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BIOL 302
Catherine Studholme

1. How does amount of yolk influence cleavage in birds? How do we call this type of cleavage? • Amount of yolk influences where cleavage occurs o Cleavage only occurs in the blastodisc and doesn’t extend into the yolk • Type of cleavage: discoidal meroblastic cleavage (yolk too large to separate cells completely 2. How is hypoblast formed during gastrulation in birds? • Primary hypoblast (polyinvagination islands) form through delamination and migration of epiblast cells • Secondary hypoblast formed from sheet of cells from posterior marginal zone of the blastoderm migrating anteriorly o Anterior Migration of posterior marginal zone cells that push primary 0 0 hypoblast cells with them = 1 and 2 merge 3. Compare and contrast gastrulation in amphibians and birds. (think about different cell movements due to the amount of yolk and the way blastocoel is formed) Both = set of morphological movements that leads to formation of 3 germ layers Birds o Blastodisc has large yolk underlying I = prevent formation of cells along yolk  First step of gastrulation = form hypoblast to form blastocoels o Delamination, migration of epibalst cells = primary hypoblast o Sheet of cells migrating from posterior marginal zone anteriorly form secondary hypoblast o Primary and secondary merge and blastocoel formed o Go through delamination o Form primitive streak at posterior epiblast  First cells through streak = endoderm, second cells = mesoderm Amphibians a. Begin gastrulationn below grey crescent with invagination to form blastopore lip 1 i. Displace cells toward inside embryo while maintaining contact w/ outside surface = form bottle cells that form blastopore > initiate gastrulation b. Marginal zone cells undergo involution to spread along outer surface and internalize middle c. Ectoderm eventually encircles embryo w/ mesoderm b/w ecto and endoderm 4. What is the relationship between primitive streak, primitive groove and Hensen’s node? • Primitive streak thickens initially then thins as it migrates to anterior end to form a primitive groove o Streak formed from migration of cell to the middle layer and posterior marginal zone thickens • Migrating cells intercalate and convergently extend from posterior to anterior embryo to cause depression in streak o Primitive groove is formed as a result of depression in streak  Groove = place where cells can migrate into blastocoel • Henen’s node is a regional thickening at the anterior end of the primitive groove which has a depression at its center that allows cells to pass into the blastocoel 5. What are the first cells which are passing through the primitive streak during gastrulation? • Endodermal precursor cells (undergo epithelial to mesechyme transition) 2 6. Compare and contrast formation of Nieuwkoop center in amphibians and birds. What is the chicken equivalent to the Xenopus Spemann organizer? What is the chicken equivalent to the amphibian blastopore lip? Explain your reasoning (in respect to their role in gastrulation). You can use diagrams. • Amphibian (Xenopus) Nieuwkoop centre o cytoplasmic rotation following fertilization  B-catenin in dorsal and protein Vg1 secreted by vegetal cells  Gravity and Site of sperm entry determine placement of b-catenin in dorsal region o Function - Induce formation of organizer cells o Overlap of b-catenin and vg1 create nieuwkoop centre • Chicken Nieuwkoop centre o B-catenin found in outer cells of the blastodisc o Vg1 producing cells located at presumptive posterior cells as result of gravity o Overlap creates Posterior Marginal Zone (PMZ) = bird version of amphibian nieuwkoop centre • Chicken “Spennan Organizer” o Koller’s sickle – induces formation of hensen node (vs. organizer inducing formation of blastopore lip?)  Anterior thickening PMZ creates Hensen’s Node  Node is equivalent to dorsal blastopore lip (Spemann’s organizer) • Induces formation of neural tube o Xenopus above, chicken below 7. What
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