BIS104 Note16

BIS104 Note16 - Lecture 16 Bio Sci 104 Winter 2010...

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Lecture 16 1 Bio Sci 104 Winter 2010 Development I: Early Development and Cell Determination I. Overview: A. Overall process mediated by fairly simple set of activities: 1. Cells grow, divide, or die 2. Form mechanical attachments and generate forces for movement 3. Generate signals to influence adjacent cells, and respond to adjacent signals 4. Differentiate by turning on or off expression of specific sets of genes 5. This process is mediated and controlled by the genome, which although identical in all cells, can define the rules by which these different cell activities are called into play. Thus, a fertilized egg generates the highly spatially ordered adult. B. Will first discuss these aspects generally for all organisms. C. Three major phases in development: 1. Phase 1: major steps include cleavage of fertilized egg to generate many smaller cells, gastrulation to generate gut cavity and general ectoderm, endoderm, mesoderm structure , and neurulation , which creates the neural tube . By end of phase one, basic body plan is established. 2. Phase 2 : organogenesis , in which the basic structure of the major body organs is formed (eyes, heart, limbs, etc). 3. Phase 3: in which the final, detailed structures that make up the neonate are generated. 4. Much of phase 3 is continuing throughout adult life: e.g. constant generation of newly differentiated skin and red blood cells to replace those that wear out. II. Cleavage and establishment of head/tail axis A. Best understood for amphibians (e.g. Xenopus) 1. egg is large, single cell with yolk concentrated toward lower end ( vegetal pole ) relative to upper ( animal ) pole. Has actin-rich cortex and a more central cytoplasmic core. 2. Vegetal and animal poles have different sets of mRNAs , and will have different fates: vegetal pole will primarily form internal tissues such as the gut, and the animal end will contribute primarily to outer structures, such as skin . This theme will continue, in that part of the determination of the final body plan often reflects an asymmetry pre-existing in the egg, and established by the mother. 3. entry of sperm initiates many important events:
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Lecture 16 2 a. A block to polyspermy. Fusion of the sperm plasma membrane with the oocyte induced structure changes in the Zona Pellucida (made of glycoproteins, protects oocyte from cross species fertilization) therefore prevent the binding to other sperms to the oocyte. b. rotation of egg cortex about 30 o . This rotation actually determines future dorsal/ ventral axis (rotation of cortex is toward future ventral side. Side opposite to sperm entry becomes future dorsal side). If rotation ( as a result of microtubule rearrangement induced by sperm entry ) is prevented, future embryo lacks dorso-ventral asymmetry. In addition, the Wnt11 proteins migrate from vegetal pole to the dorsal side (microtubule dependent) to help define the dorsal ventral polarity of the embryo. c. initiates cleavage:
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BIS104 Note16 - Lecture 16 Bio Sci 104 Winter 2010...

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