25 - Lectures 24 and 25: Development Embryonic Development...

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Required reading Chapter 47: pp. 1025-1044 Lectures 24 and 25: Development Embryonic Development Monday, 11/23/09 Bio 93 Fall 2009
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Main Concepts Embryonic development can be divided into four stages: Fertilization - combination of sperm and egg nuclei and egg activation; haploid to diploid Cleavage - rapid cell divisions without substantial growth in size to generate a multicellular embryo Gastrulation - mass cell movements to generate three germ layers Organogenesis - localized changes in tissue and cell shape
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• Cleavage partitions the cytoplasm of one large cell into many smaller cells called blastomeres Fertilized egg. Shown here is the zygote shortly before the first cleavage division, surrounded by the fertilization envelope. The nucleus is visible in the center. (a) Four-cell stage. Remnants of the mitotic spindle can be seen between the two cells that have just completed the second cleavage division. (b) Morula. After further cleavage divisions, the embryo is a multicellular ball that is still surrounded by the fertilization envelope. The blastocoel cavity has begun to form. (c) Blastula. A single layer of cells surrounds a large blastocoel cavity. Although not visible here, the fertilization envelope is still present; the embryo will soon hatch from it and begin swimming. (d) S phase, M phase; almost no G1 and G2; little or no protein synthesis
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Continued cleavage produces the morula. A blastocoel forms within the morula blastula Blastocoel - Fluid-filled cavity More yolk
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Cleavage patterns in different organisms are very distinct. - therefore the process of gastrulation can appear very dissimilar This is due to the nature of the eggs -how much yolk they contain and the environment in which they develop Despite these superficial differences, similar mechanisms underlie cleavage in vertebrates - in all cases a blastula is formed Cleavage
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Gastrulation The morphogenetic process called gastrulation – Rearranges the cells of a blastula into a three-layered embryo, called a gastrula, that has a primitive gut The three layers produced by gastrulation a re called embryonic germ layers The ectoderm -forms the outer layer of the gastrula The endoderm -Lines the embryonic digestive tract The mesoderm -partly fills the space between the endoderm and the ectoderm
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Frog gastrulation produces a triploblastic embryo with an archenteron. Gastrulation begins when an
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This note was uploaded on 11/19/2010 for the course BIOLOGY 93 taught by Professor O during the Spring '08 term at San Diego.

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25 - Lectures 24 and 25: Development Embryonic Development...

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