Cl09 - MCDB 4650 DEVELOPMENTAL BIOLOGY CLASS NOTES Class 9...

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MCDB 4650 DEVELOPMENTAL BIOLOGY CLASS NOTES Class 9 1 The roles of the organizer; induction of neural fates Reading: Chp 10: 312-323 Learning goals Be able to: Distinguish between neural induction, segregation and commitment to different fates Explain how the organizer influences formation of neural ectoderm and intermediate mesoderm, including the molecules involved in these events. Compare the signaling pathways known to be involved in further induction of neural cell fate along the D-V and A-P axes with other roles that these molecules have in development. (see class 20 for D-V) Describe the importance of lateral inhibition in neural fate determination Deduce phenotypes of embryos with deficits in these signaling molecules What the organizer does 1. Induces lateral mesoderm (via Signal 3, before gastrulation starts) 2. Induces overlying ectoderm to become neural ectoderm instead of epidermis, and helps to pattern it from anterior to posterior. 3. Initiates the movements of gastrulation (involution) 4. Maintains the movements of gastrulation: pulling dorsal mesoderm along the archenterons roof, to become head mesoderm and chordamesoderm (which becomes the notochord). How the organizer induces other tissues The master transcriptional regulator Goosecoid initiates most or all of these functions, including expression of secreted organizer factors (below) and cell movement capabilities. Now we can summarize how the organizer works, which turns out to be backwards from most previous assumptions. The organizer functions by secreting not activators, but inhibitors! Most of the apparent activations turn out to be inhibition of inhibitors. Remember the experiments described last time about transplantation of the organizer inducing a second dorsal axis? These exact results can also be obtained by injecting Goosecoid into a ventral vegetal blastomoere (Fig 10.27). In addition, goosecoid is known (by in situ hybridization and antibody labeling) to be present in the organizer at the time when these events are happening, and it is NOT present in the embryo when the embryo has been UV irradiated. So, goosecoid is the molecule that gives the organizer its special properties. Goosecoid is a transcription factor, so what does it induce? Quite a few proteins! (table 10.2). Some of them: chordin, noggin, and follistatin are all signaling ligands that act to BLOCK the TGFb pathway by directly binding to a TGFb ligand, BMP. This discovery was made possible, in part, by a surprising result from a standard “isolate” experiment.
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Cl09 - MCDB 4650 DEVELOPMENTAL BIOLOGY CLASS NOTES Class 9...

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