Neurulation-overview

Question 1

What is neurulation and how does it occur?

Answer 1

Formation of brain/SC in embryos
Notochord induces CNS formation by signalling ectoderm above it to form NEURAL PLATE (composed of neuroectoderm)> folds on itself forming NEURAL TUBE> differentiates into SC/BRAIN
Neural crest cells derive from NT and migrate to form PNS/ other structures

Question 2

Gastrulation

Answer 2

Embryoblast- development of amniotic cavity > yolk sac (hypoblast cells)
Bilaminar disc has epiblast and hypoblast layer, occured due to primitive streak and trilaminar disc formed
ECTO, MESO and ENDODERM- embryo trilaminar

Question 3

Where do notochord and neural tube arise from?

Answer 3

Notochord- MESODERM
Neural tube- ECTODERM
Notochord- some mesodermal cells migrate cranially in midline of MD layer- forms rod like notochordal processes
Neural plate folds onself into neural tube, differentiates in SC and BRAIN- NCC derive from neural tube and migrate out to fom PNS, melanocytes and cartilage in head

Question 4

Development of neural tube

Answer 4

1. NED tissue differentiate from ectoderm and thickens into NP- border separates ectoderm from neural plate
2. NP bends dorsally, with two ends eventually joining at NP borders (NEURAL CREST)
3. Closure of NT disconnects NC from epidermis, NC differentiates to form most of PNS
4. Notochord degenerates and only persists as nucleus pulposos of IV discs- other form somites (axial skeleton, skel muscle precursor)

Question 5

What are the neural crest cell derivatives?

Answer 5

Spinal ganglia (DRG)
ANS ganglia
Ganglia of some cranial nerves
Sheaths of P. Nerves
Meninges of B/SC
Melanocytes
Adrenal gland
Skel/muscular components of head
NT separates from surface ectoderm, Bone Morphogenic protein important inducting agent from notochord

Question 6

Adult derivatives of notochord

Answer 6

Notochord persists in adulthood as NP of intervertebral disc
In embryo, inductive signals from notochord stimulate conversion of surface ectoderm into neural plate- PRIMORDIUM of CNS

Question 7

Development of somites

Answer 7

Arise from MD layer and give rise to protective components of SC
Notochord and NT form alongside longitudinal column of paraxial mesoderm
Differentiates/divides into paired cuboidal bodies (somites)- by end of week 5 there are 42-44 pairs

Question 8

What do somites form?

Answer 8

Cartilage (then bone) of axial skeleton (vertebrae/ribs)
Muscles of ribcage, limbs and back
Cells contributing to CT of dermis
Determined by specific regions of somite:
-Sclerotome (bone)
- Myotome (muscle)
- Dermatome (CT)

Question 9

Differentiation within somites

Answer 9

Initially composed of sclerotome/dermomyotome cells- ST cells lose adhesion to one another and begin to migrate, leaving remaining DMT cell to divide
Layer of muscle cell precursors (myotome) forms beneath epithelial dermatome

Question 10

Period of susceptibility to teratogenesis

Answer 10

W1+2- period of dividing zygote, implantation and bilaminar embryo
Death of embryo and spontaneous abortion common
W3-8 (embryonic)- Neural tube defects, mental retardation common
Sensitive to truncus arteriosus, atrial septal defects and ventricular septal defects, alcohol, cleft lip, cleft palate etc
W9- 38 (fetal)- mental retardation, low set ears and deafness, cleft palate
Check diagram lecture 44

Question 11

Congenital abnormalities of neural tube

Answer 11

Cranioarachischis- complete open brain/ sc
Anencephaly- open brain and lack of skull vault
Encephalocele- herniation of meninges and brain
Iniencephaly- occipital skull/spine defects, extreme retroflexion of head
NT defects most common abnormalities (w3/4 in 1/1000 births)
Can be due to teratogenic drug affecting Neuroectoderm> failure of neural folds to fuse/form neural tube (contiguous neuropores)

Question 12

Spina bifida

Answer 12

SB- failure of fusion of vertebra arches- occulta and cystica (Men/myelo)
SB dysraphism- deficiency of at least 2 vertebral arches (lipoma?)

Question 13

Spina bifida occulta

Answer 13

Mildest form- split in vertebrae so small herniation of spinal cord/ meninges does not occur
Asymptomatic- bony abnormality seen in x-ray
Nerves may be involved when associated with hairy patch/other skin change
Failure in dorsal vertebrae fusion affects 2-10% population

Question 14

Spina bifida cystica

Answer 14

Meningocele- protrusion of meninges (filled w CSF) through defect of skull/spine (2/1000 live births)
Myelomeningocele- open spinal cord (with meningeal cyst)- 2/1000 live births
prognosis- succesful closure of simple meningeocele prognosis is good
MMC risk from infection- if survive after proper closure, stationary disability

Question 15

Hydrocephalus

Answer 15

Often assc. w/ SB- build up of fluid on brain, many babies born w/ congenital hydrocephalus have permanent brain damage
If left untreated can be fatal

Question 16

Encephaloceles

Answer 16

Sac like protrusion of brain and meninges through openings in cranial vault
Cranial meningocoele- meningeal sac only
Cranial encephalocele- meningeal sac plus cerebral cortex, cerebellum, brain stem

Question 17

Anencephaly

Answer 17

Absent cerebral hemispheres and cerebellum (1/1000 live births)
NT defects can be rectified by 400mg folic acid PD