Early development: fertilisation to gastrulation
Introduced| English | Meaning |
|---|---|
| fertilisation/ˌfɜːtɪlaɪˈzeɪʃn/ | The fusion of the nuclei of two gametes to form a zygote. |
| cleavage | The rapid cell divisions of the zygote without growth. |
| morula | A solid ball of about 16 cells formed by cleavage. |
| blastocyst/ˈblæstəsɪst/ | The hollow early embryo with an inner cell mass that implants in the uterus. |
| inner cell mass | inner cell mass |
| trophoblast | trophoblast |
| gastrulation | The stage that forms the three germ layers in week 3. |
| germ layer | germ layer |
| ectoderm | The outer germ layer: skin epidermis and the nervous system. |
| mesoderm | The middle germ layer: muscle, bone, heart, blood and kidneys. |
| endoderm | The inner germ layer: the gut lining, liver, pancreas and lung epithelium. |
| neurulation | The folding of ectoderm into the neural tube, the future brain and spinal cord. |
One cell to a body plan
- In three weeks a single cell lays down the body's axes and layers, faster and more precisely than at any later time.
- That speed makes these weeks the most vulnerable, and the most instructive, period of life.
The first week
- Fertilisation 受精 is day 0. Cleavage 卵裂 divisions make a solid morula 桑葚胚 by about day 3.
- By about day 5 it is a hollow blastocyst 胚泡, which implants in the uterine wall on days 6–7.
Put the stages of the first weeks in order.
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Day 0, about day 3, about day 5, days 6–7, then week 3.
Two parts of the blastocyst
- The inner cell mass 内细胞团 becomes the embryo itself.
- The outer trophoblast 滋养层 becomes the fetal part of the placenta. Mixing these two up is a classic exam error.
Which part of the blastocyst becomes the embryo itself?
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The inner cell mass forms the embryo; the trophoblast forms the fetal part of the placenta.
Gastrulation: three layers
- In week 3, gastrulation 原肠胚形成 reorganises the cells into three germ layers 胚层 and sets the body's axes.
- Ectoderm 外胚层 makes the nervous system and epidermis; mesoderm 中胚层 makes muscle, bone, blood, kidney and heart; endoderm 内胚层 lines the gut and lungs and makes the liver and pancreas.
Match each germ layer to a tissue it makes.
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Outside, middle and inside layers map onto the body's outer cover and nerves, its supporting and moving tissues, and its inner lining.
Neurulation
- In weeks 3–4, neurulation 神经胚形成 folds ectoderm into the neural tube, the future brain and spinal cord.
- The tube finishes closing about day 28 after fertilisation. Tissues are neighbours before they are organs, so one early field defect can affect several organs.
The neural tube closes about day 28 after fertilisation. About how many weeks is that in clinical terms, counted from the last period (add 2 weeks)?
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28 days = 4 weeks after fertilisation, + 2 weeks = about 6 weeks from the last period.
Worked example: why folate comes first
- Implantation about day 6–7, gastrulation about day 15, neural tube closed about day 28 after fertilisation.
- Day 28 is about two weeks after the first missed period, often before the pregnancy is known. So folic acid must start before conception to protect the tube.
Starting folic acid when a pregnancy is first confirmed at the clinic is early enough to protect neural tube closure in every case.
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The tube closes about two weeks after the first missed period, often before the pregnancy is known or confirmed; folate must start before conception.
Worked example: the tube that does not close
- (Fictional case) Raised maternal alpha-fetoprotein, and ultrasound shows an open spine: the neural tube failed to close at the end of week 4.
- The level of the defect predicts the loss: a lumbar defect affects the legs and bladder; a higher one affects more. Development wrote the prognosis into the anatomy.
A baby is born with an open lumbar spine. Which problems are expected?
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The lumbar and sacral cord supplies the legs and bladder; the level of the defect predicts the loss.
Three common slips
- Mixing up the blastocyst's parts. Inner cell mass → embryo; trophoblast → placenta.
- Germ layers as a list. They are a spatial map: a tube inside a tube inside a body.
- Counting pregnancy weeks from fertilisation. Clinical weeks count from the last period, about 2 weeks earlier.
Recap
- Day 0 fertilisation → day 3 morula → day 5 blastocyst → implantation days 6–7.
- Week 3 gastrulation makes ectoderm, mesoderm and endoderm; week 4 the neural tube closes.
- Folate must start before conception, because closure comes early.