Primitive Streak & Germ Layer Derivatives

Question 1

What is the first sign of gastrulation?

Answer 1

Formation of the primitive streak (Day 14)

Question 2

Which cells proliferate to form the primitive streak?

Answer 2

Epiblast cells

Question 3

Where does the primitive streak begin?

Answer 3

Caudal end of the embryonic disc

Question 4

In which direction does the primitive streak grow?

Answer 4

Caudal → Cranial direction

Question 5

What does the primitive streak allow the epiblast cells to do?

Answer 5

Migrate ventrally and form 3 germ layers (Gastrulation)

Question 6

Where is the ectoderm located in relation to the embryo?

Answer 6

Floor of the amniotic cavity (dorsal)

Question 7

Where is the endoderm located?

Answer 7

Roof of the yolk sac cavity (ventral)

Question 8

Where is the mesoderm located?

Answer 8

Between ectoderm and endoderm

Question 9

Which two regions are formed by ectoderm and endoderm without mesoderm?

Answer 9

1. Buccopharyngeal membrane (cranial)
2. Cloacal membrane (caudal)

Question 10

What is the role of the notochord?

Answer 10

Formed from mesoderm

Induces overlying ectoderm to become neuroectoderm → forms nervous system

Question 11

What is the primitive node (Hensen’s node)?

Answer 11

Cranial end of the primitive streak – organizer of notochord & axial structures

Question 12

What are the key regions of the primitive streak from cranial to caudal?

Answer 12

1. Primitive node (Hensen’s node)
2. Primitive pit (blastopore)
3. Primitive groove

Question 13

What is the order of germ layer formation during gastrulation?

Answer 13

1. Endoderm (first to invaginate)
2. Mesoderm
3. Ectoderm (remaining epiblast)

Question 14

What are the fates of the germ layers?

Answer 14

Ectoderm → Skin, CNS, PNS

Mesoderm → Muscle, Bone, CVS

Endoderm → Gut tube, Organs (Liver, Lungs, etc.)

Question 15

What is the embryonic “shield”?

Answer 15

Bilaminar disc (epiblast + hypoblast) at early stage; transforms during gastrulation

Question 16

What is the first structure that marks the cranial end of the embryo?

Answer 16

Prochordal plate (aka Prechordal plate)

Question 17

Which membrane appears just posterior to the prochordal plate?

Answer 17

Buccopharyngeal membrane

Question 18

What does the prochordal plate define?

Answer 18

The cranial (anterior) end of the embryo

Question 19

When does the primitive streak appear?

Answer 19

End of 2nd week (around Day 14)

Question 20

What is the location of the primitive streak?

Answer 20

Floor of the amniotic cavity, arising from epiblast cells

Question 21

What are the 3 parts of the primitive streak?

Answer 21

1. Primitive Knot / Node / Hensen’s Node (cranial end)
2. Primitive Pit / Blastopore
3. Primitive Groove (longitudinal depression)

Question 22

What forms the notochord?

Answer 22

Epiblast cells migrate into the primitive pit and move cranially to form the notochord (axial mesoderm)

Question 23

What forms before the notochord during axial development?

Answer 23

Prechordal (Prochordal) Plate

Question 24

What is located at the caudal end of the primitive streak?

Answer 24

Cloacal membrane

Question 25

What is the order of midline structures from cranial to caudal?

Answer 25

1. Prochordal Plate 2. Buccopharyngeal Membrane 3. Notochord 4. Primitive Knot 5. Primitive Pit (Blastopore) 6. Primitive Groove 7. Primitive Streak 8. Cloacal Membrane

Question 26

What defines cranio-caudal axis first — prechordal plate or primitive streak?

Answer 26

Prechordal Plate

Question 27

From which germ layer does the prechordal plate arise?

Answer 27

Hypoblast (with contribution from migrating epiblast cells)

Question 28

What’s the role of prechordal plate?

Answer 28

Marks the cranial end, contributes to forebrain induction, guides notochord growth

Question 29

Which appears first: PGCs or Prechordal Plate?

Answer 29

PGCs