Lec 6A - Lecture Set 6A Amphibian Axis Specification(Ch 7...

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1 Lecture Set 6A. Amphibian Axis Specification (Ch 7 in 9e, Ch 10 in 8e) Axis Specification in Vertebrates – Part 1 Lessons from Amphibians Time lapse of Xenopus development Figure 7.1 Reorganization of the cytoplasm and cortical rotation produce the gray crescent in frog eggs (Part 2) Site opposite sperm entry is future “dorsal” and is where gastrulation will initiate
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2 Figure 7.2 Cleavage of a frog egg Unequal radial holoblastic cleavage; yolk “slows down” cleavage in vegetal cells Figure 7.3 Scanning electron micrographs of frog egg cleavage Morula stage – note larger (yolkier) vegetal cells
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3 Figure 7.6 Cell movements during frog gastrulation Figure 7.7 Surface view of an early dorsal blastopore lip of Xenopus
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4 Figure 7.8 Early movements of Xenopus gastrulation Figure 7.13 Epiboly of the ectoderm is accomplished by cell division and intercalation Dividing cells labeled with an Ab for pHistone 3, a mitosis marker (black dots) Stage 10.5; div in An hemisphere Stage 12; div throughout ecto SEMs of blastocoel roof cells intercalating over developmental time
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5 What is so special about the vegetal cells? Vegetal cells have two jobs: 1- differentiate into endoderm 2- induce overlying cells to become Mesoderm There is a vegetally-localized mRNA ( VegT ) that is necessary for these two functions. VegT activates genes for “endoderm” Fate VegT activates genes encoding the Signals secreted by Veg cells Figure 7.15 Asymmetry in the amphibian egg
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6 Figure 7.17 Organization of a secondary axis by dorsal blastopore lip tissue (Part 1) The Spemann Mangold Experiment (DBL transplants) Figure 7.17 Organization of a secondary axis by dorsal blastopore lip tissue (Part 2)
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7 Figure 7.17 Organization of a secondary axis by dorsal blastopore lip tissue (Part 3) Movie demonstrating how to do the Spemann-Mangold DLB Transplant Experiment
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8 These cells were called the ORGANIZER Spemann called these cells “the Organizer” -Induced host ventral tissue to change their fates (ectoderm neural/dorsal and mesoderm dorsal) -Organized host and donor tissues into a complete, secondary embryo with clear A-P and D-V axes This was the first clear example of INDUCTION (Spemann won Nobel Prize for this) Questions raised by the Spemann-Mangold Experiment How does the Organizer get its properties/form? What factors are made (secreted) by the Organizer cells that cause the formation of the neural tube and to create the axes? How do the different parts of the neural tube become established, with the most anterior becoming sensory organs and forebrain and the most posterior becoming spinal cord? CLUE: Back to those original grey crescent observations and the vegetal cells that are sitting right underneath where the blastopore lip (Organizer) first forms.
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9 Embryology experiments that gave some early clues about origins of Organizer Fate Map of late blastula stage Isolation Experiments: Remove Animal Cap, culture ectodermal derivatives
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Lec 6A - Lecture Set 6A Amphibian Axis Specification(Ch 7...

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