Embryology of CNS

Question 1

what are the 3 stages of development of the neural tube

Answer 1

1. neural plate: ectodermal cells overlaying the notochord become tall columnar (in contrast to surrounding ectorderm that produces epidermis of skin)
2. neural groove
3. neural tube: the dorsal margins of the neural groove merge medially, forming a neural tube composed of columnar neuroepithelial cells surrounding a neural canal

Question 2

what does the notochord induce

Answer 2

overlaying ectoderm to become neuroectoderm and form a neural tube

Question 3

how does closure of the neural tube progress

Answer 3

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

Question 4

what are the outcomes of neural tube differentiation and the associated structures

Answer 4

1. neural tube –> CNS (brain + spinal cord)
2. cavity of tube (neural cavity) –> central canal of spinal cord and ventricles of brain
3. neural crests: arise from border of neural plate and surface ectoderm –> form a column of cells dorsolateral to neural tube

Question 5

what are the outcomes of neural crest differentiation

Answer 5

1. neurons of PNS with their cell bodies located in spinal ganglia
2. neurolemmocytes (Schwann cells) of PNS (myelination of nerves)
3. ganglionic neurons of ANS and enteric nervous system
4. others: adrenal medulla cells, melanocytes of skin, variety of structures in the face, etc.

Question 6

what are the structures shown

Answer 6

Question 7

what occurs to the neural tube

Answer 7

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)

Question 8

what are the two cell types in the differentiation of neural tube development

Answer 8

1. immature neurons
2. spongioblasts

Question 9

what are immature neurons

Answer 9

differentiate into nmature neurons (no further division occurs)

Question 10

what are spongioblasts

Answer 10

progenitors of the neurectodermal supporting cells of the nervous system = neuroglia (oligodendrocytes and astrocytes)

Question 11

what are the 3 concentric layers

Answer 11

1. mantle layers: develop into grey matter (where neuronal cell bodies are located)
2. marginal layer: becomes white matter (where axons the neurons are located)
3. the neural canal becomes the central canal in spinal cord and the ventricular system in the brain. lined by ependymal cells

Question 12

what is the neural canal lined by

Answer 12

ependymal cells originating from germinal layer –> only remnant of this germinal layer = one cell thickness

Question 13

what is the final steps of neural tube development

Answer 13

Question 14

how does the dorsal nerve roots develop in the sensory system

Answer 14

Axons emerge from the segmental spinal ganglion (neural crest)

Form afferent neurons (sensory)

Cell body is in the spinal ganglion

Question 15

how does development of ventral nerve roots in the motor system occur

Answer 15

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

Question 16

how does the brain develop in the first steps

Answer 16

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).

Question 17

what are the structures shown here

Answer 17

Question 18

what structures are shown here

Answer 18

Question 19

what structures are shown here

Answer 19

Question 20

what structures are shown here

Answer 20

Question 21

how is the forebrain organized

Answer 21

in the telecephalon the arrangement is different from the basic neural tube organization

Question 22

where are the neuronal cell bodies located in the forebrain organization

Answer 22

neuronal cell bodies are located in 2 locations

1. most of grey matter is located in cerebral cortex
2. others are located deeply in subcortical basal nuclei

Question 23

how is the white matter organized in the forebrain

Answer 23

white matter tracts (axons) are located deep inside telencephalon

Question 24

where is the commissural in the forebrain

Answer 24

between cortical areas from one hemisphere to the other

Question 25

where is the white matter association fibres

Answer 25

between cortical areas within hemispheres

Question 26

where are the white matter projection fibres

Answer 26

connecting cortical areas of a hemisphere and brainstem nuclei

Question 27

what are the white matter tracts

Answer 27

1. commissural fibres (corpus callosum) 2. projection fibres (internal capsule) 3. association fibres (long and short)

Question 28

what are the structures shown

Answer 28

Question 29

what are the 3 layers of connective tissue of the meninges

Answer 29

1. dura matter: thick, fibrous layer 2. arachnoid: thin layer, which encloses the cerebrospinal fluid within the subarachnoid space 3. pia matter: thin layer, adherent to the CNS parenchyma

Question 30

what are the 3 connective tissue layers derived from and what do they cover

Answer 30

derived from neural crests with a small mesodermal contribution cover the brain and spinal cord

Question 31

what are the main type of malformations in development

Answer 31

1. hydrocephalus 2. neural tube defects 3. migration disorders 4. encephaloclastic defects (destructive lesion) 5. others: cerebellar and spinal cord malformations

Question 32

what is anecephaly/atelencephaly

Answer 32

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

Question 33

what is cranial bifidum (two forms)

Answer 33

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

Question 34

what does cranium bifidum normally effect

Answer 34

dogs at a young age --\> median 6.5 months

Question 35

what are the presenting problems with carnium bifidum

Answer 35

seizures and behavioural abnormalities

Question 36

what are the causes of cranium bifidum

Answer 36

1. genetic: craniofacial malformation 2. teratogenic substances: griseofulvin treatment of pregnant queens 3. folic acid deficiency: well recognized cause in people 4. cause unknown

Question 37

what is diecephalus

Answer 37

due to incomplete duplication (partial twins)

Question 38

what is holoprosencephaly

Answer 38

holoprosencephaly: failure of cleavage in two separate telencephalic vesicles

Question 39

what is lissencephaly

Answer 39

smooth brain without gyri/sulci and pachygyria (thick cerebral cortex) might be associated with cerebellar hypoplasia

Question 40

what is hydranencephaly

Answer 40

–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

Question 41

what is chiari-like malformation

Answer 41

–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.

Question 42

what is spina bifida

Answer 42

–Frequent vertebral malformation due to a defect of closure of the neural tube

Question 43

what is the different degrees of severity of spina bifida

Answer 43

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)

Question 44

what is dermoid sinus

Answer 44

–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).