Lecture 49: Primary germ layers

Question 1

What are the characteristics of identical = monozygotic twins?
- in terms of placenta, umbilical cord and gametes

Answer 1

• single fertilised ovum, zygote splits to form two embryos
• usually share a placenta, but each foetus has its own umbilical cord
• genetically similar

Question 2

What are the characteristics of non-identical twin = dizygotic?

Answer 2

• 2 ova, 2 sperms, 2 zygotes

- each foetus has its own placenta and umbilical cord

Question 3

What is gastrulation ?

What are the 3 germ cell layers?

Answer 3

Is the process of forming 3 embryonic layers
- formation of three germ cell layers from the two layered structure
= ectoderm, mesoderm and endoderm
These 3 layers make all of the cells and tissues of the developing baby

Question 4

What structures arise from the ectoderm?

Answer 4

• epidermis = outer layer of skin
• brain and spinal cord = neural tube
• nerve cells
• some bones of the face and skull

Question 5

What structures arise from the mesoderm?

Answer 5

• muscle
• connective tissue
• bone
• kidneys
• gonads

Question 6

What structures arise from the endoderm?

Answer 6

• lining of the gastro-intestinal tract

- respiratory system = lining of the lungs and bronchi

Question 7

Describe the formation of the 3 primary germ layers:

- early embryo

Answer 7

Initially cells are totipotent = can develop into any type of cell in the body

Question 8

Describe the formation of the 3 primary germ layers:

- five days after fertilisation

Answer 8

A blastocyte has formed - the cells of the blastocyte are pluripotent = can form almost any cell type

• the blastocyte has two types of cells; the trophoblast and the inner cell mass
• the trophoblast is the outer cells that will develop into the placenta
• the inner cell mass will form the embryo

Question 9

Describe the formation of the 3 primary germ layers:

- gastrulation

Answer 9

• involves the formation of 3 germ cell layers
• At the end of 2nd week the primitive streak forms in the epiblast/future ectoderm
• Cells migrate down into the primitive streak to form both the endoderm and mesoderm
• cells of epiblast move toward primitive streak & migrate downwards pushing the hypoblast out of the way.
  = hypoblast forms umbilical stalk, 1st cells to migrate through streak form endoderm, cells that migrate later form the mesoderm

Question 10

The inner cell mass of the implanted blastocyte divides into what 2 layers

Answer 10

Inner cell mass —-> embryonic disc
= the epiblast(ectoderm) and hypoblast
- the layer on top is called the epiblast and it will give rise to cells inside the embryo. All three layers of the embryo arise from the epiblast
- the lower layer is called the hypoblast and will form the placenta
Above the epiblast is the amniotic space and below the hypoblast is the yolk sac.