By day 17-19 post-fertilization, the embryo is already a thin trilaminar disc (ectoderm + mesoderm + endoderm).Signal: the notochord forms in the midline mesoderm and releases Shh (Sonic Hedgehog) + BMP antagonists to the overlying ectoderm, specifying it as the neural plate.
Neural-plate cells at this point: · Proliferate rapidly + elongate → forming 'elongated neuroepithelium' · Demand for folate + B12 spikes: one-carbon metabolism is running flat-out, producing purines + thymidine as raw materials for DNA replication
Almost no women know they are pregnant at this point — the menstrual period is only 5-7 days late, and most home hCG tests are just starting to register.
2 · Day 20–22 · groove forms
The tops of midline neural-plate cells contract and narrow, so the whole plate folds upward into a U and forms the neural groove.Driven by BMP / Wnt + cytoskeletal contraction, the apical actin rings of midline neural-plate cells contract → apical narrowing → the whole neural plate folds upward into a U → forming the neural groove.
The raised ridges on either side of the groove are the neural folds — they will close toward the midline and zip the tube shut.
Critical: cells are still proliferating rapidly here, and one-carbon demand is at its peak. Folate shortage is the main preventable risk factor for failed closure; B12, choline and others sit in the same metabolic network, but the evidence for them is far thinner than for folate.
3 · Day 23–26 · zipper closure starts mid
The neural folds first meet and fuse at the cervical segment, then zip toward both ends, and only when both neuropores close is the neural tube intact.Neural folds first meet + fuse at the cervical segment (around somite 6) → closing into a tube. Then closure expands like a zipper toward both ends:
· Rostral → anterior neuropore closes by day 25 · Caudal → posterior neuropore closes by day 27
With both neuropores closed, the neural tube is intact → developing into: · Anterior → brain vesicles → cerebrum + cerebellum + brainstem · Posterior → spinal cord
Failure of closure = neural tube defect (NTD) — irreversible (once this window passes, the consequence is lifelong): · Anterior neuropore fails → anencephaly, cerebral hemispheres missing, most are stillborn or die as neonates · Posterior neuropore fails → spina bifida, varying degrees of neurologic injury; severe forms cause lower-limb paralysis + incontinence · Anywhere in between → encephalocele, skull defect with herniation of brain tissue
4 · Where folate sits · public health miracle
Folate supports neural tube development on three chemical levels, and randomized trials and staple-food fortification both show that folic acid reduces neural tube defects.Folate (folic acid) acts on this developmental sequence at least on three independent chemical levels:
1. DNA-synthesis raw materials: 5,10-methylene-THF → thymidine synthesis (TYMS pathway)10-formyl-THF → purine synthesisWithout these two routes, cells cannot divide 2. Methylation: 5-methyl-THF → methylates back to methionine (with B12, via MTR)Methionine → = the universal methyl donorThis affects DNA methylation + histone methylation (epigenetic regulation) 3. Lowering homocysteine: High homocysteine is associated with neural tube defects, but whether it is a direct cause is still debatedFolate, B12 and B6 all take part in homocysteine metabolism The evidence that folate prevents NTDs:
· MRC 1991 (The Lancet, ) — women at high risk because of a previous NTD pregnancy took 4 mg of folic acid a day → recurrence risk fell 72% · Czeizel 1992 (New England Journal of Medicine, randomized controlled trial) — women from the general population took a multivitamin with 0.8 mg of folic acid from before conception → first-occurrence NTDs fell to 0 (no cases in the vitamin group, several in the control group) · The US CDC in 1992 advised 0.4 mg a day for all women of reproductive age · From 1998 the US required 0.14 mg of folic acid per 100 g of cereal-grain products — Williams 2015: NTD birth prevalence fell by about 28% (35% in surveillance programs that count prenatal diagnoses), about 1,326 fewer affected births a year · Crider 2022 review: 71 countries require folic acid in a staple food, with reported reductions of up to 78%
Why fortifying a staple works: a single intervention lowers a whole class of birth defects, and it does not depend on anyone remembering to take a pill.
But the core problem: at days 21-28, most women still do not know they are pregnant — by the time they do, starting folic acid is already too late. That is why women planning a pregnancy should start early, usually 3 months before conception.