Chapter 8 - Lecture 11 cont.pdf

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Cell and Systems Biology
Ashley Bruce

 Noggin, chordin, and follistatin are all secreted by the organizer, and that each of them prevents BMP from binding to and inducing the ectoderm and mesoderm cells near the organizer  BMP4 is expressed initially throughout the ectodermal and mesodermal regions of the late blastula BMP4 – powerful  During gastrulation, bmp4 transcripts become restricted to the ventralizing factor ventrolateral marginal zone because the goosecoid protein is induced by the Siamois/Twin and Smad2 interactions in the dorsal mesoderm starting at the beginning of gastrulation  These TFs repress bmp4 and wnt8 transcription  In the ectoderm, BMPs repress the genes involved in forming neural tissue while activating other genes involved in epidermal specification  In the mesoderm, it appears that graded levels of BMP4 activate different sets of mesodermal genes: o An absence of BMP4 specifies the dorsal mesoderm o Low amount specifies the intermediate mesoderm o High amount specifies the lateral mesoderm  2 important sets of experiments confirmed the default model and the importance of blocking BMPs to specify nervous system 1. Used antisense morpholinos to eliminate 3 BMP antagonists (top 2) o Result: catastrophic failure of dorsal development o Lacked neural plates and dorsal mesoderm 2. Blocked BMP activity with antisense morpholinos (C & D) o Simultaneously blocked the formation of BMPs 2, 4, 7 -> neural tube became greatly expanded, taking over a much larger region of the ectoderm o Inactivation of 3 BMPS & ADMP (another BMP family protein) -> entire ectoderm became neural  Thus, epidermis is instructed by BMP signaling, and the organizer specifies the ectoderm above it to become neural by blocking that BMP signal from reaching the adjacent ectoderm The Regional Specificity of Neural Induction  Most important phenomena in neural induction: regional specificity of the neural structures that are produced o Forebrain, hindbrain, spinocaudal regions of the neural tube much be properly organized in an anterior to posterior direction  Organizer tissue also specifies the regions of the neural tube  Mangold’s husband transplanted 4 successive regionsof the archenteron roof of late-gastrula newt embryos into the blastocoels of early-gastrula embryos o Most anterior potion of the archenteron roof induced balancers and portions of the oral apparatus o Next most anterior portion induced the formation of various head structure – nose, eyes, balancers (forebrain) o Third section including notochord induced the hindbrain structures (posterior head, diencephalon, otic vesicles) o Most posterior section induced the formation of dorsal trunk and tail mesoderm  In further experiments, demonstrated that o When dorsal blastopore lips from early salamander gastrulae were transplanted into other early salamander gastrulae -> secondary heads o When dorsal lips from later were transplanted to early -> formation of secondary tails  Results show that the first cells of the organizer to enter the embryo induce the formation of brains and heads, while those cells that form the dorsal lip of later-stage embryos induce the cells above them to become spinal cords and tails  What are the molecule being secreted by the organizer in a regional fashion? o Possible model: The head inducer: Wnt antagonists  A gradient of Wnt signals is highest in the posterior and lowest in the head  Wnts made in the posterior regions and Wnt antagonists in the anterior  Most anterior regions of the head and brain are underlain by pharyngeal endoderm and head (prechordal) mesoderm  Endodermal tissue constitutes the leading edge of the dorsal blastopore lip o These cells induce the most anterior head structures by blocking the Wnt pathway as well as by blocking BMP  Wnt ant
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