Appearance
4.4 — Gastrulation and the Three Germ Layers
The embryologist Lewis Wolpert wrote that it is not birth, marriage or death that is the most important moment of your life, but gastrulation. He meant it literally. Before it, you are a disc of undifferentiated cells with no front, back, head or tail. About a day and a half later, the entire body plan exists in outline, and every cell knows which of three layers it belongs to and therefore what kind of tissue it will become.
Everything in the rest of this volume descends from that assignment, and this is the page that explains why the diaphragm is supplied by nerves from the neck, why a child with a heart defect often has a facial anomaly too, and why the retina is technically part of the brain.
Week 2: two layers and a cavity
By the start of week 2 the embryo is implanted (Chapter 4.3), and the inner cell mass flattens into a bilaminar disc — two sheets of cells stacked on top of each other.
Epiblast is the upper layer, facing a fluid-filled space that becomes the amniotic cavity. Everything in your body comes from the epiblast.
Hypoblast is the lower layer, facing the yolk sac. In humans it contributes almost nothing to the body — the yolk sac is a leftover from egg-laying ancestors, retained because it is the site where the first blood cells and the germ cells form before migrating to their final homes.
Week 3: the primitive streak, and the moment everything is decided
On about day 15, a groove appears along the midline of the epiblast at one end. This is the primitive streak, and its appearance is the single most consequential event in development, because it establishes three axes at once.
The streak defines the head–tail axis — it forms at what becomes the tail end and elongates toward the head. It defines left and right, by being on the midline. It defines back and front, because the surface it forms on becomes the back.
Then cells begin to move. Epiblast cells at the streak change shape, detach from their neighbours, and dive down through the groove into the space beneath — a process called ingression. This migration is gastrulation, and it converts two layers into three.
The first wave displaces the hypoblast entirely and becomes the endoderm. The second wave spreads out between the two layers and becomes the mesoderm. What remains on the surface becomes the ectoderm.
\text{epiblast} \;\longrightarrow\; \text{ectoderm (stays)} + \text{mesoderm (middle)} + \text{endoderm (bottom)}
Three layers, one origin, and every cell's fate now constrained by which layer it joined.
The name means it. Gastrula comes from the Greek for stomach, because in simpler animals this process creates the gut tube directly. In humans it is more elaborate, but the endoderm still becomes the gut lining.
And the primitive streak is a legal and ethical boundary as well as a biological one. In several jurisdictions the limit on research using human embryos is set at 14 days, because that is when the primitive streak appears — and before it appears the embryo can still split into twins, so it is not yet definitively one individual. Chapter 4.3 covered the twinning timings that make that argument.
One more thing forms with the streak. The notochord is a rod of cells extending forward along the midline from the streak. It is a temporary structure — it becomes only the soft centres of the intervertebral discs in the adult — but while it exists it is the organiser. It secretes signals that instruct the tissue above it to become nervous system, and signals that pattern the mesoderm around it. Remove it experimentally and no nervous system forms.
The master table
This table is the payoff of the whole chapter. Memorising the logic rather than the list is what makes it useful, and the logic is: ectoderm covers and senses, mesoderm supports and moves, endoderm digests and breathes.
| Ectoderm (outer) | Mesoderm (middle) | Endoderm (inner) |
|---|---|---|
| Epidermis of skin | All muscle | Lining of gut |
| Hair, nails, sweat glands | Bone and cartilage | Lining of airways and lungs |
| Brain and spinal cord | Blood and blood vessels | Liver |
| All peripheral nerves | Heart | Pancreas |
| Retina and lens | Kidneys and ureters | Thyroid and parathyroid |
| Inner ear | Gonads | Thymus |
| Tooth enamel | Dermis of skin | Bladder lining |
| Adrenal medulla | Adrenal cortex | Middle ear cavity |
| Pituitary (front and back, from two sources) | Spleen | Tonsils |
Several entries in that table explain something clinical.
