Neuro Block8: Development of the Nervous System

List the Order of early embryonic development

Day 1: Fertilisation → Day 5: Blastocyst → Day 8: Bilaminar disc → Day 15: Trilaminar disc + Primitive Streak Formation

What makes up the Bilaminar disc?

• ectoderm (amniotic side)

• endoderm (yolk sac side)

What are the 2 functions of the primitive streak?

• establishes body axis (cranial → caudal)

• becomes the source of mesoderm cells → allows trilaminar disc to form

What does the Rostral part of the primitive streak become?

rostral primitive streak → invaginates to become primitive pit → extends rostrally to become the notochordal process

How is the Notochord formed?

• the notochordal process (derived from rostral primitive streak mesoderm) fuses temporarily with endoderm → reforms as the notochord

What is the role of the notochord?

• by day 18 the notochord induces the overlying ectoderm to infold

• the notochord induces neural development via signalling molecules

Which structural component of the spine is derived from the notochord?

• nucleus pulposus (inside the intervertebral discs)

Define Neurulation:

• the process by which the neural tube is formed from the ectoderm during embryological development

• the CNS is derived from the neural tube

List the pathway of neurulation:

notochord induces ectoderm to form neural plate → neural plate folds → becomes neural groove → neural groove closes → becomes neural tube

At what day does the neural groove close and become the neural tube?

• day 22-24

What does the neural tube give rise to?

• the CNS

• neurons + glial cells (except microglia)

Draw Neural Tube Closure

By the end of which week are the 3 primary brain vesicles discerned?

week 4

What is the neural crest and how does it form?

• neural crest is a migratory cell population

• forms from edges of neural folds

List the derivatives of the neural crest:

1. PNS

2. sensory ganglia i.e. dorsal root ganglion

3. autonomic ganglia

4. Schwann cells

5. meninges

6. adrenal medulla

7. melanocytes

In what 2 ways is the body segmented?

1. dermatomes - an area of skin supplied by a single spinal root

2. myotome - a muscle/group of muscles supplied by a single spinal root

Draw out the brain development table

What does the spinal cord develop from?

• caudal end of the neural tube

• retains central grey matter and central canal

Draw and label the spinal cord: (C1 - coccygeal +)

Explain how the cerebral cortex develops?

• develops from neuronal cells that are initially around the inner part of the neural tube

• cerebral cortex is formed from radial glial cells

• these cells divide and produce neurons which migrate upwards

List 3 important cellular processes in CNS development:

1, Synaptogenesis

2. Apoptosis

3. Myelination

What is Synaptogenesis and when does it occur?

• connection formation between neurons

• occurs between week 20 - adulthood

What is apoptosis and when does it occur?

• programmed cell death

• occurs between week 16 to 4 months

What is myelination and when does it occur?

• axons gain myelin sheaths (via Schwann cells)

• occurs from week 28 to adulthood -L continues into early-mid 20s

Define Induction:

One tissue influences the development of nearby cells via chemicals.

List 3 key induction molecules in neuronal development:

1. Chordin and Noggin

2. BMP-4

3. Sonic Hedgehog (Shh)

What is the role of Chordin and Noggin in neuronal development?

• produced from notochord

• they diffuse to affect overlying ectoderm to INHIBIT its development into epiderms and instead cause it to form NEURONAL precursor cells

• INHIBITS BMP-4

What is the role of BMP-4?

• BMP-4 promotes epidermal fate (for ectoderm → epidermis)

What is the role of Sonic Hedgehog?

• produced from notochord tissues

• induces ventral part of neural tube to develop motor characteristics

• induces dorsal part of neural tube to acquire sensory characteristics

What is Nerve Growth Factor?

• decides which neurons survive while others undergo apoptosis

• ‘pruning’ of neural branches which went astray in development

What type of growth factor is it?

• trophic - supports survival

List 2 ways in which nerve growth factor promotes growth:

• Axon growth toward target tissues

• Survival of correctly connected neurons

Explain how the spine is formed embryologically:

• paraxial mesoderm segment into somites (craniocaudal direction) week 3-4

• somites differentiate → sclerotome → sclerotome migrates around notochord and neural tube

• caudal part of one sclerotome merges with cranial part of next → forms vertebrae and muscles bridge the vertebral joints

• chondrification + ossification week 6-8