Session 3 ILOs - Embryology

Question 1

Outline the steps of morula formation

Answer 1

The fertilised cell (oocyte plus sperm) divides from on cell into 2 and then divides again to form 4 cells and keep going until 16 cells have been made which forms the morula (mulberry). All the cells are still contained within the zona pellucida

Question 2

Outline the steps of blastocyst formation

Answer 2

A blastocyst is formed by the process of compaction - where the first cavity is formed. The cells separate in the zona pellicuda to form the embryoblast (inner cell mass) and the trophoblasts (outer cell mass) - this forms the blastocyst cavity.
Now have 2 distinct populations of cells!

Question 3

Outline the first differentiation step of intraembryonic tissues

Answer 3

NOT SURE

Question 4

Outline the first differentiation step of extraembryonic tissues

Answer 4

NOT SURE

Question 5

Introduce the embryonic period of development & what key processes occur during this phase

Answer 5

The embryonic period of development occurs between weeks 3-8 (5 week period) and this is where most of the development takes place - all major structure and organs are formed.
The processes that occur (after fertilisation and implantation):
1. Gastrulation
2. Neurulation
3. Segmentation
4. Folding

Question 6

Outline how the the 3 embryonic germ layers are established and the derivatives of the germ layers

Answer 6

In gastrulation, the 3 germ layers are formed initially from the bilaminar disc which comprises of the epiblast layer and the hypoblast layer. The epiblast layer cell multiplies and they push through the epithelium layer. The 3 layers are formed from this (trilaminar disc):

1. Ectoderm - skin (epidermis)
2. Mesoderm - muscle, cartilage, vascular system
3. Endoderm - epithelial lining of GI, respiratory tract

Question 7

Outline how a body plan is established, including segmentation

Answer 7

A body plan is established by the notochord sending signals for left or right growth. It causes the ectoderm to thicken and form a ‘slipper shaped’ neural plate of which the edges curl out and towards each other - to create the neural tube.
Somites appear in very regular, ordered fashion and form 31 somite pairs

Question 8

Explain how organisation is achieved within the mesoderm

Answer 8

The mesoderm is organised into different sectoins

1. Paraxial mesoderm
2. Intermediate mesoderm
3. Somatic mesoderm
4. Intraembryonic coelom
5. Splanchnic mesoderm

Question 9

Outline the process of embryonic folding

Answer 9

Embryonic folding occurs by cephalocaudal folding (head and tail folding) as well as lateral folding - so you end up with a package with a small opening which is the yolk sac. The ectoderm folds around the endoderm (so you can see the ectoderm on the outside).