Gastrulation

Question 1

What are monozygotic twins?

Answer 1

One egg for the two foetuses

Question 2

What are dizygotic twins?

Answer 2

Two eggs for the two foetuses

Question 3

What is sesquizygosity?

Answer 3

A third form of twinship whereby twins are genetically indentical to each other with respect to one parent but differ from each other by approx 50% with respect to the composition of DNA inherited from the other parent

Question 4

Describe the time periods for implantation?

Answer 4

Spermatozoon penetrates into the oocyte- day 0 
Two cell stage- day 1
Four cell stage - day 2
Eight cell stage - day 3 
Morula (16-32 cells)- day 4 
Free blastocyst- day 6

Question 5

What are the different ectoptic implantation zones and what does it lead to?

Answer 5

Ovary 
Infundibulum
Fallopian tube 
Isthmus
Cervix 
Abdominal/pelvic cavity
Abdominal peritoneum

Results in no viable zygote - usually self aborts

Question 6

What is a yolk sac?

Answer 6

Vesicle which develops in week 2

Question 7

What is the floor of the yolk sac?

Answer 7

Heuser’s membrane

Question 8

What is the ceiling of the yolk sac?

Answer 8

Hypoblast

Question 9

What is the function of the yolk sac?

Answer 9

Nourishment for the egg

Question 10

How is a secondary yolk sac formed?

Answer 10

When the extraembryonic mesoderm separates to form the extraembryonic coelom; cells from the mesoderm pinch off an area of the yolk sac, and what remains is the secondary yolk sac

Question 11

How is the final yolk sac formed?

Answer 11

Week 4 - part of the yolk sac is surrounded by endoderm and incorporated into the embryo as the gut - remaining part is the final yolk sac

Question 12

How is the chorionic cavity formed?

Answer 12

Extra- embryonic mesoblast cells, derived from epiblast cells, grow into the extra-embryonic reticulum.
Beginning of the reticular vacuole formation that will form the future chorionic cavity

Question 13

How is the amniotic cavity formed?

Answer 13

Fusion of the parts of the amniotic fold

Question 14

What are the two layer of flat cells of the embyro at week 2 and what are these collectively known as?

Answer 14

Epiblast and hypoblast

Bilaminar disc

Question 15

What happens to the bilaminar disc at week 3?

Answer 15

Gastrulation occurs
Bilaminar disc is converted to trilaminar disc and formation of three germ layers - Ectoderm
Mesoderm
Endoderm

Question 16

What is gastrulation?

Answer 16

Formation of the three embyronic germ layers

The emnbryogenesis where basic organisation of the organism is established

Question 17

What does gastrulation begin with?

Answer 17

Formation of the primitive streak

Question 18

What are the three things that make up the primitive streak?

Answer 18

Primitive groove, primitive node and primitive pit

Question 19

What happens at the primitive groove?

Answer 19

Immigration of epiblast cells, which form the future mesoblast, as well as the endoblast, which replaces hypoblast

Question 20

What are features of epithelial cells?

Answer 20

Cell polarity - always sit on the basement membrane
Cell ahesions- always uniform
Stationary 
High levels of E-cadherin
Low levels N- cadherin

Question 21

What are features of mesenchymal cells?

Answer 21

No cell polarity 
Loss of cell adhesion 
Ability to migrate and invade 
Low levels of E- cadherein 
High levels of N- cadherin

Question 22

What is the process whereby epithelial cells become mesenchymal cells?

Answer 22

Epithelial- mesenchymal transition

EMT

Question 23

What is derived from the endoderm?

Answer 23

Epithelial lining of the GIT 
Respiratory tract bladder and urethra 
Thyroid and parathyroid glands
Liver 
Pancreas

Question 24

What is derived from the mesoderm?

Answer 24

CT
Striated and smooth muscles 
Heart
Blood and lymphatic vessels 
Kidneys 
Ovaries and testes 
Spleen 
Serous membranes lining the body cavities

Question 25

What is derived from the ectoderm?

Answer 25

CNS
PNS
Sensory epithelia of the eye
Ear 
Nose 
Epidermis and its appendages
Mammary glands
Pituitary gland

Question 26

What are the results of gastrulation?

Answer 26

The primary germ layers are formed
Creation of the bodies axes
(Anterior posterior)
(dorsal ventral)
(Left right)