Gastrulation, 3rd week

Annotations:

• source of all of the germ layers. Cells in these layers will give rise to all of the tissues and organs in the embryo

1. primitive streak
   1. primitive node

      Annotations:

      • cephalic end of the streak. Slightly elevated area surrounding the primitive pit
      1. prenotochordal cells

         Annotations:

         • invaginate in the primitive node
         1. prechordal plate
            1. notochordal plate
               1. detachment from the endoderm
                  1. notochord
   2. invagination

      Annotations:

      • cells detach from the epiblast and slip beneath it. Cell migration and specification are controlled by fibroblast growth factor 8 (FGF8)
      1. endoderm

         Annotations:

         • cells that displace the hypoblast
         2. form AVE which expresses- head-forming genes
            1. OTX2
            2. LIM1
            3. HESX1
      2. mesoderm

         Annotations:

         • cells that lie between the epiblast and newly created endoderm
      3. ectoderm

         Annotations:

         • cells that remain in the epiblast