Lecture 2- Development of the nervous system 1/2

Question 1

Day 18- Primitive disc

Answer 1

• At day 18 primitive node and streak form in the epiblast cells (will become trilaminar disc)
  
  • Epiblast from superior view= future ectoderm (skin and NS)

Question 2

what occurs along the primitive streak

Answer 2

• process of invagination will occur–> gastrulation

Question 3

Gastrulation

Answer 3

• formation of trilaminar embryo
  
  • Epiblast cells approach primitive streak and invaginate
  • Go onto form the other 2 embryonic layers
    
    • Mesoderm
    • Ectoderm

Question 4

how is the notochord formed

Answer 4

• Part of the mesoderm will grow cranially from the primitive pit –> forming notochord

Question 5

role of notochord

Answer 5

• inducts the overlying ectoderm to form the neural tube and the rest of the CNS

Question 6

In adult CNS the ……………. ………………….is the only remnant of the notochord

Answer 6

nucleus pulposus

Question 7

Day 20- development of ectoderm to create CNS

Answer 7

• Skin ectoderm (pink)
• Neuroectoderm (green)
  
  • Will form neural tube
• Cranial end is bigger and more sophisticated than caudal end will become brain

Question 8

outline how the neural tube is created

Answer 8

• Under direction of the notochord, the ectoderm is rising up, folding and then invaginating down forming the neural tube

​Ectoderm will start to elevate (green sides) to form neural folds and groove

Question 9

why does the neural tube run along the axis of the foetus

Answer 9

Notochord runs cranially to caudally (grey line) invagination along the axis of the foetus

Question 11

Day 22- Neurulation

Answer 11

• Where the skin ectoderm crosses the green neuroectoderm = where neural tube has fully invaginated (in the cervical region)
  
  • If we look at a cross section at this level= formation of a complete tube (neural tube)
  • Skin ectoderm has healed up over this point forming a body wall
  • From this cervical point, the neural tube will ‘zip up’ along its length
    
    • Cranially to form the brain
    • Caudally to form the spinal cord

Question 12

neurulation- a stage where things can go wrong

Answer 12

• If the neural tube fails to ‘zip up’ in the cranial direction –> anencephaly
  
  • If cranial tube doesn’t close- brain wont form
  • Incompatible with life
• If the neural tube fails to ‘zip up’ in the caudal direction–> spina bifida
  
  • Failure of closure of neural tube in caudal direction

Question 13

neural crest cells

Answer 13

At the apices of the neural folds we have a further population of cells called the neural crest cells

• Once the neural tube has closed these neural crest cells become free and, migrate to many places in the embryo
• Diverse tissue sets
  
  • Head and neck
  • Heart
  • Melanocytes in skin
  • Sensory and autonomic ganglia

Question 14

once complete invagination of the cranial and caudal ends has occured what can be seen

Answer 14

somites

• We can see somites which will form future, skin, muscles and vertebrae

Question 15

Day 26- Formation of 3 brain vesicles

Answer 15

• Very simple at this stage and bears little resemblance to adult CNS
  
  • Except we can see cranial end has developed three swellings (vesicles) and flexures
    
    • Forebrain
    • Midbrain
    • Hindbrain
  • Flexures: midbrain (cephalic) flexure and cervocal fleuixre (junction between brainstem and cervical spinal cord

Question 16

imaging showing early development of the three primary brain vesicles

Answer 16

Question 17

Forebrain vesicles- day 26

Answer 17

• We can start to see laterally directed outgrowths of the diencephalon
  
  • These outgrowths (grey) will become the optic nerves
  • Growing towards the developing eyes to connect up with them

Question 18

label this embryo at day 35-38

Answer 18

Question 19

Focussing in on the nervous system at 35-38 days

Answer 19

Question 20

forebrain is known as the

Answer 20

prosencephalon

Question 21

midbrain is also known as

Answer 21

mesencephalon

Question 22

hind brain is also known as the

Answer 22

rhombencephalon

Question 23

development of the forebrain (prosencephalon) vesicle at 35-38 days

Answer 23

• Divided into two subdivision:
  
  • Cranially- telencephalon (cerebral hemispheres)
    
    • Dark green
  • Caudally- Diencephalon (thalamus, hypothalamus and optic nerve)
    
    • Light green

Question 24

development of the midbrain (mesencephalon) vesicle at 35-38 days

Answer 24

• Only has 1 division

Question 25

development of the hindbrain (rhombencephalon) vesicle at 35-38 days

Answer 25

* **2 subdivision** * Metencephalon (light green) * Pons and cerebellum * Myelencephalon * medulla

Question 26

summary of vesicle organisation

Answer 26

Question 27

telencephalon

Answer 27

cerebral hemispheres

Question 28

diencephalon

Answer 28

thalamus, hypothalamus and optic nerve

Question 29

mesencephalon

Answer 29

midbrain

Question 30

etencephalon

Answer 30

pons and cerebellum

Question 31

* Myelencephalon

Answer 31

medulla

Question 32

**Day 45- Further elabortion of the neural tube esp at cranial end**

Answer 32

* **Telencephalic outgrowth forming cerebral hemisphere** * **Will develop frontal, parietal, temporal, occipital and limbic lobes (5 lobes of the brain)** * **Diencephalon**

Question 33

skeletal system at day 45

Answer 33

**Skeletal system** * Not fully developed yet * **At this stage the neural tub fills the entirety of the vertebral canal** (1:1 correspondence between spinal cord segments and vertebral levels) * As growth proceeds the vertebral bodies will grow much faster than the spinal cord, resulting in the spinal cord being pulled down leading to the formation of the cauda equina (L1)

Question 34

Day 60

Answer 34

* **We can start to see:** * Cerebellum * Cerebral hemispheres * Spinal cord and dorsal root ganglia * Peripheral nerves are growing into the upper and lower limbs

Question 35

cranial nerves

Question 36

**Ventricular system development (at day 60)** *

Answer 36

1. Neural canal dilates within the cerebral hemisphere- corresponding to the lumen of the neural tube (like a mould)- forming the lateral ventircles 2. Chororid capillary plexus which invaginates the roof plates of the ventricles secretes CSF which flows through ventricular system 3. These communicate with the third ventricle contained within the diencephalon via the interventiruclar foramen (foramen of monro) 4. 3^rd and 4^th ventricles communicate through the cerebral aqueduct (aqueduct of sylvius) in the midbrain 5. CSF exits 4^th ventricle through 3 apertures in its roof known as foramen magendie (in midline) and the right and left foramen luschka)

Question 37

* Developmental defects in the ventricular system can cause

Answer 37

* congenital hydrocephalus * Blockage of drainage of CSF * Also blocked by tumours and stenosis

Question 38

why does the ventricular system look proprotionally larger in developing embryos than in humans

Answer 38

* Much larger than in the adult ventricular system due to neurones not having proliferated yet * Shape corresponds to the cerebral hemispheres * Quite large cavity in brain at this point since we haven’t had much growth of the neural tissue yet