6.1 Chicken Development

Question 1

5 reasons Chick embryos as model systems

Answer 1

1. Large eggs
2. Easy to obtain
3. Easy to keep and easy to grow
4. Relatively robust, thus good subjects for microsurgery and other manipulations
5. Have many organ systems and developmental genes in common with human embryos

Question 2

What kind of cleavage in chickens

Answer 2

Meroblastic cleavage:
Teleoecithal, Discoidal Cleavage

Telolecithal: Only a small area that is free of yolk

Meroblastic: type -incomplete or partial due to the presence of large volumes of yolk -seen in centrolecithal, telolecithal.

Discoidal meroblastic cleavage: Cleavage is restricted to a small portion of the cytoplasm present on top of the large yolk. Example includes the embryo of aves, where the eggs are telolecithal

Question 3

Basic Chick Egg Anatomy

Answer 3

• Chalaza: hold egg right way
• 3 layers: amnion yolk sac allantoris chorion albumen
• Blastodisk: small amount of non yolk in chick egg

Question 4

Explain Egg Layers

Answer 4

Amnion: prevents the embryo from desiccation and stay afloat in fluid enviornment

Yolk Sac: enables nutrient uptake, development of circulatory system

Allantois: stores waste products

Chorion: contain blood vessels that exchange gases with the environment

Albumin: protect the yolk and provide additional nutrition for the growth of the embryo. Stabalizes in center

Question 5

Where is cleavage in Chickens done

Answer 5

Blastodisk. There’s the area pellucid the inside, the area opaca the outside and the marginal zone which is inbetween. Forms the blastoderm

Question 6

5 Steps in Chick Cleavage

Answer 6

1) happens at blastodisk
2) forms single layer of cell called blastoderm made of epiblast cells
3) equatorial division causes 5-6 layed thick cells
4) sub germinal layer seperates blastoderm from yolk
5) SGL gives blastoderm a translucent appearance

Question 7

fluid filled cavity under the 5-6 layer cells of blastoderm that seperates it from the yolk

Answer 7

sub-germinal cavity

Question 8

thickning of the epiblast at the edge of the area pellucida

Answer 8

koller’s sickle

Question 9

posteior marginal zone

Answer 9

in between the koller’s sicker and area opaqa

Question 10

Formation of the second layer of cells

Answer 10

Cells from the epiblast delaminate into the subgerminal space and cells also migrate forward from the posterior marginal zone - now the blastoderm is two layered!

The delaminating and migrating cells combine within the subgerminal space—a definitive blastocoel is formed. Epiblast forms the embryo and the hypoblast forms the extraembryonic membranes

Question 11

epiblast cells delaminate and stay attached to posterior marginal zone are called

Answer 11

hypoblast island then 5-20 cells migrate and become the primary hypoblast

Question 12

sheet of cells from the Koller’s sickle grow anteriorly form

Answer 12

secondary hypoblast or endoblast

Question 13

secondary hypoblast + primary hypoblast

Answer 13

complete hypoblast layer

Question 14

entire embryo is derived from (along with extra embryonic membranes)

Answer 14

epiblast

Question 15

Formation of the primitive Streak

Answer 15

• thickening of epiblast (and the Kohler’s sickle) [3-4hrs]
• cell converge form depression called primitive groove [7-8hrs streak, 15-16hrs groove]
• cells move through groove to the deeper layers cells in anterior layer thicken and form hensons’ node (primitive knot) [19-22hrs]

The streak starts at the posterior margin of the epiblast, and elongates anteriorly