Development of the CNS Flashcards
What is neurulation?
Formation of the neural tube
When does neurulation occur?
during the third week of development
along with gastrulation during formation of trilaminar disc
Which germ layer gives rise to the central and peripheral nervous systems?
The ectoderm
Where does the notochord form from?
From the primitive node in the opposite direction of the primitive streak
What relationship does the notochord have with the ectoderm?
Inductive
What are the stages of development of the notochord?
- tube extension
- axial process
- notochordal process
- notochord
What germ layer is the notochord derived from?
The mesoderm
What does the notochord induce the ectoderm to do, once formed?
To thicken and form the neural plate
What cells make up the neural plate?
Neuroectoderm
What is formation of the neural plate a process of?
neurulation
What is the first thing to happen to the neural plate after formation?
The plate lengthens and the lateral edges elevate forming the neural folds
The depressed midregion forms the neural groove
How is the neural tube formed?
When the neural folds approach each other in the midline and fuse
What happens to the shape of the cells during neurulation?
The cells become more columnar in appearance
Which factors effect the bending of the neural plate?
> Intrinsic
- cytoskeleton
- stage of cell cycle
> Extrinsic
- adhesion points
Which hinge-points are present during neural plate bending?
The middle aspect becomes known as the medP) once the lateral edges begin to rise up
Dorsal-lateral hinge-points form in the dorsal lateral regions
What changes in the cells facilitate the tube formation in neurulation?
The cells at the hinge points become wedge-shaped and allow convergence
What are the four stages in neural tube formation?
- shaping and folding
- elevation
- convergence
- closure
What needs to be inhibited to allow for induction of the neural plate?
BMP-4
What needs to be promoted to allow for induction of the neural plate?
FGF
What does Chordin do?
Also acts to inhibit BMP-4
What does Noggin do?
Also acts to inhibit BMP-4
What cell adhesion molecule is expressed to regulate the formation of the neural tube?
Switching from E-cadherin to N-cadherin
What does N-cadherin do?
helps with the joining of the neural folds by allowing the teo ends of the folds to recognise each other.
also prevents fusion of the newly formed neural tube to the overlying ectoderm
What are the neural crest cells?
Temporary group of cells that are highly migratory
List the neural crest cell derivatives
These neural crest cells are destined to form the melanocytes in the skin and the hair follicles
Neural crest cells also go on to form dorsal root ganglia.
The cells can migrate in a ventral pathway through the anterior half of the somite and they form sensory ganglia, sympathetic neurons, Schwann cells, or adrenal medulla cells
The neural crest cells also migrate from the cranial end of the embryo before the neural tube is fully fused and contribute to the cranio-facial skeleton as well as neurons for the cranial ganglia, glia and melanocytes in this region
How does the neural tube fuse?
From the middle, and proceeds in cephalic and caudal directions simultaneously
What are the open ends of the tube called?
Anterior and posterior neuropores
What do the anterior and posterior neuropores connect with?
The overlying amniotic cavity
When does the anterior neuropore close?
By day 25
When does the posterior neuropore close?
By day 27
Which cells are found in the walls of the neural tube?
The walls of the newly closed neural tube consist of neuroepithelial cells, which extend over the entire thickness of the neural tube.
Describe the histology of the neural tube
Thick pseudostratified epithelium
What cells doe the neuroepithelial cells give rise to?
neuroblasts
What are the features of neuroblasts?
Large round nuclei
Pale nucleoplasm and dark stained nucleoli
What are neuroblasts?
Primitive nerve cells
What do the neuroblasts form?
The mantle layer and marginal layer of the spinal cord
What is the mantle layer of the spinal cord?
The zone around the neural epithelial layer that will go on to form the grey matter of the spinal cord
What is the marginal layer?
The outer layer of neuroblasts, consisting of nerve fibers emerging from the mantle layer
As a result of myelination, this will go on to be white matter
What happens to the neural tube after neuroblast formation?
Differentiation of the neural tube wall in the ventral and dorsal aspects
What are the basal plates?
The ventral aspect thickening of the neural tube wall
What is derived from the basal plates?
The motor area of the spinal cord
What are the alar plates?
The dorsal aspect thickening of the neural tube wall
What is derived from the alar plates?
The sensory area of the spinal cord
What is the sulcus limitans?
A longitudinal groove marking the boundary between the basal and alar plates
What is the dorsal mid-line plate called?
The roof plate
What is the ventral mid-line plate called?
The floor plate
What is the function of the floor and roof plates?
Serve primarily as pathways for nerve fibers to cross from one side of the spinal cord to the other
Do not contain neuroblasts
What is the function of the floor and roof plates?
