What is gastrulation?
-The process of forming all three germ layers from the epiblast
What are the main events which occur in week 3 of development?
- Gastrulation
- Defining of the axes
What is the first event in gastrulation?
-Formation of the primitive streak
Where does the primitive streak form?
-In any direction on the surface of the epiblast
What is the structure of the primitive streak?
- A streak of cells which appears on the surface of the epiblast which has narrow bulging at the edges
- The primitive node is an elevated area located at the cranial end of the streak
- The primitive pit is a central depression in the node
What is the first event that occurs involving the primitive streak?
-Invagination-> cells of the epiblast migrate towards the streak, detach from the epiblast and depress down into the streak
What does the process of invagination into the primitive streak cause?
-Displacement of the hypoblast by the differentiated cells of the epiblast known as endoderm
What happens to the primitive streak as invagination is occuring?
-Begins to regress towards the caudal end
How are the mesoderm and ectoderm layers formed during gastrulation?
-Once the endoderm has formed, the epiblast cells begins to diferentiate into ectoderm, whilst invagination of epiblast cells which are differentiating to mesoderm fills the space inbetween the two layers
What happens to the primitive yolk sac during gastrulation?
-The hypoblast regresses and endoderm now lines the primitive yolk sac
What is the end result of gastrulation?
-Formation on the trilaminar disc containing the three germ layers
Does mesoderm fill the entire space between the ectoderm and endoderm?
-Yes, except from 2 areas, one at the cranial end and one at the caudial end which will be the future mouth and anus
How are the openings for the mouth and anus produced?
- These areas are not filled with mesoderm which gives rise to the CVS
- Without CVS these areas will die and rupture creating openings
What is the notochord?
-A solid rod of cells which runs down the midline of the trilaminar disc
How is the notochord formed?
-Cells migrate from the primitive node and form a line down the midline without spreading out
What two events does the notochord permit?
- Defining of the axes
- Formation of the nervous system
In what direction does the notochord form?
-Cranial to caudal
Which event defines the formation of the cranial/caudal axes?
-Formation of the primitive streak
Which axes does the notochord define?
- Dorso-ventral
- left-right
How does the notochord define the left-right axes
-Signals for the beating action of cilia which produce a side-specific signalling cascade
What is situs invertus and what causes it to occur?
- Complete mirror image of the organs and structures of the body-> often not associated with morbidity
- Immotile cilia
What are the derivatives of ectoderm?
- Organs and structures which remain in contact with the outside world
- Epidermis
- Nervous system
What are the derivatives of mesoderm?
- Supporting structures
- CVS, muscles, bone, cartilage
What are the derivatives of the endoderm?
- Internal structures
- Epithelia of the GI tract and respiratory tract
- Parenchymal cells
What are the three possible causes of monozygotic twins?
- Zygote separates at two cell stage
- Zygote separates at the inner cell mass
- Appearance of second primitive streak
In twinning, why do two embryos have two placentas is separation occurs at the two cell stage?
-Embryo has split before any differentiation, meaning that each ambryo can then differentiate to produce a placenta of their own
In twinning, if separation occurs at the inner cell mass, why are two embryos produced which share a placenta?
- The embryo has undergone differentiation into inner cell mass and outer cell mass
- Outer cell mass forms placenta, separation of the inner cell mass inside the outercell mass will produce two embryos sharing a placenta
In twinning, why does the appearance of another primitive streak lead to embryos sharing an amniotic cavity?
-The primitive streak appears on the epiblast which lies inside of the amniotic cavity which has already developed
In twinning, when can the appearance of a second primitive streak produce conjoined twins?
-If separation (of the bilaminar disc) is not complete
What is teratogenesis?
-The disruption of embryonic development by whatever means
Why does each organ system have a particularly sensitive window for teratogenesis?
-They all develop at different times
Give an example of a chemical teratogenic agent
- Thalidomide
- Alchol
- Drugs
Give an example of an infectious teratogenic agent
-Rubella
