Download Early Development and Axis Formation in Amphibians

Biology 4361 Early Development and Axis Formation in Amphibians October 25, 2006
Overview Cortical rotation Cleavage Gastrulation Determination the “Organizer” mesoderm induction
Setting up the axes: dorsal/ventral anterior/posterior left/right Cortical Rotation
Unequal Radial Holoblastic Cleavage Cell cycles regulated by mitosis­promoting factor (MPF) ­ no G phases Mid­blastula transition ­ promoters demethylated ­ transcription factors, e.g. VegT formed in vegetal cytoplasm
Xenopus Fate Map Fate mostly dependent on whether the cells are located in superficial or deep layers in the blastula. ­ ectoderm and endoderm precursors: superficial layer on surface ­ mesodermal precursors: mostly in deep layers ­ in Xenopus, notochord lies beneath the surface in the marginal areas Gastrulation – moves cell layers into proper conjunction
Gastrulation ­ Vegetal Rotation
Involuting marginal zone motive force Xenopus Gastrulation
Xenopus Gastrulation Cell movements:
­ invagination ­ involution ­ epiboly (animal cap) IMZ = involuting marginal zone ­ intercalation and ­ convergent extension ­ migration Xenopus Gastrulation NIMZ
Determination Spemann’s Ligation Experiment ­ demonstrated that early nuclei were capable of generating an entire organism ­ however, further experimentation complicated story
Asymmetrical Egg Ligation if one blastomere received no gray crescent material ­ resulted in “belly piece”
“Belly piece” – blood, mesenchyme, gut cells ­ no dorsal structures (notochord, somites) Gray crescent area (future dorsal lip of the blastopore) was critical to development Dorsal Lip Transplant Doral lip = “Organizer” ­ organizes secondary D­V axis ­ induced ventral cells to change fates
The Organizer Spemann and Mangold’s experiments established the organizing properties of the dorsal blastopore lip. ­ self­differentiates ­ establishes Dorsal­Ventral axis ­ specifies multiple tissues ­ dorsal mesoderm, which includes: ­ head mesoderm (prechordal plate) ­ chordamesoderm (notochord) ­ dorsalizes surrounding mesoderm into paraxial mesoderm ­ induces the neural tube ­ initiates the movements of gastrulation How is the dorsal lip specified? …… back to the fate map for re­orientation and overview:
Xenopus Fate Map The general fate map is imposed by transcription factors and paracrine factors in the vegetal region. Anterior­posterior, dorsal­ventral, and left­right axes specified by events triggered at fertilization and realized during gastrulation. Of all tissues in the early Xenopus gastrula, only the dorsal lip of the blastopore has its fate determined. The dorsal lip will determine the fates of notochord and head endomesoderm and determine the primary embryonic axis.
Mesoderm & Organizer Induction Animal cap factors released by vegetal cells Neither animal fragments nor vegetal fragments alone produce mesoderm ­ but placed in conjunction = mesoderm induction Vegetal cells induce mesoderm VegT ­ transcription factor Vg1 ­ paracrine factor Mesodermal fate depends upon origin of vegetal inducing cells ­ ventral vegetal cells induce mesenchyme, blood
­ intermediate – muscle, kidney VegT Vg1 ­ dorsal (organizer) – notochord, somites Dorsal Signal ­ $­catenin $­catenin ­ anchor for cadherins ­ nuclear transcription factor (Wnt pathway) ­ in sea urchins, specifies micromeres, endomesoderm ­ in Xenopus $­catenin is initially synthesized throughout the embryo from maternal mRNA ­ accumulates in dorsal cells ­ concentrates in the Nieuwkoop center and organizer How? It starts in the egg…… organizer dorsal signal $­catenin VegT Vg1
Nieuwkoop center Initially, $­catenin is evenly distributed throughout the egg But eventually appears just in dorsal cells Dorsalization is accomplished by: 1. protecting $­catenin in dorsal area only 2. degrading $­catenin everywhere else Disheveled/$­Catenin/Cortical Rotation $­catenin induces cells to dorsal fates
$­catenin is initially distributed throughout oocyte Dsh – Disheveled GBP – GSK3 binding protein GSK3 ­ glucose synthase kinase Disheveled/GSK3/$­Catenin At the blastula stage, $­catenin is located exclusively in the future dorsal region
GSK3 degrades $­catenin…… but Disheveled and GBP block GSK3 activity ­ resulting in $­catenin survival only in the marginal area opposite the point of sperm entry Organizer Induction $­catenin acts with Tcf3 (a transcription factor) to stimulate expression of several dorsalizing genes
Twin and Siamois – TFs activate Xlim, goosecoid ­ dorsal determinants Goosecoid protein – TF responsible for organizer properties ­ goosecoid also plays a part in specifying dorsal mesoderm ­ however, additional vegetal factors are needed: vegetal TGF­$ signals Mesodermal Signals Xnr – Xenopus nodal related genes Xnrs: TGF­$ genes
Determination of Ectoderm early cells are uncommitted ­ exhibit regulative development
later cells are determined ­ exhibit autonomous development Establishment of Axes D­V axis – set up at fertilization A­P axis – established by gastrulation movements across the dorsal lip of the blastopore L­R axis – (still to come)
Dorsal Lip Inductions dorsal mesoderm (paraxial mesoderm ­ somites) chordamesoderm ­ notochord
head mesoderm (prechordal plate) pharyngeal endoderm Left – Right Axis wild­type Xnr1 ­/­
Left–Right axis established by Xenopus nodal­related (Xnr1) Nodal expression: ­ common to all vertebrates ­ expressed on the left side! Xnr1 expression is limited to the left side in a process involving cortical rotation and Vg1 heart loops to the left gut coiling – counter­clockwise Block Xnr1 expression = random gut coiling, heart looping