The frog blastula has superficial and deep cells superficial cells blastocoel deep cells Superficial cells are epithelial cells joined by junctions Deep cells are more loosely organized adapted from Alberts et al 3e Fig 21 4 Amphibian Gastrulation Overview Animal cap Gilbert 10e Fig 8 7 The animal cap makes ectoderm Gastrulation initiates at the dorsal blastopore lip Bottle cells help to initiate lip formation Amphibian Gastrulation Overview Gilbert 10e Fig 8 7 The mesoderm involutes beneath the ectoderm The archenteron primitive gut begins to form The blastocoel and what was the animal pole AP are displaced towards the future anterior at the left Amphibian Gastrulation Overview Anterior Gilbert 10e Fig 8 7 Endomesoderm comes to lie at the future anterior Amphibian Gastrulation Overview Gilbert 10e Fig 8 7 Head and other mesodermal mesenchyme forms Amphibian Gastrulation Overview Blastocoel Animal cap Dorsal blastopore lip Archenteron Ewald A J et al Development 2004 131 6195 6209 see Gilbert 9e Fig 7 6 Gastrulation closes the yolk plug The ectoderm spreads to cover over the endoderm leaving a round yolk plug Gilbert 10e Fig 8 9 Amphibian Gastrulation Ray Keller some ugly guy John Gerhart Ray Keller Marginal zone morphogenesis is crucial for gastrulation Marginal zone adapted from Gilbert 10e Fig 8 7 The marginal has two cell types 1 deep cells and 2 superficial cells The superficial cells of the marginal zone form endoderm The deep cells form mesoderm Superficial and deep cells have different fates in Xenopus Marginal zone superficial cells endoderm Marginal zone deep cells mesoderm Dorsal marginal zone mesoderm Organizer Gilbert 9e Fig 7 5 see 10e Fig 8 3 Amphibians Fate Map Surface dorsal AP ventral anterior NIMZ IMZ courtesy Ray Keller VP BCs posterior NIMZ Non involuting marginal zone Dorsal NIMZ becomes neural ectoderm ventral NIMZ becomes epidermis Amphibians Fate Map Surface dorsal AP ventral anterior NIMZ IMZ courtesy Ray Keller VP BCs posterior IMZ Involuting marginal zone Deep cells of the dorsal IMZ are Organizer mesoderm superficial dorsal IMZ becomes part of the roof of the archenteron Amphibians Fate Map Surface dorsal AP ventral anterior NIMZ IMZ courtesy Ray Keller VP BCs posterior BC Bottle cells They constrict at the beginning of gastrulation and later respread to form part of the roof of the archenteron Amphibians Fate Map Interior dorsal ventral animal cap NIMZ anterior blastocoel IMZ deep A BCs LEM deep zone vegetal base blastopore posterior courtesy Ray Keller Deep cells of the dorsal IMZ are Organizer mesoderm Amphibians Fate Map Interior dorsal ventral animal cap NIMZ anterior blastocoel IMZ deep A BCs LEM deep zone vegetal base blastopore posterior courtesy Ray Keller Dorsal leading edge mesoderm LEM becomes head and other mesenchyme Amphibians Fate Map Interior dorsal ventral animal cap NIMZ former BCs anterior blastocoel IMZ deep A BCs LEM deep zone vegetal base blastopore posterior courtesy Ray Keller BC Bottle cells form part of the roof of the archenteron A Amphibians Deep Cell Fates dorsal dorsal IMZ IMZ ventral anterior LEM ventral DIMZ LEM posterior Hd Ht LV Deep cells of the dorsal IMZ are Organizer mesoderm LEM courtesy Ray Keller Amphibians Deep Cell Fates dorsal dorsal IMZ IMZ ventral anterior LEM ventral DIMZ LEM posterior Hd Ht Dorsal leading edge mesoderm LEM becomes mesenchyme It forms head Hd heart Ht and liver Lv LV LEM courtesy Ray Keller Gastrulation involves region specific behaviors Adapted from Gilbert 10e Fig 8 10 Gastrulation involves region specific behaviors Animal cap cells epiboly via radial intercalation of deep cells and spreading of superficial cells Radial intercalation involves intercalation of cells throughout the thickness of a tissue leading to its thinning courtesy Ray Keller Gastrulation involves region specific behaviors Animal cap cells epiboly via radial intercalation of deep cells and spreading of superficial cells courtesy Ray Keller Gastrulation involves region specific behaviors Adapted from Gilbert 10e Fig 8 10 Gastrulation involves region specific behaviors Bottle cells Invagination by apical constriction then respreading courtesy Ray Keller Bottle cells cutaway view courtesy Ray Keller Bottle cells surface view Bottle cells constrict their apical ends on the surface They constrict down to small points in some cases courtesy Ray Keller Bottle cells undergo apical constriction Activated myosin and actin are found at apical surfaces as bottle cells constrict Lee and Harland 2007 Dev Bio 311 40 52 The blastopore forms where bottle cell constrict see Gilbert 10e Fig 8 8 Bottle cells help rotate the blastopore lip Bottle cells constrict pushing their cells bodies into the embryo The cells nearby rotate orienting the mesoderm red pink orange see Gilbert 10e Fig 8 8 Bottle cells later respread The book is wrong see Gilbert 10e Fig 8 10 Bottle cells later respread Bottle cells later respread to form part of the roof of the archenteron as mesodermal cells involute see Gilbert 10e Fig 8 10 Gastrulation involves region specific behaviors Adapted from Gilbert 10e Fig 8 10 Gastrulation involves region specific behaviors Non involuting marginal zone radial intercalation and convergent extension Radial intercalation thins the NIMZ Later dorsal NIMZ undergoes convergent extension Dorsal NIMZ becomes neural ectoderm Gilbert 9e Fig 7 10 Wolpert et al 2e Fig 8 26 Ectodermal movements dorsal AP ventral anterior NIMZ IMZ courtesy Ray Keller VP BCs posterior NIMZ Non involuting marginal zone Dorsal NIMZ becomes neural ectoderm ventral NIMZ becomes epidermis Deep cell movements dorsal dorsal IMZ IMZ ventral anterior LEM ventral DIMZ LEM posterior Hd Ht LV Notice how much the cells of the dorsal IMZ elongate LEM courtesy Ray Keller Gastrulation involves region specific behaviors Involuting marginal zone convergent extension The dorsal IMZ undergoes dramatic convergent extension resulting in explosive elongation IMZ forms mesoderm Gilbert 10e Fig 8 10 Wolpert et al 2e Fig 8 26 IMZ undergoes convergent extension Gilbert 4e Fig 4 24 Fluorescently labeled cells mix with labeled cells Conclusion The dorsal IMZ undergoes dramatic convergent extension Keller explants allow analysis of convergent extnesion Pieces of the early gastrula that include what would normally become neural ectoderm and axial mesoderm notochord and somites are excised A practical issue these are not happy on their own so two of these are placed together back to