Chickens Flashcards
Why is the amniotic egg an important adaptation to the terrestrial environment?
Eggs were no longer tied to water
What are the 4 extra-embryonic membranes in amniotes?
Chorion, yolk sac, amnion and allantois
What is the function of the amnion?
Surrounds the embryo and provides an aqueous environment
What is the function of the chorion?
Surrounds everything and allows gas exchange. Keeps the inside of the egg sterile
What is the function of yolk sac?
Nutrition
What is the function of allantois?
Holds metabolic waste products
How does the egg get formed in a chicken?
Chickens have only 1 ovary, and ovulation happens from there. The egg gets fertilized and the albumen is added. Then the egg passes the shell gland and get rotated as it is covered with a calcified shell. The egg is laid out of the cloaca
What type of yolk distribution do chickens have?
Telolecithal
What type of cleavage do chickens have?
Meroblastic, discoidal
What are the two zones of the blastodisc?
Area opaca and area pellucida
What is the subgerminal cavity?
The cavity between the animal cells in the blastodisc and yolk
How is the primary hypoblast created?
Delamination from the epiblast to create hypoblast islands, which then expand
What does the primary and secondary hypoblast become in chicken embryos?
Nothing, they act as scaffolding
Where does the secondary hypoblast come from? What does it do?
Comes from the posterior marginal zone. It pushes the primary hypoblast out of the way and causes expansion of the primitive streak
What does Koller’s sickle do?
Directs cell movements
What is the primitive streak? How it is formed?
A darkened line on the dorsal side of the embryo that is the site of gastrulation. Created by delamination and intercalation of epiblast cells along the midline
What causes the elongation of the primitive streak?
Movement of the secondary hypoblast beneath it and convergent extension of the posterior epiblast cells
Where does gastrulation occur?
Front end of the primitive groove
What is the organizer in the chicken embryo? Where is it? What does it become?
Hensen’s node, located at the anterior end of the primitive groove. Becomes the notochord
Why does gastrulation occur through epithelial-mesenchyme transitions?
There is too much yolk for the cells to move around freely, so they need to become mesenchymal cells to be able to move
What is the fate of the cells that go through the primitive groove first? Where along the primitive streak do those cells go through?
The first cells are future endoderm. They go through at the posterior end, far away from Hensen’s node
What is the fate of the cells that go through the primitive groove second? Where along the primitive streak do those cells go through?
Future mesoderm cells. They go through between Hensen’s node and the posterior end, but aren’t particularly close to Hensen’s node
Where are the future ectoderm cells found relative to the primitive streak?
They stay on the dorsal side and don’t go through the primitive groove. The cells surrounding Hensen’s node become neural ectoderm and the ones further away become epidermal ectoderm
What is happening to the primitive streak during gastrulation?
It is regressing back towards the posterior end
What is happening at the same rate as the regression of the primitive streak?
Elongation of the notochord
What are the ectoderm cells doing while the primitive streak is regressing?
Expanding over the yolk through epiboly through the use of fibronectins signals
What structures are homologous to Hensen’s node in frogs and fish?
The dorsal blastopore lip in frogs and the embryonic shield in fish
What two signals are the marginal cells in the posterior marginal zone expressing?
Wnt8c and Vg1
What type of signal is Wnt8c? What does it do?
Paracrine factor in the Wnt pathway. It allows beta-catenin activity
What signal gets expressed when both Vg1 and Wnt8c are expressed? Where is it expressed?
Nodal. Gets expressed in epiblast near Koller’s sickle
What transcription factor is expressed in primary hypoblast cells?
Cerberus
What does Cerberus do?
Inhibits Nodal
How does Nodal expression expand towards the anterior end when Cerberus is inhibiting it?
When the secondary hypoblast expands, it pushes away the primary hypoblast that is expressing Cerberus and Nodal is no longer inhibited
Why is the expansion of Nodal expressing cells and the regression of Cerberus expressing cells so critical?
Makes sure the organizer is in the right place
What are 6 signals expressed by Hensen’s node?
Goosecoid, chordin, noggin, nodal, BMP antagonists and FGF
What does Goosecoid do?
A transcription factor that shuts off non-brain genes
Where is Smad1 expressed?
Epidermal ectoderm
How does FGF help specify the neural plate cells? (4 things)
It promotes Nodal activity and makes cells less sensitive to BMP signalling molecules. It regulates the transcription of ectoderm cells to stop them ingressing through the primitive streak and produce molecules that are involved in neural tissue production
