zoo 19 th lecture

zoo 19 th lecture - Week One: November 17 November 21...

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Week One: November 17 November 21 Cleavage and Gastrulation – Avian embryos - Meroblastic dicoidal cleavage pattern – Cleavage is initially incomplete. The egg is a very large, yolky egg that displaces to one side of the cytoplasm. It makes a cytoplasmic disc or blastodisc. - Initially produces a subgerminal cavity – this is not analogous to the blastocoel . - Must establish a two layered blastoderm before gastrulation – defines a blastocoel cavity. This is distinctly different from the subgerminal cavity. - Resembles gastrulation in both the fishes and in mammals Cleavage – discoidal - The cells gradually begin to define themselves as they produce a membrane across the yolk layer. There is going to be a clear space that opens up – this is referred to as the area pellucida.
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Formation of the blastoderm - At one edge of the egg, there is a group of cells that begin to proliferate that is referred to as the posterior marginal zone. It will ultimately elaborate the next layer of cells – it will split the subgerminal cavity in half. - Area over the subgerminal cavity is the area pellucida (semitransparent) - Area at marginal zones overlie yolk – area opaca - Subgerminal cavity is NOT analogous to the blastocoel - Layer of cells forming the area pellucida is the presumptive epiblast (cells from which the embryo proper will be formed). No parts of the embryo proper will be from the hypoblast. Formation of the blastocoel and Koller’s sickle - The posterior marginal zone will ultimately give rise to a group of cells that will be called the Koller’s sickle. - In the formation of the blastocoel, certain cells will delaminate from the area pellucida/epiblast. They will become embedded and form poly-invaginated islands. The islands are typically 5-20 cells. These form the primary hypoblast . - After these poly-invaginated islands form, the group of cells sticking out from Koller’s crescent begins to project a single cell layer across the subgerminal cavity. As they do so, they will interact with the islands and push them towards the anterior end. These are the secondary hypoblast .
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Formation of the Blastoderm - (A) – At the very beginning – The blue cells are going to contribute to the secondary hypoblast cells as they project out. The Koller sickle is just a small group of cells that does not change in any situations – but seems to be acting as an inducer (not analogous to the Neiuwkoop center or the primary organizer). -
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This note was uploaded on 07/22/2011 for the course BIOL 3156 taught by Professor Lynn,j during the Spring '08 term at LSU.

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zoo 19 th lecture - Week One: November 17 November 21...

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