Organogenesis and congenital anomalies
Introduit| English | Sens |
|---|---|
| heart septation | heart septation |
| foramen ovale | foramen ovale |
| ductus arteriosus | ductus arteriosus |
| patent foramen ovale | patent foramen ovale |
The busiest building site
- In weeks 4–8 three flat layers become recognisable organs. It is the most error-prone window of human life.
- Reasoning about birth defects is development read backwards: from the defect, work out the week and the process that failed.
Construction windows
- Weeks 3–4: the neural tube and heart tube form. Weeks 4–8: the heart's septa, the limbs and the face take shape; the diaphragm closes around week 7 and the palate fuses in weeks 7–10.
- After week 8 organs mostly grow and mature rather than form, so a harmful exposure then is less likely to cause a major structural defect.
Match each event to its window.
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A harmful exposure damages whatever is being built at that moment, so the window dates the defect.
A harmful drug first taken in week 20 of pregnancy is most likely to cause a major structural malformation such as a cleft palate.
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By week 20 the palate and other major structures have formed; later exposures more often affect growth and function.
Building the heart
- The heart tube folds in week 4. Then heart septation 心脏分隔 divides it: septa grow to separate the atria and the ventricles.
- The atrial septum keeps a flap-valve hole, the foramen ovale 卵圆孔, open throughout fetal life.
Why must the foramen ovale stay open before birth?
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Before birth oxygenated blood comes from the placenta, so the lungs are bypassed; the foramen is one of two bypasses.
The fetal bypass
- Before birth the lungs are not used, so blood bypasses them twice: right atrium → foramen ovale → left atrium, and pulmonary artery → ductus arteriosus 动脉导管 → aorta.
- At the first breath lung resistance falls, left atrial pressure rises above right, and the foramen's flap is pushed shut. The ductus closes over the next few days.
Put the events at the first breath in order.
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The pressure change, not a muscle, closes the flap; tissue seals it over the following months.
Worked example: the hole that stays
- The foramen closes by pressure at once, but it is sealed by tissue only over months, and in about 25% of adults the seal is never complete: a patent foramen ovale 卵圆孔未闭.
- It usually does no harm. It matters only when right atrial pressure rises above left, letting blood (or a clot) cross from right to left.
About 25% of adults have a patent foramen ovale. In a group of 120 adults, about how many would you expect to have one?
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120 × 0.25 = 30. It is common and usually harmless, so finding one does not by itself explain a symptom.
Worked example: one cause, several systems
- (Fictional child) A cleft palate and a murmur from a ventricular septal defect. The palate and the ventricular septum were being built in overlapping weeks.
- Some causes hit both: a harmful exposure in that window, or a gene change in a signal both use (the 22q11 deletion is the classic one). So a doctor who finds a cleft always listens to the heart.
A baby has a cleft palate. Why should the heart be examined too?
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A harmful exposure or a shared gene change, such as the 22q11 deletion, can affect both; one defect prompts a look for its neighbours.
Three common slips
- "Congenital" means "genetic". Most defects have mixed causes; many are about timing.
- Dating a defect by birth. The error happened weeks or months before birth.
- Forgetting the placenta is fetal. Its vessels are part of the fetus's circulation.
Recap
- Weeks 4–8 build the organs; each septum and fold has its window.
- The foramen ovale and ductus arteriosus are open before birth and close after it; about 25% of adults keep a patent foramen ovale.
- Organs built in the same weeks can share one cause.