Human Gastrulation

Question 1

What is blastocyst?

Answer 1

• Embryoblasts clump at one end of embryo and this leaves a fluid filled cavity (blastocoel) at the other end
• The outer layer trophoectoderm will develop into structures that help the growing embryo implant the mother’s uterus
• Zona pellucida dissolves here
• At this point, the inner cell mass are pluripotent

Question 2

When does gastrulation occur?

Answer 2

• Week 3

- Trilaminar disc

Question 3

What happens at the 2nd week?

Answer 3

• ICM differentiate into hypoblast and epiblast which are the 2 layers of the bilaminar disc
• The hypoblast is the layer facing the blastocoel whilst the epiblast is on the other side
• On the side of the hypoblast, the primitive yolk sac forms
• Amniotic cavity forms on the side of the epiblast
• The amniotic cavity will eventually surround the fetus
• The hypoblast doesn’t contribute to the embryo

Question 4

What is gastrulation?

Answer 4

• Formation of the 3 germ layers from the bilaminar disc
• The 3 layers are ectoderm, mesoderm and endoderm - flat trilaminar disc
• The 3 layers arise from epiblast

Question 5

What does ectoderm give rise to?

Answer 5

-Outerlayer

• Epidermis
• Skin appendages e.g hair, nails, CNS, PNS

Question 6

What does the mesoderm give rise to?

Answer 6

-Middle layer

• Muscle
• bone
• connective tissue
• kidneys
• gonads

Question 7

What is the endoderm?

Answer 7

• Innermost layer

- Forms epithelial lining of GIT, lungs

Question 8

What are the steps of gastrulation?

Answer 8

• Cells from the epiblast undergo epithelial mesenchyme transition and and move downwards
• This makes a groove on the surface of the epiblast, called the primitive streak
• The presence of the primitive streak marks the beginning of gastrulation. It defines the major axis of the body: cranial-caudal and left-right
• At the cranial end of the primitive streak, cells migrate thickly creating a circular depression called the primitive node (expansion of primitive streak)
• Cells of the epiblast migrate inwards, towards the primitive streak and slip beneath it into the interior of the embryo - this is invagination.
• These cells invade the hypoblast (also known as primitive endoderm) and displace it. The hypoblast becomes completely replaced by a new proximal cell layer called the definitive endoderm
• The remaining cells of the epiblast are now referred to as the ectoderm (most external layer)
• Some of the invaginated epiblast cells remain in the space between the ectoderm and definitive endoderm —> this forms the middle layer which is the mesoderm
• After all this, epiblast cells no longer migrate towards primitive streak
• Cells migrating through the primitive node begin to form a hollow rod which curves towards the cranial end of the embryo - this is the notochord (part of mesoderm). This connects to the primitive node
• The notochord fuses with the endoderm to form the notochordal plate
• By being continuous with the primitive node, it allows free passage between amniotic cavity and yolk sac
• Notochord plays a big role is defining major axis of the body and is important in neurulation.

Question 9

What is embryonic folding?

Answer 9

• Flat trilaminar disc becomes cylinder in shape
• Occurs due to transverse and longitudinal folding (horizontal and median plane)

-Occurs as a result of differing rates of growth of embryonic structures

• Folding in horizontal plane results in the 2 lateral body folds
• Folding in median plane results in caudal and cranial folds
• Folding in these planes occur simultaneously, resulting in rapid development of embryo
• As the folding continues, the endoderm moves towards the midline and fuses, incorporating the dorsal part of the yolk sac to create the primitive gut tube
• This is split into foregut, midgut and hindgut
• Foregut can be seen at the cranial end - it is temporarily closed by oropharyngeal membrane which eventually ruptures from the mouth
• Midgut remains connected to the yolk sac until the 5th week of development
• Hindgut lies at the caudal end of the embryo which is temporarily closed by the cloacal membrane. This membrane ruptures to form the urogenital and anal openings

-As embryonic folding continues, the connection to the yolk sac narrows into a stalk, known as the vitelline duct

• Septum and heart move from margin to centre
• Yolk sac, allentois and stalk make the umbilical cord

Question 10

What is Bicoid?

Answer 10

• A morphogen that isn’t secreted
• it’s a transcription factor
• It patterns anterior posterior axis in the drosophila embryo

-It can still create a gradient in the embryo because the early Drosophila embryo is a syncytium

• Bicoid mRNA is deposited in the oocyte maternally during oogeneis
• Found at anterior part of oocyte
• As Bicoid mRNA is translated into protein, the protein diffuses through the embryo creating a concentration gradient
• The gradient means different levels of Bicoid work in different nuclei, determining different cell fates along the lengths of the embryo

Question 11

What factors are involved in epithelial Mesenchyme transition?

Answer 11

-Snail which downregulates cadrhins which reduce cell to cell contact which allows migration