How to Make a Brain

Question 1

Describe the process of neurulation

Answer 1

The neural plate (derived from ectoderm) is stimulated to undergo neurulation when the notochord forms ventral to the neural plate.

A thickening is formed by the cells on the edge of the plate. The actin in the apex of these cells constrict which fuses the edges to form a neural tube with a rostral neuropore (closes day 25) and a caudal neuropore (closes day 28).

Question 2

Describe what can happen when there are defects in neuropore closure

Answer 2

Rostral neuropore = anencephaly

• no forebrain
• unreactive to light and sound
• usually stillborn

Caudal neuropore = spina bifida (3 types)

• characterised by a displaced cerebellum and hydrocephalus
• occulta (mild): no spinal cord protrusion but incomplete closure of vertebrae)
• meiningomyelocele : protrustion of spinal cord but undamaged
• myeloschisis: part of spinal cord unfused, protrusion into sac formed by meninges (severe disability)

Question 3

What is the role of folate in pregnancy?

Answer 3

Supplements of folate are taken in maternal diet to reduce the risk of neural tube defects. This is because folate is involved in the DNA synthesis pathway in cell division.

Question 4

Describe how the walls of the forebrain, midbrain and hindbrain are formed

Answer 4

3 swellings form at the end of the neural tube;

Forebrain (prosencephalon): forms lateral expansions that becomes the telencephalon (cerebral hemispheres) and diencephalon (thalamus)

Midbrain (mesencephalon): forms a single central tube

Hindbrain (rhomboencephalon): forms the metencephalon (pons, cerebellum) and myelencephalon (medulla)

Question 5

Describe the derivation of the ventricles of the brain

Answer 5

Lateral and III ventricle = forebrain
cerebral aqueduct = midbrain
IV ventricle = hindbrain

Question 6

Describe the development of the IV ventricle

Answer 6

It sits closed before widening in development and forming a roof that contains specialised cells that secrete CSF. With widening the dorsal elements (sensory) become lateral.

Question 7

Describe the development of the spinal cord

Answer 7

In the neural tube, the ventricular layer (progenitor cells) divide to form neurons/glia that form the mantle layer. Here, some neurons grow processes and communicate to make the white matter layer (marginal layer).

A sulcus forms (sulcus limitans) which separates the ventral (motor) elements from the dorsal (sensory) elements.

Motor grey matter sprout to form the ventral root of the spinal cord. DRG then enter the dorsal horn of the spinal cord.

Question 8

Describe the origin of the neural crest cells and its derivatives

Answer 8

Origin: cells on the neural groove (of the developing neural tube) change from epithelial to migratory cells which form the neural tube cells

Derivatives:

• skull (bone = odontoblasts)
• sensory and autonomic NS (ganglia)
• pigment cells (melanocytes)