Unit 14 Axis Specification Gastrulation in Other Vertebrates Amphibians are not the only model Fish Cleavage Gilbert 10e Fig 8 39 Zebrafish Overview Most of the embryo proper forms on top of the large multinucleate yolk cell Gilbert 10e Fig 8 36 Zebrafish Cleavage The enveloping layer EVL is a single cell layer on the surface It eventually covers the entire embryo It forms extraembryonic tissues later Gilbert 10e Fig 8 40 The yolk cell is multinucleate mRNAs produced from nuclei near the animal edge of yolk cell in the yolk syncytial layer produce proteins important for axial patterning The YSL also contracts to help pull the EVL vegetally Zebrafish Overview The hypoblast forms a floor for the embryo at the edges of the cellular portion of the embryo The epiblast is the surface layer fo the embryo Some epiblast cells will eventually involute Gilbert 10e Fig 8 36 Zebrafish Overview The shield forms due to convergence of cells toward the future dorsal midline of the embryo It is the equivalent of the Organizer in fish Gilbert 10e Fig 8 36 Zebrafish bozozok is important for dorsal vegetal differentiation As in frogs b catenin accumulates on the dorsal side of the embryo in the vegetal region including nuclei in the yolk cell These function somewhat similarly to the dorsal vegetal cells of a frog embryo Bozozok from a Polish word is a transcription factor expressed in the dorsal vegetal region yolk cell in a region analogous to the Nieuwkoop Center Schier and Talbot 2005 Ann Rev Gen 39 561 613 Zebrafish Catenin is in dorsal nuclei As in frogs b catenin accumulates on the dorsal side of the embryo in the vegetal region including nuclei in the yolk cell dorsal view See Gilbert 10e Fig 8 45 Zebrafish Endodermal signals induce mesoderm Dorsal vegetal signals appear to generate the Organizer Other regions of the yolk cell emit nondorsal mesoderm inducing signals Nodal related proteins are important for mesoderm specification as they are in frogs Gilbert 10e Fig 8 44 Zebrafish The shield is the Organizer It is possible to transplant bits of the shield from one embryo to another Gilbert 10e Fig 8 43 Zebrafish The shield is the Organizer An extra set of axial structures results from the shield transplant Conclusion The shield acts like an Organizer Gilbert 10e Fig 8 43 Zebrafish Shield counteracts ventral signals BMPs e g BMP2 are found on the ventral side of the embryo Chordin chordino in fish is produced by the shield Organizer Gilbert 10e Fig 8 44 Zebrafish BMP pathway is required for ventral structures somitabun WT dorsalized embryo Loss of Smad5 leads to loss of ventral structures Conclusion The shield acts like an Organizer Myers et al 2002 Trends Genetics 18 447 455 Zebrafish Radial intercalation causes thinning of the epiblast The enveloping layer is somewhat like the superficial cells of the frog gastrula Deep cells undergo radial intercalation to thin the epiblast This is similar to deep cells of the animal cap in frogs Montero Heisenberg 2004 Trends Cell Bio 14 620 627 Zebrafish Radial Intercalation Requires half baked E Cadherin hab In half baked mutants radial intercalation fails leading to incomplete gastrulation Kane et al 1996 Development 123 47 55 Zebrafish Radial Intercalation Requires half baked E Cadherin WT hb In half baked mutants radial intercalation fails leading to incomplete gastrulation Kane et al 2005 Development 132 1105 1116 Zebrafish Mesodermal deep cells involute Gilbert 10e Fig 8 41 Zebrafish Mesodermal deep cells involute delaminate Gilbert 10e Fig 8 41 The hypoblast may form by delamination Epiblast cells appear to involute A small group of cell forerunner cells do not involute and remain on the outside Zebrafish Overall cells converge and extend Mesodermal cells converge and extend Note they largely appear to migrate as individuals which differs from frogs Gilbert 10e Fig 8 42 Zebrafish Axial mesoderm undergoes convergent extension Mesodermal cells converge and extend The overall pattern is similar to frogs Gilbert 10e Fig 8 42 Zebrafish PCP is necessary for normal migration WT Silberblick Wnt11 Mesodermal cells require the planar cell polarity In Wnt11 mutants which are defective in PCP migration is not as directional Courtesy C P Heisenberg Zebrafish Gastrulation drives axial extension Dorsal mesodermal cells converge and extend to extend the anterior posterior axis Gilbert 10e Fig 8 41 Zebrafish retinoic acid acts in the posterior Otx2 is a transcription factor that is expressed in future brain Cells that experience high levels of retinoic acid FGFs and Wnts are in the future posterior Gilbert 810e Fig 8 46 Zebrafish retinoic acid acts in the posterior anterior posterior Cells that experience high levels of retinoic acid are found in the brain and the posterior Shimozono et al 2013 Nature doi 10 1038 nature12037 Chick Cleavage Birds have the ultimate meroblastic eggs Chick cleavage occurs on top of the yolk Cells form on top of the yolk cells they appear clear area pellucida Gilbert 10e Fig 9 2 Chick rotation in the shell is involved in specifying the posterior The posterior forms orthogonal to the long axis of the eggshell Rotation occurs in the oviduct Gilbert 10e Changing rotation orientation changes where the posterior forms Fig 9 8 Birds The hypoblast forms from two sources Some epiblast cells ingress to form hypoblast Others migrate forward from the posterior marginal zone not the same structure as in frogs Gilbert 10e Fig 9 3 Birds The hypoblast forms from two sources Gilbert 10e Fig 9 3 Koller s sickle seems to act in an analagous fashion to the Nieuwkoop Center in frogs Birds Definitive endoderm forms from some of the first epiblast cells to ingress Gilbert 10e Fig 9 3 Mesoderm ingresses through the primitive streak Definitive endoderm invades the hypoblast to form embryonic endoderm Blastocoel Birds The blastodisc is the blastula equivalent Blastocoel Epiblast The blastocoel form between the epiblast and the yolk cell Gilbert 7e Fig 11 13 Birds The primitive streak is like the blastopore Though shaped differently the primitive streak is the equivalent of the amphibian blastopore Gilbert 10e Fig 9 4 Convergence of primitive streak cells involves pollonaise movements Cells converge and extend as the streak moves anteriorly Gilbert 10e Fig 9 6 Birds Fate map Gilbert 10e Fig 9 4 The primitive streak is the equivalent of the blastopore Henson s node is the equivalent of the dorsal blastopore lip Birds Overview The embryo elongates extensively