Formation of the Basic Body Plan Flashcards
Explain formation of the primitive streak and its components
Day 15
- Thickening, primitive streak, containing a midline groove forms along the midsagittal plane of the embryonic disc
- Along the length of the primitive streak, the primitive groove becomes deeper and more defined
- the cranial end of the primitive streak is expanded to form the primitive node
- the primitive then forms a depression called the primitive pit, which is continuous caudally with the primitive groove
What does the primitive streak define?
All major body axes
- primitive streak forms midline of embryonic disc, thus the cranial to caudal axes, and medial- lateral axis
left-right axis
Gastrulation definition
The process of forming endoderm, ectoderm and mesoderm from the epiblast layer
Stages of gastrulation
- epiblast cells undergo ingression, moving in through the primitive streak
- Epiblast cells undergo epithelial to mesenchymal transformation EMT
- The ingressing epiblast cells invade the hypoblast and displace the cells
- Hypoblast is eventually completely replaced by a new layer of cells called the definitive endoderm
- Epiblast cells migrate through the primitive streak and diverge into the space between the epiblast and definitive endoderm- form third layer called intraembryonic mesoderm
- Remaining epiblast cells constitute the ectoderm
- By week 4 primitive streak disappears
What is the difference between epithelial cells and mesenchymal cells?
Epithelial- sheets of regular shaped cells tightly interconnected to one another at their lateral surfaces.
Mesenchyme- more more irregular, stellate loosely connected cells
What happens during EMT?
epiblast cells become more flask shaped, elongate, extend rootlike processes called pseudopodia which allow them to migrate through the primitive streak
What happens between the ectoderm and endoderm?
Two depressions are formed, one at cranial and one at caudal end.
- oropharyngeal membrane and cloacal membrane, exclude mesoderm
- eventually bind to gut tube which form the oral cavity and opening of the anus respectively
How does the endoderm induce the formation of the mesoderm?
Mesoderm induction requires TGF beta and FDF families of secreted growth factors. Deregulate production of E cadherin, preventing the binding of epiblast cells so they can move into the primitive streak
Two important regions of the embryo explained
Nieuwkoop centre- induces the formation of the organiser-
Spemann-Mangold organiser- sends out signals to pattern the newly formed mesoderm into its medial and lateral subdivisions
-primitive node in humans
What does the ectoderm differentiate into?
Central neural plate and peripheral surface ectoderm
In which direction does the differentiation of cells take place?
Cranial to caudal
Teratogenesis definition
Abnormalities of physical development
Example of a teratogenic condition + explained
Sirenomelia
- insufficient caudal mesoderm
notochord definition
Flexible rod made out of a material similar to cartilage, which eventually forms the vertebral column
Stages of notogenesis
- Prospective notochord cells, formed of mesoderm, migrate cranially at the midline to form the notochord process
- Day 18 the notochordal process fuses with the endoderm to then become the notochord plate
- provides a passage from the amniotic sac to the yolk sac- forming the neurenteric canal
- Eventually, two edges of notochordal plate fuse to become the notochord which ends up in the mesoderm
What does the notochord eventually form?
Nucleus pulposus