Wk10 Embryological Development Of The Spinal Cord Flashcards
What happens in gastrulation?
- trilaminar disc = 3 layers:
- ectoderm- nervous tissue and skin
- mesoderm- notochord, muscles, circulatory system etc
- endoderm- digestive/ respiratory tracts
Notogenesis
- During gastrulation we have the appearance of the notochord- important in setting up neurulation.
- notochord is made from primitive node and migrates to form rod-like structure in mesoderm, formed during gastrulation
- elongated midline structure in the mesoderm of the trilaminar disc
- Notochord is instrumental in signalling and inducing the neural plate of ectoderm above to start to fold in on itself to generate the neural tube
Neurulation
- Neurulation= neural tube formation; also developmental process of forming nervous system
- embryo called neurula at this stage
- 1st step is induction of neural plate by the notochord
- Notochord (and paraxial mesoderm) sends signals to induce overlying ectoderm to thicken and form the Neural plate (noggin/ chordin/follistatin= BMP-4 inhibitors)
- Neural plate formation has cranial to caudal formation.
- This neural plate are cells of the neuroectoderm
Neurulation
Neural plate> Neural tube
Next steps:
Neural plate starts folding- neural folds forming midline neural groove
neural fold fuse together> neural tube
neural tube formation in the middle of embryo and move in both cranial and caudal directions
Neural tube is the precursor to the entire CNS
Closure of the neural tube
Neural tube closes firstly in cervical region – day 21
Neural tube closes towards the cranial and caudal ends leaving two openings:
• Anterior neuropore
• posterior neuropore
Anterior neuropore closes around day 25
Posterior neuropore closes around day 28
White/grey matter
Neural tube = neuroepithelial cells
neuroblasts= nerve cell precursors arise in the mantle/ intermediate zone
Intermediate zone= future grey matter
marginal zone= future white matter
Dorsal-Ventral Patterning of Neural Tube
Spinal cord is highly organised
Neural tube is patterned both segmentally and dorsal/ ventrally
Patterned by signals from surface ectoderm, paraxial mesoderm and notochord
Grafting experiments in mice and chick embryos important for our understanding of patterning within the developing neural tube.
Notochord produces Sonic hedgehog (Shh) induces floor plate to produce Shh
Shh= morphogen
Wnts/ BMPs > dorsalisation> induces roof plate Patterns Dorsal side of tube (alar plate)
Shh (notochord) > ventralisation> induces floor plate
Patterns ventral side- basal plate
Neural crest cells
During neurulation cells at the lateral border/ crest begin to dissociate from their neighbouring cells: Neural crest cells
Epithelial to mesenchymal transition migrate
Trunk Neural crest cells (level of the sixth somite to the most caudal somites) migrate and develop into:
1) sympathoadrenal> sympathetic ganglia and adrenal gland
2) Dorsal root ganglia> sensory neurons
3) Melanocytes
Lumbosacral neural crest cells: parasympathetic neurons and enteric nervous system
Cranial neural crest cells: sensory CN nuclei, parasympathetic ganglia, facial skeleton
Sympathetic from trunk NC cells!
Parasympathetic from lumbosacral, cranial, vagal and circumpharyngeal Neural crests!
Neural crest cells: DRG
Neural crest cells form the DRG
Two lineages : sensory neurons and glia
From there the sensory neurons axons project dorsomedially to neural tube and ventrolaterally into growing spinal nerve
where they join with motor neurons from the basal plate to form the trunk of a spinal nerve.
Cranial neural tube
Upper/ rostal neural groove enlarges before fusion
forms 3 swellings, then later 5 swellings (vesicles)
neural tube central canal become ventricular system
Brain forms around these swellings.
Brain development
Neural tube still organised into sensory alar and motor basal plates
however in some places the arrangement of the tube changes- e.g. brainstem
medulla: where arrangement forms cranial nerve motor and sensory nuclei in columns
midbrain: alar plates migrate from canal to form nuclei
Neural tube defects
Neural tube defects (NTDs)- failure of the neural tube to close, results in:
• cranial defects
• spinal dysraphism
Open NTDs occur in about 0.1% of live births
Can be detected by antenatal Ultrasound and or screening
Risk factors: Mothers age (older and younger mothers)
Folic acid deficiency
Maternal diabetes/ obesity
NTDs
Rachischisis- most severe, complete inability to close Failue of the cranial neuropore to fuse Anencephaly Cranial bifida/ cranial meningocele Failure of the caudal neuropore to fuse: spina bifida • Spina bifida occulta • Spina bifida cystica • Meningocele- Spinal cord not involved • Myelomeningocele- Spinal cord involved
Spina Bifida
Different forms of spina bifida cystica: menigocele- Spinal cord not involved
Myelomeningocele- herniating meninges and neural tissue (most common form of spina bifida cystica):
• co presents with Arnold- Chiari type 2 malformation and hydrocephalus
• motor and sensory impairments to lower limb, bowels and bladder
Spina bifida occulta
Spina bifida occulta – hidden • 2% population • May not be diagnosed until adulthood • failure of vertebral arches to fuse • may be associated with a cyst • Site may have overlying tuft of hair
