Embryology Flashcards
What % of newborns have major structural abnormalities?
3%
What forms the bilaminar disc?
Epiblasts and hypoblasts
What is gastrulation?
Formation of the 3 germ layers (ectoderm, mesoderm and endoderm)
During gastrulation, how does the mesoderm form?
Epiblast cells migrate through the primitive groove to lie above the newly formed endoderm and form the mesoderm.
During gastrulation, how does the endoderm form?
Epiblast cells invaginate through the primitive groove displacing the hypoblast laterally and forming the endoderm.
Which signalling molecules are important for gastrulation?
BMP, FGF and Wnt
What is the role of BMP in gastrulation?
Establishing the rostro-caudal axis
What do HOX genes control?
The cranio-caudal patterning of the body
When does primary neurulation occur?
3-4 weeks
What is primary neurulation?
Formation of the neural tube (plate, grooves and folds)
When does the primitive streak form?
Day 13
What is Hensen’s node?
The cranial end of the primitive streak
When does the notochord form?
Day 17
What releases sonic hedgehog?
The notochord
What forms the neural plate?
Notochord induces the primitive streak to form the neural plate
What causes the neural plate to form the median hinge and hence form neural groove?
SHH
When do the neural folds fuse to form the neural tube?
Day 22 - the prox 2/3 form the brain and distal 1/3 forms the spinal cord
Where does the neural tube start to close from?
The hindbrain (CCJ)
When does the anterior neuropore close?
24 days (forming the lamina terminalis)
When does the posterior end of the neural tube (at L1/2) close?
Day 26 - a problem at this stage causes neural tube defects and chiari malformations.
What is dysjunction?
Separation of cutaneous ectoderm from the neuroectoderm after neural tube formation. This forms intact dorsal skin.
What type of cells form the notochord?
Mesenchymal
What does the mesenchyme after dysjunction form?
Dura, neural arches and paraspinal muscles
What is the consequence of dysjunction occurring too early?
Mesenchyme can enter the neural tube and form lipomas / lipomyelomeningoceles
What is the consequence of a focal failure of dysjunction?
Ranging from a Dermal sinus (as the skin does not close over the top) to a myelomeningocele.
How do lipomyelomeningocoeles form?
The lipid and dura are mesenchyme. if dysjunction occurs too early before the neural tube has closed, then mesenchymal cells can enter the neural tube which form lipomas / meningoceles and combinations etc
When does secondary neurulation occur?
4-5 weeks
What is the consequence of failures of secondary neurulation?
Spinal dysraphism below L1/2
How do you know if a spinal dysraphism occurred due to primary or secondary neurulation failure?
If above L1/2 (encephalocoeles etc) then primary neurulation.
If below L1/2 then secondary neurulation.
What contributes to the branchial arches?
Neural crest cells
What conditions occur with the failure of ventral induction?
Holoprosencephaly, septo-optic dysplasia and dandy-walker malformation
What is holoprosencphaly?
Failure to form two hemispheres. Can be just between the frontal lobes (lobar), between frontal and parietal lobes (semi lobar) or fronto-parieto-occipital (alobar).
This is due to a failure of ventral induction.
What is septo-optic dysplasia?
Under development of the optic nerve, pituitary gland and absence of the septum pellucidum. (2 of the 3 need to be present to make the diagnosis). This is due to a failure of ventral induction.
When does neuronal proliferation and differentiation occur?
2-4 months. A failure at this stage causes AVMs and neurocutaneous syndromes.
When does cellular migration occur?
2-5 months with cells migrating from ventricular zones to the periphery (mantle layer). A problem at this stage causes callosal agenesis, schizencephaly and FCD / GM heterotopias.
What pathologies arise with a failure of cellular migration?
A problem at this stage causes callosal agenesis, schizencephaly and FCD / GM heterotopias.
When does neuronal organisation and myelination occur?
From the 5th month - proceeds from caudal to cephalad, dorsal to ventral and central to peripheral with sensory before motor.
When does myelination complete?
2 years
At birth, what are the normal GM and WM signals on MRI?
On T1 WM is dark and GM is light, this is the opposite in adults as the WM has not myelinated yet/
What is ventral induction?
Formation of 3 primary vesicles from the neural tube (prosencephalon, mesencephalon and rhombencephalon). Occurs between 5-10 weeks of gestation.
What do the prosencephalon, mesencephalon and rhombencephalon form?
Prosencephalon = Telencephalon and diencephalon
Mesencephalon = Mesencephalon
Rhombencephalon = Metencephalon and myelencephalon
What is the adult derivative of the telencephalon?
The cerebral hemispheres and lateral ventricles including the caudate, putamen, fornix, hippocampus, anterior commissure and corpus callosum.
What is the adult derivative of the diencephalon?
The thalamus and third ventricle including the GP, posterior pituitary, optic nerves and posterior commissure.
What is the adult derivative of the mesencephalon?
Midbrain and cerebral aqueduct
What is the adult derivative of the metencephalon?
Pons, cerebellum and upper 4th ventricle
What is the adult derivative of the myelencephalon?
The medullar and lower 4th ventricle
What does the germinal matrix form?
Neurons and glia from the 7th week of life. It involutes at the 30th week of life.
How does the corpus callosum form?
Genu > Body > Splenium > Rostrum. This is why partial agensis includes the splenium and rostrum.
When does the brainstem form?
Between 2-6 months
What does the alar plate of the brainstem contain?
Sensory nuclei
What does the basal plate of the brainstem contain?
Motor nuclei for developing cranial nerves
What are the three layers of the developing spinal cord?
Neuroepithelial
Mantle
Marginal
What does the neuroepitheal layer of the developing spinal cord form?
The ependyma that lines the central canal and also gives rise to neuroblasts that form the mantle and marginal layers.
What does the mantle layer (neuroblasts) form?
Gray matter of the spinal cord