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#animal hemisphere

3 public questions tagged with this topic.

Sea Urchin -ll

The animal hemisphere predominantly gives rise to:

Animal hemisphere encompasses mesomeres derived from animal tier blastomeres after third equatorial division inherits minimal vegetal determinants. These cells express animal transcription factors such as FoxQ2, Six3 and SoxB1 that promote ectodermal differentiation and repress endomesoderm. Classical fate mapping and isolation show they autonomously and conditionally produce apical plate, ciliary band, stomodeal region, and aboral plus oral ectoderm covering pluteus larva. Under lithium-induced vegetalization they can be fully respecified to endoderm and mesenchyme. Under normal physiological conditions they predominantly generate ectoderm, covering embryo exterior, while vegetal Veg and micromere lineages generate internal gut and mesenchyme derivatives.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 8: Animal hemisphere fate map - ectoderm derivation.

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Sea Urchin -ll

Animal hemisphere cells without micromeres form:

When micromeres surgically removed from 16-cell stage sea urchin embryo, remaining animal hemisphere and macromeres lack sufficient vegetal Wnt/β-catenin and Delta-Notch signals needed for endomesoderm induction. Without these inductive cues, archenteron never forms, no primary or secondary mesenchyme ingresses, and gut differentiation fails. Instead embryo becomes permanently ciliated, hollow epithelial ball composed predominantly of expanded apical ectoderm and ciliary band, termed dauerblastula or animalized embryo. It swims but never gastrulates, analogous to β-catenin inhibition phenotype. This demonstrates conditional specification of animal cells requiring micromere signals; replacement of micromeres rescues gastrulation and normal pluteus formation.

Ref: Gilbert, Developmental Biology, Chapter 8: Animalization after micromere removal - dauerblastula phenotype.

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Sea Urchin -ll

Transplanting micromeres to the animal hemisphere induces:

Micromeres function as embryonic organizer in sea urchins comparable to amphibian dorsal lip. Horstadius in 1935 demonstrated transplanting fluorescently labeled micromeres from 16-cell embryo to animal pole of otherwise intact host embryo causes adjacent animal cells, normally fated to ectoderm, to change destiny, invaginate and form second archenteron with associated pigment cells and secondary mesenchyme. Induction requires Delta-Notch juxtacrine presentation and Wnt8 plus early β-catenin paracrine signals that reprogram host ectoderm toward endomesoderm. Resulting twinned gastrulation mimics Spemann organizer experiment. Transplant thus induces secondary archenteron formation, proving conditional specification of animal hemisphere under micromere influence.

Ref: Hörstadius 1935 organizer experiment; Gilbert, Developmental Biology, Chapter 8: Micromere transplantation and induction.

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