BSC2011_Slide Set 3

BSC2011_Slide Set 3 - SLIDE SET III Animal Development IV....

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SLIDE SET III Animal Development IV. Cleavage V. A. Introduction B. Example: gastrulation 1. sea urchin 2. frog 3. chick
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I. GENERAL PATTERN OF DEVELOPMENT EGG (n) + SPERM (n) Zygote (2n) Fertilization Cleavage Mitotic cell division Blastula Gastrulation & Morphogenesis Mitotic cell division & cell differentiation Gastrula Neurulation & Organogenesis Mitotic cell division & cell differentiation Embryo 1. 2. 3. 4. 1 2 3 4 & morphogenesis
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SEA URCHIN DEVELOPMENT: 1. fertilization, 2. cleavage 2. cleavage
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1 cell (zygote) 4 cells 8 cells 16 cells *compare overall size vs. # of cells CLEAVAGE - a rapid series of miototic cell divisions that occurs in the absence of cell growth
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ZYGOTE Sea urchin cleavage Cleavage proceeds to a morula (solid ball of cells) & ends with a blastula (a “hollow ball” of cells) the hollow cavity = a blastocoel each cell = a blastomere Fig. 47.6
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Differences in cleavage patterns & in appearance of blastula among species arise due to differences in the amount and distribution of yolk in the egg: From: Gilbert’s Developmental Biology type of blastula seen with holoblastic cleavage Because sea urchin egg is microlecithal (little yolk), it has holoblastic CLEAVAGE (cells divide completely)…
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Taken from Wolpert, Principles of Development ANIMAL POLE of egg cell is mostly cytoplasm VEGETAL POLE is mostly dense yolk …but a FROG egg has an UNEVEN yolk distribution & it is mesolecithal (has a moderate amount of yolk). But cleavage remains holoblastic (cells divide completely) as in sea urchin.
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(strictly speaking, morula is 16-64 cells) blastula (with blastocoel cavity inside in animal pole) Cleavage in a Frog Embryo First 2 cleavages are vertical (division is slower through dense yolk but eventually divides completely); third cleavage is horizontal; Blastocoel is reduced and offset in frog blastula because of uneven yolk distribution ( high-yolk cells divide more slowly ). Fig. 47.7; 8th: 47.8
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BSC2011_Slide Set 3 - SLIDE SET III Animal Development IV....

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