CNS Development Flashcards
First stage of gastrulation?
- primitive streak formed on surface of epiblast (narrow groove wi bulging regions on either side)
- cells of epiblast move towards primitive streak, and when arriving at streak become flask shaped, detach from epiblast and slip beneath it (invagination)
When does gastrulation begin? And what is this?
At 3 weeks gestation, when the three germ layers (mesoderm, ectoderm, endoderm) are formed
What chemical controls the cell change and movement during invagination? (Gastrulation)
Fibroblast Growth Factor 8 (FGF8)
This is synthesised by streak cells.
FGF8 downregulates e-cadherin which binds epiblasts together.
FGF8 also regulates Brachyury expression, which controls the cell specification into mesoderm.
Second stage of gastrulation? Formation of separate layers.
- once cells have invaginated, some displace the underlying hypoblast and create endoderm.
- other cells lie inbetween the epiblast and endoderm to make the mesoderm layer.
- remaining cells in the epiblast become the ectoderm.
How is the notochord formed?
Prenotochordal cells that move from primitive node cranially and forwards in midline towards prechordal plate form the notochord.
What is the neuroectoderm?
The ectoderm that lies immediately above the notochord, that gives rise to entire nervous system.
How is the neural plate formed and when?
Formation of neural plate is induced by primitive node.
Notochord extends forwards whilst the chemicals are being released, causing cells of central part of ectoderm to rapidly proliferate and thicken.
This thickened part is the neural plate. Formed on day 18.
Which chemicals cause induction of neural plate?
FGF is upregulated which inhibits the BMP-4 (a member of TGF-B transforming growth factor B). This causes the ectoderm to be neuralized.
FGF upregulates chordin, noggin and follistatin which antagonist BMP-4. These chemicals are released by notochord, primitive steak and prechordal plate.
Which types of tissue do chordin, noggin and follistatin correspond to?
Only induce forebrain and midbrain tissue types.
What chemicals induce hindbrain and spinal cord type tissue?
WNT-3a and FGF
What effect does BMP-4 have on the ectoderm?
In presence of BMP-4 ectoderm becomes epidermis and mesoderm forms intermediate and lateral plate mesoderm.
Therefore inhibition causes ectoderm to return to its default state - neural tissue
Process of neurulation and when does it start?
At the end of the 3rd week the lateral edges of neural plate are elevated to form neural folds.
These folds approach each other at midline and then fuse at cervical region - 5th somite. Fusion proceeds cranially and caudally.
When do the separate neuropores (open ends) close?
On 25th day gastrulation anterior neuropores close.
On 27th day gastrulation posterior neuropores close.
This gives rise to the neural tube.
What happens to cells that develop on edges of the neural folds?
Form the neural crest on either side of the tube.
These then detach themselves, undergo epithelial-to-mesenchymal transition and enter the underlying mesoderm through either dorsal or ventral pathways.
These become
Melanocytes, sensory ganglia, symp neurones, DRG, glial and schwann cells
What does abnormal closing of the neuropores lead to?
Abnormal closure of anterior neuropores leads to anencephaly.
Abnormal closure of posterior neuropores leads to spina bifida.