Serve primarily as pathways for nerve fibers to cross from one side of the spinal cord to the other
Do not contain neuroblasts
What structure develops in addition to the dorsal and ventral horns?
The intermediate horn
What is the intermediate horn responsible for?
Contains sympathetic neurons from the ANS
Which regions is the intermediate horn found?
The thoracic and upper lumbar regions of the spinal cord
What structures develop following the development of dorsal and ventral horns?
The ventral fissure and the dorsal median septum
When do motor nerve fibers first appear?
Week 4
Where do motor nerve fibers arise from?
The nerve cell bodies present in the basal plate of the ventral horn
What are ventral nerve roots?
The outgrowing motor axon fibers that collect into bundles and extend into the periphery
Where are the nerve cell bodies of the sensory fibers found?
In the dorsal root ganglia
Where do the dorsal root ganglia arise from?
The neural crest cells that have migrated to this area
Where do the nerve fibers found in the dorsal horn stem from?
The processes are formed within the dorsal root ganglion and form bundles that grow into the dorsal horn
Where do the peripheral sensory nerve fibers stem from?
The processes are formed within the dorsal root ganglion and form bundles that protrude out to the periphery
What fibers are found in spinal nerves?
Sensory and motor
How many brain vesicles are present at week 3?
3
What are the three primary brain vesicles?
Forebrain - prosencephalon
Midbrain - mesencephalon
Hindbrain - rhomencephalon
What are the derivatives of the prosencephalon?
Forebrain
- Telencephalon = cerebrum
- Diencephalon = eye, thalamus, hypothalamus and epithalamus
What are the derivatives of the mesencephalon?
Midbrain
What are the derivatives of the rhombencephalon?
Hindbrain
- Metencephalon = pons, cerebellum
- myelencephalon = medulla oblongata
How many brain vesicles are present at week 5?
5 secondary vesicles
What are the 5 secondary vesicles?
- telencephalon
- diencephalon
- mesencephalon
- metencephalon
- myelencephalon
Which cells form the meninges?
The meninges develop from the mesenchymal cells and neural crest cells
During what time period does the meninges form?
day 20-35
How is the dura mater formed?
The mesenchymal cells and neural crest cells migrate to surround the tube forming an external layer that forms a membrane called the dura mater
How is the arachnoid mater formed?
The neural crest cells migrate to surround the tube forming an internal layer that forms the arachnoid mater
ONLY FORMED FROM NEURAL CREST CELLS
How is the pia mater formed?
The neural crest cells migrate to surround the tube forming an internal layer that forms the pia mater
ONLY FORMED FROM NEURAL CREST CELLS
How is the subarachnoid space formed?
When the small spaces fill with CSF and coalesce to form larger spaces
Where is CSF made?
Produces in the choroid plexus in the brain ventricles
Where does the choroid plexus form?
Where the pia mater and the ependyma come into direct contact
- mainly in the roof of the 4th ventricle
Which disorders result from the failure or incomplete closure of the neural tube?
- anencephaly
- spina bifida
What is anencephaly?
- failure of closure in the anterior neuropore
What is spina bifida?
- failure in closure of the posterior neuropore
How may neural tube defects be prevented?
- folic acid supplementation
What are the different forms of spina bifida?
- spina bifida occulta
- meningocele
- myelomeningocele
Describe spina bifida occulta
- mildest form
- the outer part of the vertebrae is not completely closed
- no protrusion of the spinal cord
- sometimes the only indication is a small tuft of hair or dimple
Describe meningocele
- least common form of spina bifida
- vertebrae form normally
- the meninges protrude through gaps of the vertebrae
Describe myelomeningocele
- most serious form
- whole are involved in te lesion has a flattened plate like mass of nervous tissue
- no overlying protective membrane protruding from the spinal cord
- associated nerves at the site of the lesion are damaged or do not form properly => loss of sensation or paralysis
- the more cranial the defect the more serious loss of function will be
What is hydrocephalus?
- water on the brain
- associated with spina bifida
- large head shape
- cerebral anomaly
- raised pressure
What is anencephaly?
an absence of a major portion of the skull and scalp
Normally the telencephalon structures that fail to form when this condition is present
- The cerebral hemispheres do not develop
Normally not compatible with life longer than a few hours after birth
How is hydrocephalus treated?
relief of intracranial CSF pressure by shunting fluid
What is spina bifida cystica
meningocele
How does raised ICP occur in spina bifida cystica?
CSF builds up in head due to obstruction of the foramen magnum by the cerebellum
- CSF gets trapped
- = raised ICP
