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Embryology · General embryology: weeks 1 to 8

Neurulation and closure of the neural tube !

  • Process

In weeks 3 and 4 a plate of cells rolls up into the tube that becomes brain and spinal cord; if it fails to close, spina bifida results.

Updated 2026-11-01

See it in 3D: Spinal cord · Cauda equina · L4 vertebra · L5 vertebra · Sacrum

What it is

Neurulation is the formation of the neural tube from ectoderm. The notochord induces the neural plate, whose edges rise as neural folds and meet in the midline. Closure starts in the future cervical region and proceeds toward both ends; the last openings are the cranial and caudal neuropores. It happens in the fourth week after fertilization, often before a woman knows she is pregnant, which is why folic acid must be present from before conception.

Key facts

  • Day 18–19: the neural plate appears over the notochord.
  • Day 22: the neural folds first fuse at the level of the future cervical region.
  • Day 25: the cranial neuropore closes.
  • Day 27–28: the caudal neuropore closes.
  • Secondary neurulation: the sacral and coccygeal cord forms later, from the caudal eminence.
  • Neural crest cells leave the edges of the folds as they close.

Step by step

  1. The notochord blocks BMP signaling and the overlying ectoderm thickens: neural plate.
  2. Its edges rise (neural folds) and the center sinks (neural groove).
  3. The folds come together and fuse in the midline, starting in the cervical region.
  4. Closure runs cranially and caudally like a zipper.
  5. The tube separates from the surface ectoderm, which covers it as future skin.
  6. The cranial (day 25) and caudal (day 27–28) neuropores close.
  7. The tube's lumen becomes the ventricles and central canal; its wall, the brain and cord.

Markers

  • SHH
  • BMP4
  • Noggin
  • Chordin
  • AFP

Clinical notes

  • Open cranial neuropore → anencephaly; open caudal neuropore → spina bifida (myelomeningocele).
  • Clues: high alpha-fetoprotein in maternal serum and amniotic fluid, and acetylcholinesterase in amniotic fluid.
  • Ultrasound: lemon sign (skull) and banana sign (cerebellum) from the associated Chiari II malformation.
  • Risk factors: low folate, valproate, carbamazepine, maternal diabetes, obesity and hyperthermia in early pregnancy.
  • Colombia fortifies wheat flour with folic acid, and the national health institute (INS) tracks birth defects, including neural tube defects.

Test yourself

Think of the answer, then tap the question to see it.

On about which day do the cranial and caudal neuropores close?

The cranial around day 25 and the caudal around day 27–28 after fertilization.

Which defect results from failed closure of the caudal neuropore?

Spina bifida, whose severe form is myelomeningocele.

Which lab finding suggests an open neural tube defect?

Raised alpha-fetoprotein in maternal serum or amniotic fluid, with acetylcholinesterase in the amniotic fluid.

Why must folic acid be present before conception to prevent these defects?

Because the neural tube closes around the fourth week after fertilization, often before the pregnancy is known.

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See it in 3D

The structures of Durgical's 3D anatomy where it is found.

In Pharmacology

← All of embryology

Educational content on embryonic development; not a guide to prenatal diagnosis or to medicines in pregnancy and no substitute for a health professional's advice. No doses are given.

Sources: Langman's Medical Embryology, 13th ed., ch. 6: Third to Eighth Weeks: The Embryonic Period (Wolters Kluwer, 2015); About Spina Bifida | Spina Bifida | CDC (Centers for Disease Control and Prevention, 2026); About Folic Acid | Folic Acid | CDC (Centers for Disease Control and Prevention, 2026).

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