#ectoderm

Eye Lens Induction -ll

What happens when FGF8 beads are implanted near ectoderm in lens development?

Implantation of fibroblast growth factor 8-soaked beads near competent head ectoderm mimics optic vesicle signal and triggers lens transcriptional program. FGF8 activates MAPK signaling inducing L-Maf expression in ectoderm, which directly transactivates delta-crystallin and beta-crystallin enhancers driving lens fiber differentiation. Implantation experiments in chick demonstrated bead-induced ectopic lens placodes expressing Sox2 and crystallins, whereas inhibition of Pax6, suppression of Six3, or activation of Wnt blocks not observed. This bead assay evidences sufficiency of FGF8 as lens inducer upstream of L-Maf-crystallin cascade and Sox2 activation.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 12: FGF8 beads inducing L-Maf and crystallin expression.

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amphibian -ll

The 'default fate' of ectodermal cells is to become:

Early Xenopus animal caps isolated before gastrulation cultured in simple saline express neural markers NCAM, Sox2 and neurofilament spontaneously without added growth factors, indicating intrinsic tendency. In intact embryo BMP4 from ventral mesoderm and ectoderm actively suppresses neural fate inducing epidermal keratin via Smad1, Dlx3 and p63. Organizer transplantation provides BMP antagonists removing suppression. Thus default or ground state of competent ectoderm is neural tissue, epidermal fate requires active BMP induction. This default model revolutionized understanding of neural induction as disinhibition rather than instructive activation, concept conserved across vertebrates.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 10: Default fate of ectoderm is neural tissue when BMP inhibited.

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frog development

High levels of BMP specify amphibian ectodermal cells to become:

BMP morphogen gradient patterns ectoderm: highest ventrally, lowest dorsally. High BMP4/7 activates Smad1/5/8 complexing with Smad4, translocating to nucleus inducing epidermal keratin genes XK81, AP2 and GATA2 while repressing Sox2 and Neurogenin. Animal caps exposed to BMP become ciliated epidermis. In intact embryo lateral and ventral ectoderm receiving high BMP becomes epidermis covering embryo, dorsal ectoderm protected by organizer antagonists becomes neural plate. Neural crest forms at intermediate BMP border. Thus high BMP specifies epidermis rather than neural, crest or endoderm fates. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.

Ref: Gilbert, Developmental Biology 12th ed., Chapter 12: BMP gradient and ectodermal patterning.

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Drosophila -l

Neurogenic ectoderm in Drosophila embryo gives rise to:

Ectodermal patterning along dorsoventral axis separates dorsal amnioserosa, dorsal ectoderm and ventrolateral neurogenic ectoderm. Intermediate levels of nuclear Dorsal activate proneural genes achaete-scute complex in neurogenic region. Under Notch-mediated lateral inhibition, single neuroblasts delaminate from neurogenic ectoderm through de-epithelialization, proliferate asymmetrically to generate neurons and glia of ventral nerve cord. This lineage establishes central nervous system. Non-neurogenic dorsal ectoderm forms epidermis, mesoderm forms muscles. Therefore neurogenic ectoderm specifically gives rise to nervous system representing neural progenitor domain specified by Dorsal gradient.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 10: Neurogenic ectoderm nervous system development.

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