#chick development

chick development-l

Sonic hedgehog (Shh) signaling during chick development is crucial for:

Sonic hedgehog secreted from Hensen's node and left paraxial mesoderm plays pivotal role in left-right asymmetry, not just dorsoventral neural patterning. Shh induces Caronte, a Cerberus family BMP antagonist, on left side near node. Caronte inhibits BMP signaling locally, relieving BMP-mediated repression of Nodal in left lateral plate mesoderm via FoxH1 transcription. Right side FGF8 maintains BMP activity blocking Nodal. This Shh-Caronte-Nodal-Pitx2 cascade drives heart looping and gut coiling. Loss of asymmetric Shh randomizes situs, demonstrating crucial role in left-right asymmetry specification beyond neural tube closure.

Ref: Levin, Cell 1995; Gilbert, Developmental Biology, Chapter 12: Sonic hedgehog signaling in left-right asymmetry regulation.

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Chick development-ll

Chick embryo epiblast cells are precursors for:

Classic fate mapping using carbon particles and fluorescent dyes shows chick epiblast before gastrulation is pluripotent sheet. Single labelled epiblast cells can contribute descendants to neural ectoderm, epidermis, all mesodermal subtypes including paraxial, lateral, extraembryonic and definitive endoderm depending on ingress timing through streak. Hypoblast only yields extraembryonic yolk sac endoderm that eventually replaced. Thus epiblast at stages X-XIII harbors progenitors for all three germ layers, reflecting epiblast as embryonic source homologous to amphibian animal and marginal zones combined, not restricted to only one layer.

Ref: Wolpert, Principles of Development, Chapter 5: Chick epiblast fate as precursor of all germ layers.

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Chick development-ll

Hensen's node transplantation into another gastrula location:

Hensen's node transplantation to ectopic location in host gastrula demonstrates organizer properties analogous to Spemann experiment. Donor node labelled with quail nucleolus self-differentiates into notochord and floor plate while emitting signals including chordin, noggin, Shh that neuralize surrounding host epiblast converting it into secondary neural tube and recruit lateral mesoderm into ectopic somites. Full secondary embryonic axis forms, sometimes incomplete but containing dorsal axial structures. Controls show lateral epiblast alone lacks ability. Therefore transplantation induces secondary axis formation confirming node as avian organizer. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.

Ref: Waddington 1932 chick organizer transplantation - Hensen's node secondary axis induction, Developmental Biology.

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Chick development-ll

Development of the chick primary hypoblast involves:

Primary hypoblast development in chick originates from polyinvagination of centrally located epiblast cells during EGK stages X-XII that individually delaminate down toward yolk forming islands of cuboidal hypoblast. Subsequently posterior marginal zone contributes secondary hypoblast cells migrating anteriorly under epiblast to complete continuous sheet. Live imaging shows localized cell migration rather than rapid proliferation or apoptosis, with cells moving via filopodia on vitelline membrane. This dual origin hypoblast transiently inhibits streak formation anteriorly before being displaced by definitive endoderm ingressing through streak. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.

Ref: Eyal-Giladi and Kochav, From cleavage to primitive streak formation in chick, Hypoblast migration origin.

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Chick development-ll

Which pathway regulates primitive streak migration in chick embryos?

Coordinated extension of primitive streak and polarized movement of epiblast cells requires Wnt planar cell polarity pathway including ligands Wnt5a, Wnt8c, receptors Frizzled, co-receptors Vangl2, Prickle and downstream RhoA, ROCK and JNK regulating cytoskeletal anisotropy and convergent extension. FGF8 expressed in streak regulates cell chemotaxis repelling ingressed mesoderm laterally, interacting with Wnt to orient streak. BMP signaling patterns dorso-ventral mesoderm but does not drive streak migration, Hedgehog later patterns neural tube, so Wnt PCP and FGF8 together regulate primitive streak migration and elongation. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.

Ref: Voiculescu et al., Wnt and FGF regulate chick primitive streak, Nature Cell Biology 2007.

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Chick development-ll

Posterior Marginal Zone (PMZ) in chicks is equivalent to amphibian:

Posterior marginal zone is extraembryonic ring of cells between area opaca and area pellucida posteriorly expressing Vg1, Nodal, Wnt8c and Tbx6. It secretes signals inducing primitive streak and Koller's sickle in adjacent epiblast and can induce axis when grafted anteriorly, demonstrating inducing rather than responding character. Like amphibian Nieuwkoop center in dorsal vegetal cells, PMZ is vegetal signaling center containing Nodal-related determinants and requiring Wnt/beta-catenin input, inducing organizer fate in overlying marginal zone. Thus PMZ is avian equivalent of Nieuwkoop center rather than blastocoel or node.

Ref: Gilbert, Developmental Biology 12th ed., Chapter 12: PMZ as avian Nieuwkoop center homologue.

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