The third week of embryonic development

What you will learn

• Describe the transition from a bilaminar to a trilaminar disc through gastrulation.
• Explain formation of the primitive streak, mesoderm and notochord.
• Relate notochordal BMP4 inhibition to induction of the neural plate.
• Describe the allantois, primordial germ cells, blood islands and tertiary villi.

Course outline

1. Entering the third week

   Place gastrulation and neural induction between days 15 and 21.

   6 min

2. Gastrulation and FGF8 control

   Understand epiblast migration that establishes the three primary germ layers.

   9 min

3. Primitive streak and Hensen’s node

   Describe the first axial structure and establishment of embryonic laterality.

   9 min

4. Establishing the three germ layers

   Follow hypoblast replacement and the fates of migrating and remaining epiblast.

   10 min

5. Notochord formation

   Describe the three stages from the chordal process to the solid notochord.

   12 min

6. Roles of the notochord

   Relate the notochord to the embryonic axis and neural and vertebral induction.

   7 min

7. Neural induction and BMP4

   Understand how BMP4 inhibition directs ectoderm toward the neural plate.

   11 min

8. Neural plate, groove and tube

   Describe the morphological sequence and origin of neural crests.

   8 min

9. Allantois and primordial germ cells

   Follow the allantoic diverticulum and initial migration of germ cells.

   9 min

10. Wolff and Pander islands

    Understand the first organization of extra-embryonic vascular and blood-forming cells.

    7 min

11. Tertiary villi and extra-embryonic circulation

    Describe villous vascularization and the beginning of extra-embryonic circulation.

    9 min

12. Mesoderm division and neural closure

    Identify the three mesoderm territories and continuing cervical neural closure.

    9 min

13. Third-week summary

    Connect gastrulation, notochord, neural induction and annex evolution.

    7 min