Gastrulation and neurulation

Question 1

What happens in the first week after conception

Answer 1

• Ovulation
• Conception in uterine tube
• Migration to uterus
• Cell division to morula, then blastocyst
• Implantation in uterine mucosa complete by day 9

Question 2

What happens at the end of week 2

Answer 2

• Bilaminar disc with connecting stalk
• Within chloronic sac
• Amniotic sac on “dorsal” side
• Yolk sac on “ventral” side
• Epiblast and hypoblast are in contact
• Day 14 primitive streak appears - cells start to move
• Gastrulation has begun
• Next 3-4 days a lot happens

Question 3

What is the primitive groove

Answer 3

• A groove with raised edges and a pit, form on the epiblast along ahead-tail axis
• Groove - primitive streak
• Pit - primitive pit associated with primitive node - a key signalling region in gastrulation

Question 4

Describe the first stage of cell migration

Answer 4

• Epiblast cells divide and migrate through primitive streak
• Displace/replace hypoblast cells
• Becoming endoderm

Question 5

Describe the second stage of cell migration

Answer 5

• Second wave of migration “fills in” between the 2 layers
• Establishes a trilaminar disc except at 2 locations
• Epiblast now ectoderm

Question 6

What is the notochord

Answer 6

It is a tube shaped structure that grows into the mesoderm from the primitive pit. It is essential for setting up dorsal and ventral axes and inducing neurulation, forming neural tissue

Question 7

What is neurulation

Answer 7

• Epithelial cells become columnar in an area called the neural plate
• The neural plate converts to a groove then a tube

Question 8

How are the fold and groove formed

Answer 8

Cells on the edge of the plate thicken forming a groove and then a fold

Question 9

How is the tube formed

Answer 9

The edges of the fold roll over to make a tube

Question 10

Describe the closure of the folds

Answer 10

The folds close like a zip in cephalic and caudal directions, the open ends being the Anterior and Posterior
Neuropores

Question 11

How are neural crest cells derived

Answer 11

From the edges of the neural tube

Question 12

What do neural crest cells undergo

Answer 12

An epithelial to mesenchymal transition become motile and migrate away from the neural tube to widespread destinations

Question 13

How is the mesoderm formed

Answer 13

• Cells from the epiblast flow into the primitive streak
• These cells replace the hypoblast cells to from endoderm
• Cells lie between the hypoblast and epiblast form mesoderm
• Cells that remain in the epiblast become ectoderm

Question 14

What are somites

Answer 14

Paired condensations of paraxial mesoderm in the future trunk

Question 15

Describe somite formation

Answer 15

• Cells of the paraxial mesoderm have an internal timer
• They go through cycles every 90 minutes defined by a notch signalling clock
• A wave of FGF signal that passes along the embryo in a rostral caudal direction
• When the FGF wave passes cells, they are programmed to change into part of a somite
  according to where they are in the clock cycle

Question 16

What are hox genes

Answer 16

They direct formation of body structures at the right place along the body

Question 17

What forms the primitive gut tube

Answer 17

Craniocaudal folding and lateral folding pinches off endoderm lined yolk sac becoming the primitive gut tube