The nervous system is ectoderm — the same layer as skin. This is not a coincidence of filing; both come from the surface, and the nervous system forms by the surface folding inward. It is why several inherited conditions affect skin and nervous system together — a family of disorders called the neurocutaneous syndromes, including neurofibromatosis and tuberous sclerosis, in which characteristic skin marks accompany brain lesions. If you see the skin signs, you look at the brain.
The retina is ectoderm, and specifically it is an outgrowth of the developing brain. So the retina is central nervous system tissue, not peripheral. This is why the optic nerve is not really a nerve but a brain tract, why it does not regenerate after damage the way a peripheral nerve can, and why looking into the back of the eye with an ophthalmoscope is the only place in the body where you can see brain tissue and blood vessels directly, without cutting anything. Raised pressure inside the skull shows up there as a swollen optic disc, which is why a doctor looks in your eyes when worried about your head.
The adrenal gland is two organs from two germ layers. The cortex is mesoderm and makes steroid hormones. The medulla is ectoderm — specifically neural crest — and makes adrenaline. It is essentially a modified sympathetic nerve ganglion, which explains why it releases adrenaline on nervous command in a fraction of a second, rather than on the slow hormonal schedule the cortex uses (Chapter 12.4).
The liver, pancreas, thyroid and lungs are all endoderm, because all of them start as outpouchings of the primitive gut tube. That is why they all begin as buds from one tube and why blockage of a duct is such a common theme in their diseases.
Neurulation: how the nervous system forms
Days 18 to 28. The notochord signals the ectoderm above it to thicken into the neural plate. Its edges rise into folds, curl toward each other, meet in the midline and fuse, forming the neural tube. The tube then detaches from the surface and sinks beneath it, and the surface ectoderm closes over the top.
Fusion starts in the middle and zips in both directions, so the two ends close last: the head end around day 25, the tail end around day 27.
And that is why neural tube defects occur at the ends.
Failure to close at the head end gives anencephaly — the brain does not form. It is invariably fatal.
Failure at the tail end gives spina bifida. Severity depends on how much is exposed: the mild form is a vertebral arch defect covered by skin and often symptomless; the severe form has the spinal cord itself exposed, with paralysis below that level, loss of bladder and bowel control, and usually hydrocephalus.
Folic acid prevents most of them, and the timing is the point. Supplementation reduces neural tube defect risk by around 70 percent — but the tube closes by day 28, which is often before a woman knows she is pregnant. Taking folic acid after a positive pregnancy test is too late. This is why the recommendation is 400 micrograms daily for any woman who could become pregnant, and 5 mg for those at higher risk, and why many countries fortify flour with folic acid at population level. Fortification in the United States reduced neural tube defects by around a third, and it is one of the clearest public health wins available from an embryology fact.
The neural crest
As the neural folds fuse, a strip of cells at their crest breaks away and migrates all over the embryo. These neural crest cells are ectoderm, and they travel remarkable distances to become an implausibly varied list: all sensory and autonomic nerves outside the brain and cord, the adrenal medulla, the pigment cells of the skin, the bones and cartilage of the face and skull, the outflow tract that divides the heart's exit into aorta and pulmonary artery, and part of the thymus and parathyroid glands.
This is why several syndromes look bizarre until you know the embryology. DiGeorge syndrome (Chapter 2.7) affects neural crest cells migrating into the pharyngeal region, and produces heart outflow defects, absent thymus with immune deficiency, absent parathyroids with low calcium, and cleft palate — four apparently unrelated problems from one migration failure. Waardenburg syndrome combines deafness with a white forelock and pale eyes, because both the inner ear pigment cells and the hair and skin pigment cells are neural crest.
And it is why a child born with a significant facial anomaly is examined carefully for a heart defect. The same cells build both.
Mesoderm organises itself into blocks
The mesoderm on either side of the notochord divides into repeating blocks called somites, forming head to tail at a rate of about one pair every 90 minutes, from day 20 to day 30, ending with 42 to 44 pairs.
Each somite splits into three:
- Sclerotome → the vertebra and ribs at that level.
- Myotome → the skeletal muscle at that level.
- Dermatome → the dermis of the skin over that level.
