Session 2

Question 1

Discuss the development of the primitive streak

Answer 1

1) Appears in In the third week on the dorsal surface of the epiblast
2) As gastrulation proceeds and the three germ layers are established, the primitive streak regresses

Question 2

What are the four steps of the migration of mesoderm through the primitive streak

Answer 2

1) Cells on the epiblast migrate towards the primitive streak.
2) They then invaginate into the epiblast and displace the hypoblast creating a 3rd layer – the mesoderm.
3) As more cells migrate through the streak they spread laterally and upwards
4) Mesoderm spreads out between the ectoderm and endoderm like a sandwich filling, leaving two gaps and holes

The fate of invaginating epiblast depends on where in the streak or node they invaginate

Question 3

Discuss the significance and features of the notochord

Answer 3

• The Notochord define the phylum Chordata
• It’s the basis for the axial skeleton and axial skeleton forms around it
• It drives the formation of the nervous system(neurulation)
• Prenotochordal cells of epiblast migrate through the cranial part of the primitive pit
• Forms a solid rod of cells running in the midline with an important signalling role
• It defines the midline
• It regresses. The vestigial remnant in the adult is the nucleus pulposus of intervertebral discs.

Question 4

Describe the embryonic disc at the end of the third week

Answer 4

1) Three germ layers in place anteriorly
2) Because development proceeds cephalocaudally (head to tail) gastrulation is not complete in the caudal region until week 4.
3) Axes are established

Question 5

Discuss the characteristics of the primitive streak, node and pit (location etc.)

Answer 5

1) The streak is a narrow groove with bulging edges
2) Primitive node is located at the cranial end of the streak.
3) Primitive pit is located at the centre of the node
4) Development proceeds in a cranial/rostral to caudal direction. (top to bottom)