embryo cell cleaves into 8 to 16 cells?
By the time it gets to the 8 to 16 cell stage it becomes a morula
It is ready to undergo its first reorganisation called compaction
What happens during compaction?
During compaction, cells in the embryo sort themselves
Some remain in the exterior and some will move within the embryo
The outside cells will acquire strong cell-cell contacts with the neighbouring cells and they will have one of their surfaces to the external environment
This provides cell polarity
The cells in the inside of the embryo are apolar because all sides of the cell are in contact with other cells
What are the different zones that develop after compaction?
The outside cells give rise to the trophectoderm, and the inside cells become the inner cell mass
The outside cells start pumping fluid inside the embryo leading to a fluid filled cavity called a blastocoele, pushing the inner cell mass to one side and starts giving an asymmetry
Now it is called a blastocyst which is now ready to implant into the uterus
The inner cell mass cells will give rise to two different cell types:
- Cells lining up at the edge of the inner cell mass with the blastocoele
- The rest of the cells that remain not in contact with the blastocoele
This gives rise to the hypoblast and the epiblast
What happens in the full implantation stage?
As the embryo fully implants cells are becoming more distinguished
The epiblast and hypoblast form a bilaminar germ disc
The epiblast cells organise to leave another cavity which will gives rise later on to the amniotic cavity
What happens in the gastrulation stage of embryo development?
Gastrulation: from 2 layers to 3
At one of the ends of this bilaminar germ disc, a groove forms on the epiblast called the Primitive groove
Cells from the epiblast will ingress from this groove and spread beneath the epiblast
They will displace the hypoblast and will form the endoderm and the mesoderm underneath the original epiblast layer that will give rise to the ectoderm
What happens in the neuralation stage of embryo development?
Next the neural plate forms to give rise to the neural tube
Neural tube closure
As it folds over itself to give rise to the tube it becomes buried inside the embryo
Closure of the neural tube doesn’t occur all at once
It starts at the edge between the hindbrain and the spinal cord and spreads in opposite directions
The mesodermal cells start to become segmented in tissue blocks called somites
These somites will be the precursors of the bones, muscles and tendons
What else happens while the embryo undergoes neurulation?
Neurulation is concomitant with other form-shaping (morphogenetic) processes, particularly gut formation and body folding (“silk purse” model)
The mesoderm and endoderm will gives rise to all the internal organs so they need to be enclosed by the embryo
So the embryo needs to fold over on itself
What is the cloth purse model?
The ‘cloth purse’ model
Septum and heart move from margin to centre
Yolk sac, allantois and stalk male umbilical cord
Prochordal and cloacal plates delimit gut tube
What are the different things that happen during organogenesis?
Differentiation of somitic derivatives - bones, muscles, tendons Development of sensory organs - ears, eyes, olfactory pits Limb formation - forelimbs first, hindlimbs next - establishment of pattern in the limbs: proximodistal, anterior-posterior, dorsal-ventral Formation of face structures - jaws, nose, tongue, palate Formation of genital structures
