General embryology

Neurulation and the neural tube defects

6 steps · 2 anomalies · Week 3 to Week 4

Read this first

A story with only two dates worth memorising, and both of them explain a disease. The neural tube zips shut from the middle outwards, so it has two ends left open — and each open end, if it fails to close, gives a different and recognisable defect. Learn the two neuropores and their days and you have the whole topic.

Week 3

Step 1 of 6

What happens: The notochord induces the ectoderm lying over it to thicken into the neural plate, at about day 18.
What it forms: A pear-shaped plate of neuroectoderm, broad at the cranial end.
What remains in the adult: Everything in the central nervous system comes from this plate.

Nothing in this set goes wrong at this step — it is here because the story does not make sense without it.

Every anomaly, and the step it blames

This is the table the examiner is working from. Read the clue column first and see whether you can name the step before you look.

Anencephaly
You are told: An antenatal scan showing absent cranial vault, with a raised maternal alpha-fetoprotein.
Failed step: The cranial neuropore closes at about day 25. (week 4)
The cranial neuropore fails to close at about day 25, so the brain and the vault of the skull do not form.
Spina bifida with myelomeningocele
You are told: A midline swelling over the lower back of a newborn, with weak legs and bladder involvement.
Failed step: The caudal neuropore closes at about day 27. (week 4)
The caudal neuropore fails to close at about day 27, leaving the lower cord and meninges exposed or covered only by a sac.

Viva voce

1 of 5 · 0 answered

Say your answer out loud before you open it. That is the exam skill.

When and where does the neural tube begin to close?

If you remember nothing else

Closure begins in the cervical region and zips both ways
Two ends are left open — the cranial and caudal neuropores
Neural crest cells leave as the tube closes

Do not confuse it with

Spina bifida occulta
Occulta is not a failure of neurulation at all: the neural tube closes normally and it is the vertebral arches behind it that fail to fuse, most often at the fifth lumbar or first sacral level. That is why there is no neurological deficit, only a dimple or a tuft of hair.
Meningocele versus myelomeningocele
A meningocele contains meninges and cerebrospinal fluid only, so the legs usually work; a myelomeningocele contains neural tissue as well, and the child has weak legs and a neuropathic bladder.

Examiner trap

Calling spina bifida occulta a neural tube defect of closure. The tube closed; the vertebral arches did not.

Memory hook

Twenty-five for the head, twenty-seven for the tail.

Senior’s 30-second takeaway

Neurulation and the neural tube defects — the whole bench in four lines. Read this on the way in.

If you see this: A defect of the back or the cranial vault in a newborn · A question naming a neuropore · A raised maternal alpha-fetoprotein
If asked to identify it: The tube closes from the cervical region outwards, leaving two neuropores, and each one gives its own defect.
If asked one more: Cranial neuropore about day 25, caudal about day 27.
If asked why it matters: Folic acid must be started before conception, because the tube has closed before most pregnancies are recognised.

Then stop. Let the examiner ask the next question.

🧬 Read this first

Every anomaly, and the step it blames

🎙️ Viva voce

Senior’s 30-second takeaway

Read this first

Viva voce