Neurulation & Embryonic Folding Flashcards
What happens as the primitive streak regresses?
The primitive node lays down the notochord
How is neurulation initiated?
What is this process called?
By signals from the notochord that cause the overlying ectoderm to thicken and form the neural plate
This is induction
What does the ectoderm become once it has thickened?
What happens to it as it thickens?
The neuroectoderm
As the ectoderm thickens, the lateral edges elevate to form neural folds
What happens when the neural folds elevate?
They meet in the midline and fuse together to form the neural tube
The neural tube goes on to form the spinal cord and the brain
On which day do the neural folds begin to elevate?
day 19
They are almost ready to fuse together on day 20
When the neural folds fuse, in which regions do they not fuse?
They fuse along the entirety of their length to form the neural tube
Where does fusion of the neural folds commence?
Fusion of the neural fold begins in the cervical (neck) region of the embryo and continues cranially and caudally
What does the tube separate from as the neural folds fuse?
The neural tube separates from the surface ectoderm
This will go on to form the epidermis of the skin
By which day has fusion of the neural folds in the midline completed?
Fusion of neural folds in the midline to form the neural tube is complete by day 22
What are the openings at the top and bottom of the neural tube?
The anterior and posterior neuropores
On which days to the anterior and posterior neuropores fuse?
The anterior neuropore fuses on day 25
The posterior neuropore fuses on day 28
What will the cranial and caudal ends of the neural tube form?
The cranial end will form the brain
The caudal end will form the spinal cord
When does anencephaly occur?
When the anterior neuropore fails to fuse
What happens in anencephaly?
There are no cerebral hemispheres (forebrain) in the brain, but there is still a brainstem
The brainstem is usually exposed
What happens to a baby that is born with anencephaly?
It is usually a stillbirth or the newborn dies a few hours after birth
Why may a baby with anencephaly rarely be able to survive for a few years?
The brainstem controls the heart, lungs and some reflexes
What is rachischisis caused by?
Failure of the posterior neuropore to fuse
What is rachischisis?
The spinal cord does not form properly, leading to paralysis
The open neural tube also prevents the formation of vertebrae leading to spina bifida
What is different about the neural tube in rachischisis?
The neural tissue forms a flat plate, rather than a tube
This plate is exposed at birth so there is a high risk of infection
What types of defects are anencephaly, rachischisis and spina bifida?
neural tube defects
Why is spina bifida classed as a neural tube defect?
The neural tube is correctly formed
But it is caused by abnormal induction of the sclerotome by the notochord
By how much and why can folic acid reduce neural tube defects?
Folic acid reduces neural tube defects by 50-70%
It is important for correct neurulation and development of the CNS
What happens to cells as the neural folds elevate and fuse?
Cells at the lateral edge separate from the neural tube
These are neural crest cells
What is the role of the neural crest cells?
They migrate laterally and ventrally and differentiate into a variety of structures throughout the body
What happens at the same time as migration of the neural crest cells?
The neural tube separates from the mesoderm
What is neurofibromatosis caused by?
It is a genetic condition caused by a mutation in the neurofibromin 1 gene
What is the role of the neurofibromin 1 (NF1) gene?
It is a tumour suppressor gene
It prevents neural crest cells from differentiating too much
What happens in neurofibromatosis?
NF1 gene is switched off leading to increased cell division
Overproliferation of neural crest cells leads to formation of benign tumours of the nervous system, skin and cranial bones
What else is occurring at the same time as neurulation?
The embryo goes from being a trilaminar disc to form a three-dimensional structure
It does this through embryonic folding
What are the 2 ways in which the embryo folds and when does this occur?
- craniocaudal folding
- lateral folding
This occurs during the 4th week of pregnancy
What brings about embryonic folding?
Differential growth of the amniotic sac and the yolk sac
The amniotic sac grows rapidly, whilst the yolk sac remains the same size
What is the difference between craniocaudal and lateral folding?
Craniocaudal folding results in the C-shape of the embryo
Lateral folding causes the edges of the trilaminar disc to come together and fuse in the midline
What is lateral folding caused by?
The amniotic cavity expands very quickly
Amniotic fluid surrounds the embryo and pushes the lateral edges together so that they fuse and form the body cavity
In which regions of the embryo does the lateral body fold?
everywhere except for the level of the yolk sac
During lateral folding, what will the surface ectoderm go on to form?
The epidermis of the skin
outer layer of the body cavity
After craniocaudal folding, what will the ectoderm and connecting stalk go on to form?
Ectoderm will form the spinal cord
The connecting stalk will form the umbilical cord
What is the allantois and the vitelline duct?
The allantois is the foetal bladder
The vitelline duct is the connection between the yolk sac and the midgut
What is the process behind craniocaudal folding?
The amniotic cavity gets larger and pushes the head and tail end in closer proximity
This causes the embryo to become a C-shape
What is ectopia cordis?
The heart develops outside of the body
This is due to the lateral folds failing to fuse correctly in the thoracic region
What happens if the lateral folds fail to fuse?
There is a gap in the anterior wall
How many live births does ectopia cordis occur in?
1 in 5 million
What is gastroschisis?
The intestines are outside of the body wall
This is due to the lateral folds failing to fuse in the abdominal region
What is the only area where the lateral walls should not fuse?
The level of the midgut
How many live births does gastroschisis occur in?
3.5 per 10,000
What is the treatment for gastroschisis?
The intestines are put into an aseptic bag until there is enough space in the abdominal cavity for them to be put back
