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Module 07

Weeks 2 and 3: the embryonic disc

How the inner cell mass becomes a flat disc of two layers, then three, gains a head end and a tail end, and starts to build the nervous system, all before a first scan.

  • 4lessons
  • 16 minreading
  • 6quiz questions
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The lessons and the quiz in this module are part of the Fertility with Kara Package. Module 1 is free to read.

Your way through Module 07

  1. Lesson 7.1 · 4 min Week 2: a disc of two layers While the embryo buries itself in the lining, its inner cell mass becomes a flat disc of two layers, with a fluid-filled space above it and the yolk sac below.
  2. Lesson 7.2 · 4 min Week 3: the primitive streak How a groove called the primitive streak gives the embryo a head end and a tail end, and how cells pour through it to make three layers out of two.
  3. Lesson 7.3 · 4 min Three layers that build a body What the ectoderm, mesoderm and endoderm each go on to build, and how the middle layer sorts itself into the blocks that become the spine and muscles.
  4. Lesson 7.4 · 4 min The notochord and the start of the neural tube How a rod of cells in the midline tells the layer above it to become the nervous system, and why the neural tube closing so early is the reason folic acid matters before pregnancy.
  5. Quiz6 questions · 5 to passNot taken BadgeModule 07Earned with the quiz

Learning aims

By the end of this module you will be able to:

  • Describe the two-layered disc of week 2, with the amniotic cavity above it and the yolk sac below.
  • Explain how the primitive streak sets the body plan and how gastrulation makes three germ layers.
  • Recognise what the ectoderm, mesoderm and endoderm each go on to build.
  • Describe how the notochord triggers the neural plate and how the neural tube closes.
  • Explain why folic acid is advised before conception, and ask your healthcare professional which dose is right for you.

The module in brief

The module in brief
  • In week 2 after fertilisation the inner cell mass becomes a two-layered disc: an upper layer (the epiblast), which forms the whole baby, above a lower layer (the hypoblast).
  • The amniotic cavity opens above the disc and the yolk sac hangs below it; a connecting stalk ties the embryo to the future placenta.
  • At the start of week 3 the primitive streak appears and gives the embryo a head end, a tail end, a left and a right.
  • Cells of the upper layer move through the streak to form three germ layers: ectoderm, mesoderm and endoderm.
  • Ectoderm builds the skin and nervous system, mesoderm the heart, blood, muscles, bones and kidneys, and endoderm the linings of the gut and lungs.
  • The notochord signals the ectoderm above it to form the neural plate, which folds into the neural tube by about 6 weeks of pregnancy.
  • Because the neural tube closes so early, folic acid is advised from before conception.
A note from Kara

Thank you for choosing to learn with me

I know you had many places to look for answers, and you chose this course. Thank you. It means a lot to me that you trust me with something this close to your heart.

Every lesson here is something I wish every couple could hear before they sit down with their clinic: how a life begins, what happens in the lab, and what the words on your report mean. Take it slowly. Short lessons, one at a time, are the way the science sinks in.

If a lesson raises a question about your own body or your own embryos, write it down and take it to your healthcare professional. That is exactly what this course is for.

Warmly, Kara

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