Early development and organizer in Amphibians

Xenopus laevis as model

easy to manipulate (large embryo), obtain in large numbers, easily cultured embryo, rapid development (4 day to tadpole), external development

Xenopus laevis unfertilized egg

has polarity, Dense yolk - Vegetal (lower), Less yolk - animal (upper)

Xenopus laevis fertilization

sperm entry anywhere, point of entry marks ventral side and 180 marks dorsal side

Cortical Rotation

sperm centriole organize egg microtubules, microtubular track allows cortical cytoplasm to rotate in respect to inner cytoplasm, cause grey crescent

Cleavage in Xenopus laevis

3 cleavage furrow - Vegetal yolk impedes so begins before other ends, 1st bisects exposed grey crescent, 2nd right angle to 1st and meridional (animal pole to vegetal pole), 3rd equatorial but asymmetrical towards animal pole (split animal into top-bottom)

Cleavage result in Xenopus laevis

Vegetal hemisphere has fewer but larger blastomere (macromeres), Animal hemisphere has more but smaller blastomeres (micromere)

Blastocoel

cavitary in animal hemisphere, prevent cell beneath from prematurely interacting with cells above it

Animal cap

blastocoel roof, differentiate into ectoderm, spread to cover entire embryo during vegetal rotation

Amphibian Gastrulation

surround embryo with cell that will become ectoderm, bring in those with endodermal organ fate, place mesoderm inbetween ecto and endo

Epidermis

outer layer, derives from ectoderm. initially solely in animal hemisphere

Epiboly

movement of epithelial sheet → spread as unit to enclose deeper layer, occur by cell dividing/changing shape/ layer intercalating

Vegetal rotation

animal cap spread to cover embryo (epidermis), vegetal asymmetrically press up against blastocoel roof on dorsal side

Bottle cells

form the dorsal lip of blastopore, invaginate using apical constriction

Invagination

infolding of epithelium sheet, bottle cells, form dorsal lip of blastopore

Involution

inward movement and expanding of outer layer, spread over internal surface → Mesodermal, under animal cap

Archenteron

forms and displaces blastocoel

Mid to end gastrulation

Archenteron forms and displaces blastocoel → obliterated, cell migrate from lateral and ventral lips into embryo, convergent extension of dorsal mesoderm → between ecto and endo, ectoderm surrounds embryo, endoderm internalized

Specification of germ layer

Ecto and ENdo specified by maternal factors, Animal cap into ectoderm, Vegetal in endoderm, Mesoderm induced by interaction of vegetal (immediately above) and animal cap

Differentiation of vegetal into endoderm

mRNA for Tf VegT in vegetal cells → activates Sox17 Tf before mid-blastula transition, Sox17 specify endodermal fate

Differentiation of vegetal into mesoderm

VegT activates paracrine factor Nodal, Nodal signal cell above to accumulate phosphorylated Smad2, P-Smad2 upregulates eomesodermin, Enomes and Smad2 collaborate to activate Zygotic VegT → maintain feedforward loop

Organizer formation

dorsal most vegetal cells in Nieuwkoop centre induce, Beta-Catenin specifies Nieuwkoop centre, synergizing with vegetal signals, dorsal blastopore lip (bottle?) and descendants,

Nieuwkoop center

Dorsal vegetal cells, induce dorsal mesoderm → somites ad notochord, organizer like properties

Beta-Catenin

from maternal mRNA, accumulates on dorsal side → cover Nieuwkoop center and organizer region, due to translocation (Cortical rotation) of Dishevelled protein/GSKS-binding protein/Wnt11 mRNA → prevent GSK3 degradation of it, Binds Tcf3 to activate Siamois and Twin → bind gene involved in organizer (Nogging/Chordin/Goosecoid)

Organizer activation synergy with vegetal signal

With Siamois and Twin, VegT activated Nodal that induces mesoderm, high level Beta-Catenin activated Nodal related genes

BMP

induce ectoderm to become epidermal tissue → blocked by organizer tissue, BMP4 powerful ventralizing factor

Organizer tissue functions

self differentiates into dorsal mesoderm, induce ectoderm to form neural tube, induce mesoderm to become dorsal (somite forming) by blocking BMP4, initiates movement of gastrulation