This segmentation persists into the adult and is the basis of neurological examination.
A dermatome is the strip of skin supplied by one spinal nerve, and the map of dermatomes is a direct record of the somites. When shingles reactivates in one spinal nerve's sensory ganglion, the rash appears in exactly one dermatome and stops abruptly at the midline — a striking sight that makes no sense anatomically until you realise you are looking at the territory of one somite from week 4 (Chapter 11.8).
And it is why a spinal cord injury has a level. Damage at a particular vertebra causes loss of sensation below a specific line on the body, and that line is a somite boundary.
The diaphragm explains itself the same way. It begins developing in the neck region, around the level of cervical segments 3, 4 and 5, and takes its nerve supply — the phrenic nerve — from there. As the embryo grows, the diaphragm descends to the base of the chest but drags its nerve with it. So the diaphragm, sitting at the bottom of the ribcage, is controlled by nerves from the neck.
That single fact has two consequences you will meet again. A spinal cord injury above C3 paralyses the diaphragm and the person cannot breathe unaided; injury below C5 spares it. And irritation of the diaphragm's undersurface — by blood, infection or gallbladder inflammation — is felt as pain in the shoulder tip, because the brain interprets a signal arriving on the C4 nerve as coming from the skin that nerve normally supplies. Shoulder tip pain in someone with abdominal trauma means blood under the diaphragm until proven otherwise, and it is a sign that has saved lives in emergency departments.
The folding
During week 4 the flat trilaminar disc folds in two directions at once — head to tail, and side to side — converting a disc into a cylinder.
Side-to-side folding wraps the ectoderm around the outside and brings the edges together at the front, enclosing the endoderm as a tube. That tube is the gut, and the point where the folds meet at the front is where the umbilical cord attaches.
When that closure fails, abdominal contents remain outside. Gastroschisis is a defect beside the cord with bowel floating free in amniotic fluid; omphalocele is a defect at the cord with the contents covered by a membrane and frequently associated with other anomalies. Both are diagnosed on antenatal ultrasound and repaired after birth.
The critical period
Weeks 3 to 8 are when organs form, and this is when the embryo is most vulnerable to anything that disrupts development — a teratogen, from the Greek for monster-maker.
Before day 15, the effect tends to be all or nothing: the embryo either dies or repairs completely, because the cells are still interchangeable.
Weeks 3 to 8 is when structural birth defects are caused, and which defect depends on exactly which week — because different organs have different windows. The heart is most sensitive around weeks 3 to 6; the limbs around weeks 4 to 7; the palate around weeks 6 to 9.
Thalidomide is the case that established all of this. Marketed from 1957 as a safe sedative and anti-nausea drug, and given widely for morning sickness. Around 10,000 children were born with severe limb defects, most commonly phocomelia — hands or feet attached directly to the trunk with the long bones absent. The specific effect depended on the exact day of exposure: taken around day 24 it caused ear defects, around days 27 to 30 arm defects, days 30 to 33 leg defects. Outside roughly days 20 to 36, no limb effect at all.
The disaster changed drug regulation permanently — modern requirements for reproductive toxicity testing and post-marketing surveillance exist because of it. Thalidomide itself was later found effective in leprosy complications and in multiple myeloma, and is used today under strict pregnancy-prevention programmes.
Other established teratogens: alcohol (fetal alcohol spectrum disorder, with no established safe threshold), isotretinoin for acne (severe defects, requiring documented contraception), sodium valproate for epilepsy (neural tube defects and neurodevelopmental effects, roughly 10 percent risk), warfarin, ACE inhibitors, and rubella infection.
After week 8 the organs exist, so exposure causes growth and functional problems rather than structural malformation — which is still serious but different in kind.
What the next page fixes
This page assigned every cell to a layer and gave the map of what each layer becomes. It did not follow any single organ from a flat sheet to a working structure. Chapter 4.5 does exactly that for the heart, the gut, the kidney, the limbs, the face and the brain — and shows how each of the commonest birth defects is a specific step of that process failing at a specific week.