Anatomy chapters
The second and third weeks
Week two makes the bilaminar disc and its cavities; week three makes the three germ layers, the notochord and the neural plate.
- Week 2 is the week of twos: two layers (epiblast, hypoblast), two cavities (amniotic, yolk sac), two trophoblast layers.
- Week 3 is gastrulation: the primitive streak forms and the three germ layers appear.
- The notochord induces the neural plate, which folds into the neural tube by week 4.
On this page
Overview
In the second week, the embryo finishes implanting and becomes a flat, two-layered disc.
In the third week, the disc turns into three layers by gastrulation. These three germ layers form every tissue of the body.
The third week also lays down the notochord and starts the nervous system.
Week 2: the week of twos
The inner cell mass splits into two layers:
- Epiblast: tall cells next to the new amniotic cavity.
- Hypoblast: small cells facing the blastocyst cavity, which becomes the yolk sac.
Together they form the bilaminar germ disc.
The trophoblast has two layers: cytotrophoblast and syncytiotrophoblast. Spaces (lacunae) in the syncytiotrophoblast fill with maternal blood, and the uteroplacental circulation begins.
Extraembryonic mesoderm appears between the trophoblast and the yolk sac and amnion. Cavities in it join to form the chorionic cavity. The embryo hangs in this cavity by the connecting stalk, the future umbilical cord.
Week 2 is the "week of twos": two germ layers, two cavities, two trophoblast layers.
Week 3: gastrulation
Gastrulation makes three germ layers from the bilaminar disc.
It begins with the primitive streak, a groove on the surface of the epiblast at the tail end of the disc. Its head end is the primitive node.
Epiblast cells move to the streak and dip down through it:
- Some replace the hypoblast and form the endoderm.
- Some spread between the layers and form the mesoderm.
- Cells left on the surface form the ectoderm.
All three germ layers come from the epiblast.
The streak also fixes the body axes: head and tail, front and back, left and right.
The notochord
Cells moving forward from the primitive node form the notochord, a rod along the midline.
It gives the embryo an axis and sends signals that make the overlying ectoderm thicken into the neural plate.
The notochord disappears where the vertebral bodies form. It persists as the nucleus pulposus of the intervertebral discs.
Notochord remnant in the adult = nucleus pulposus.
Neurulation
The neural plate folds up to form the neural groove with a neural fold on each side. The folds meet and fuse to make the neural tube.
Fusion starts in the neck region and spreads both ways. The cranial end (anterior neuropore) closes at about day 25 and the caudal end (posterior neuropore) at about day 28.
The neural tube forms the brain and spinal cord.
Neural crest
Cells at the edges of the neural folds break away to form the neural crest. These cells wander widely.
| Neural crest derivatives |
|---|
| Sensory (dorsal root) and autonomic ganglia |
| Schwann cells |
| Adrenal medulla |
| Melanocytes |
| Pia and arachnoid mater |
| Bones and connective tissue of the face |
| Parts of the heart outflow tract |
Mesoderm and somites
The mesoderm on each side of the notochord divides into three parts:
- Paraxial mesoderm: forms blocks called somites.
- Intermediate mesoderm: forms the kidneys and gonads.
- Lateral plate mesoderm: forms the body wall, limbs and the lining of body cavities.
Each somite splits into:
| Part | Forms |
|---|---|
| Sclerotome | Vertebrae and ribs |
| Myotome | Skeletal muscle of the back, body wall and limbs |
| Dermatome | Dermis of the skin of the back |
Somites can be counted to estimate the age of the embryo.
Germ layer derivatives
| Ectoderm | Mesoderm | Endoderm |
|---|---|---|
| Epidermis, hair, nails | Muscle (skeletal, smooth, cardiac) | Lining of the gut and its glands |
| Brain, spinal cord, retina | Bone, cartilage, connective tissue | Liver and pancreas cells |
| Lens, inner ear | Heart, blood vessels, blood | Lining of the respiratory tract |
| Enamel of teeth, anterior pituitary | Kidneys, gonads, spleen, adrenal cortex | Thyroid, parathyroid, thymus |
| Neural crest (see above) | Dermis | Lining of bladder and urethra |
The adrenal cortex is mesoderm but the adrenal medulla is neural crest.
Clinical relevance
Sacrococcygeal teratoma
How it happens: remnants of the primitive streak persist and grow.
What you see: a large mass over the sacrum and coccyx in a newborn, more often in girls. It can contain tissues of all three germ layers.
Caudal dysgenesis (sirenomelia)
How it happens: too little mesoderm forms at the tail end of the embryo.
What you see: fused lower limbs and defects of the lower spine, kidneys and genitalia. It is linked with maternal diabetes.
Neural tube defects
How it happens: the neural tube fails to close.
What you see: anencephaly if the cranial end fails; spina bifida if the caudal end fails. Raised alpha-fetoprotein in maternal blood and amniotic fluid is a screening clue.
Folic acid before conception and in early pregnancy lowers the risk of neural tube defects.
The usual questions are the source of the three germ layers (epiblast), the remnant of the notochord, neural crest derivatives, the parts of a somite and the tumour from a primitive streak remnant.
Quick revision
- Week 2: epiblast and hypoblast; amniotic cavity and yolk sac.
- Week 3: gastrulation begins with the primitive streak.
- All three germ layers come from the epiblast.
- Notochord remnant: nucleus pulposus.
- Neural tube closes: cranial end day 25, caudal end day 28.
- Somite: sclerotome, myotome, dermatome.
- Primitive streak remnant: sacrococcygeal teratoma.
Written by The House Job from standard embryology references (Langman's Medical Embryology).
FAQ
What is gastrulation?
The process in week 3 that turns the two-layered disc into the three germ layers: ectoderm, mesoderm and endoderm.
What does the notochord become?
It disappears except as the nucleus pulposus of the intervertebral discs.
Which structures come from the neural crest?
Sensory and autonomic ganglia, Schwann cells, the adrenal medulla, melanocytes and much of the facial skeleton.
What causes a sacrococcygeal teratoma?
Persistence of part of the primitive streak.