Embryology of CNS Flashcards
what are the 3 stages of development of the neural tube
- neural plate: ectodermal cells overlaying the notochord become tall columnar (in contrast to surrounding ectorderm that produces epidermis of skin)
- neural groove
- neural tube: the dorsal margins of the neural groove merge medially, forming a neural tube composed of columnar neuroepithelial cells surrounding a neural canal
what does the notochord induce
overlaying ectoderm to become neuroectoderm and form a neural tube
how does closure of the neural tube progress
rostrally and caudally from the level of the most caudal division of brain
the caudal closure forms the majority of the spinal cord
defect of closure of neural tube are the cause of various embryological malformations
what are the outcomes of neural tube differentiation and the associated structures
- neural tube –> CNS (brain + spinal cord)
- cavity of tube (neural cavity) –> central canal of spinal cord and ventricles of brain
- neural crests: arise from border of neural plate and surface ectoderm –> form a column of cells dorsolateral to neural tube
what are the outcomes of neural crest differentiation
- neurons of PNS with their cell bodies located in spinal ganglia
- neurolemmocytes (Schwann cells) of PNS (myelination of nerves)
- ganglionic neurons of ANS and enteric nervous system
- others: adrenal medulla cells, melanocytes of skin, variety of structures in the face, etc.
what are the structures shown
what occurs to the neural tube
initially one cell thick
division of the cells located close to the central canal = germinal layer (start dividing and form other cells)
migration towards the periphery and differentiation of the post-mitotic cells (resulting in formation of a concentric 3 layer structure)
what are the two cell types in the differentiation of neural tube development
- immature neurons
- spongioblasts
what are immature neurons
differentiate into nmature neurons (no further division occurs)
what are spongioblasts
progenitors of the neurectodermal supporting cells of the nervous system = neuroglia (oligodendrocytes and astrocytes)
what are the 3 concentric layers
- mantle layers: develop into grey matter (where neuronal cell bodies are located)
- marginal layer: becomes white matter (where axons the neurons are located)
- the neural canal becomes the central canal in spinal cord and the ventricular system in the brain. lined by ependymal cells
what is the neural canal lined by
ependymal cells originating from germinal layer –> only remnant of this germinal layer = one cell thickness
what is the final steps of neural tube development
how does the dorsal nerve roots develop in the sensory system
Axons emerge from the segmental spinal ganglion (neural crest)
Form afferent neurons (sensory)
Cell body is in the spinal ganglion
how does development of ventral nerve roots in the motor system occur
Axons grow from the basal plate (ventral horn of the neural tube) out of the neural tube
Form efferent neurons:
–General somatic efferent neurons (muscles)
–General visceral efferent neurons (autonomic nervous system)
Cell body is in the spinal cord
how does the brain develop in the first steps
The rostral end of the neural tube develops rapidly into 3 vesicles then 5 vesicles
Early in its development, the prosencephalon develops lateral enlargements:
optic vesicles (will lead to the formation of the eye and optic nerves).
what are the structures shown here
what structures are shown here
what structures are shown here
what structures are shown here
how is the forebrain organized
in the telecephalon the arrangement is different from the basic neural tube organization
where are the neuronal cell bodies located in the forebrain organization
neuronal cell bodies are located in 2 locations
- most of grey matter is located in cerebral cortex
- others are located deeply in subcortical basal nuclei
how is the white matter organized in the forebrain
white matter tracts (axons) are located deep inside telencephalon
where is the commissural in the forebrain
between cortical areas from one hemisphere to the other
where is the white matter association fibres
between cortical areas within hemispheres
where are the white matter projection fibres
connecting cortical areas of a hemisphere and brainstem nuclei
what are the white matter tracts
- commissural fibres (corpus callosum)
- projection fibres (internal capsule)
- association fibres (long and short)
what are the structures shown
what are the 3 layers of connective tissue of the meninges
- dura matter: thick, fibrous layer
- arachnoid: thin layer, which encloses the cerebrospinal fluid within the subarachnoid space
- pia matter: thin layer, adherent to the CNS parenchyma
what are the 3 connective tissue layers derived from and what do they cover
derived from neural crests with a small mesodermal contribution
cover the brain and spinal cord
what are the main type of malformations in development
- hydrocephalus
- neural tube defects
- migration disorders
- encephaloclastic defects (destructive lesion)
- others: cerebellar and spinal cord malformations
what is anecephaly/atelencephaly
failure of development of telencephalic vesicles
presence of a calvarial defect = cranioschisis
brainstem and cerebellum are present –> reduced in size
CSF leakage from calvarial defect
what is cranial bifidum (two forms)
defect of closure of cranium
various form depending on nature of tissue protruding through defect
meningocele: protrusion of meninges, covered by skin through cranium bifidum
meningoencephalocele: as above but also contains brain parenchyma in protrusion
what does cranium bifidum normally effect
dogs at a young age –> median 6.5 months
what are the presenting problems with carnium bifidum
seizures and behavioural abnormalities
what are the causes of cranium bifidum
- genetic: craniofacial malformation
- teratogenic substances: griseofulvin treatment of pregnant queens
- folic acid deficiency: well recognized cause in people
- cause unknown
what is diecephalus
due to incomplete duplication (partial twins)
what is holoprosencephaly
holoprosencephaly: failure of cleavage in two separate telencephalic vesicles
what is lissencephaly
smooth brain without gyri/sulci and pachygyria (thick cerebral cortex)
might be associated with cerebellar hypoplasia
what is hydranencephaly
–Almost complete absence of cerebral cortex, reduced to a thin layer
–Frequently secondary to viral infection, leading to destruction of germinal layer (e.g. BVD or Blue tongue viruses).
–The cerebrospinal fluid fills the “gap” left by the missing cortex
what is chiari-like malformation
–Common in Cavalier King Charles Spaniels and Brussels Griffon
–caudal cranial fossa that is too small for the volume of brain tissue that it contains ->
–herniation of the caudoventral cerebellar vermis into the foramen magnum in toy dogs and other small-breed dogs ->
–dilation of the central canal is hydromyelia; a cavitation of the spinal cord parenchyma is syringomyelia.
what is spina bifida
–Frequent vertebral malformation due to a defect of closure of the neural tube
what is the different degrees of severity of spina bifida
Can be only a defect in formation of vertebra (split vertebra) –> usually not associated with clinical signs
Can have various degree of protrusion of neuroectodermal and associated structures through the defect (e.g. meningocoele = only meninges protrude or meningomyelocoele = meninges and spinal cord or nerves protrude through the defect)
what is dermoid sinus
–Incomplete separation of neural tube from surface ectoderm
–Different degree of severity depending on whether the tract reaches or not the vertebral canal
–Frequent in certain breeds (e.g. Rhodesian Ridgeback).
