#chick embryos

chick development-l

The equivalent of the amphibian Spemann organizer in chick is:

Spemann organizer induces neural tissue and dorsalizes mesoderm through BMP antagonists Chordin, Noggin, Follistatin and Shh. Equivalent capacity in chick resides at Hensen's node at anterior primitive streak, not along entire streak, hypoblast or yolk plug. Grafting node laterally induces ectopic notochord and neural plate expressing Sox2, secreting identical BMP antagonists and Shh to block BMP4. Molecular signature includes Goosecoid, FoxA2, Chordin identical to amphibian organizer. Primitive streak forms mesoderm broadly but organizer function localized to node, confirming Hensen's node as avian Spemann organizer homolog across vertebrate evolution.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 9: Hensen's node as avian equivalent of Spemann organizer.

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chick development-l

Gastrulation in chicks primarily occurs through:

Due to enormous yolk mass preventing circular blastopore formation, chick gastrulation utilizes linear primitive streak as blastopore equivalent. Epiblast cells converge via Wnt-PCP mediated mediolateral intercalation toward posterior midline, undergo EMT, ingress through streak to generate definitive endoderm displacing hypoblast and mesoderm layers. Hensen's node at anterior tip contributes chordamesoderm forming notochord. Molecularly driven by Nodal, Wnt3a, Brachyury and FGF signaling. This ingression mechanism efficiently internalizes mesoderm around yolk while keeping blastoderm flat. Blastocoel, neural tube and hypoblast not gastrulation portals but products or separate processes.

Ref: Wolpert, Principles of Development, 5th ed., Chapter 5: Gastrulation through primitive streak in chick and amniotes.

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chick development-l

Chorion in chick embryos functions in:

Chorion is outermost extraembryonic membrane formed from somatic lateral plate mesoderm plus ectoderm, lining inner surface of shell membrane. While avascular alone, it fuses with highly vascular allantoic mesoderm creating chorioallantoic membrane where fetal capillaries appose shell pores. Oxygen diffuses inward and carbon dioxide outward through porous calcareous shell across this membrane driven by partial pressure gradients. Chorion also transports calcium from shell for bone ossification. Unlike yolk sac providing nutrition or allantois storing uric acid waste, chorion primary physiological role remains respiratory gas exchange throughout incubation.

Ref: Balfour and Gilbert, Developmental Biology, Chapter 13: Chorion and chorioallantoic membrane gas exchange function.

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chick development-l

Amnion in chick embryos is derived from:

Avian amnion forms protective fluid-filled sac from somatopleure. Extraembryonic epiblast-derived ectoderm plus underlying somatic lateral plate mesoderm originating from epiblast ingressing through posterior primitive streak elevates as amniotic folds that fuse above embryo dorsally. Resulting inner layer ectoderm facing amniotic fluid secreting amniotic fluid, outer somatic mesoderm contacts chorion. Splanchnic mesoderm plus endoderm forms splanchnopleure of yolk sac and allantois, not amnion. This dual somatic mesoderm-ectoderm composition distinguishes amnion from chorion which shares same origin but encloses outermost cavity, ensuring cushioning and preventing desiccation.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 12: Extraembryonic membranes - amnion from somatic mesoderm and ectoderm.

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chick development-l

Primitive groove in chick embryos is equivalent to amphibian:

Primitive groove is a midline depression along primitive streak where epiblast undergoes epithelial-mesenchymal transition regulated by SNAI2, downregulates E-cadherin, ingresses to replace hypoblast and become mesoderm and definitive endoderm. Functionally it serves as portal for internalization, comparable to amphibian blastopore through which bottle cells apically constrict and involuting marginal zone mesoderm enters to form archenteron. Both represent blastoporal openings for gastrulation; chick groove is linear due to massive yolk constraint while frog blastopore is circular, but molecular regulation by Brachyury and Nodal is homologous.

Ref: Wolpert, Principles of Development, 5th ed., Chapter 5: Primitive groove homologous to amphibian blastopore.

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Chick development-ll

Excess FGF8 expression in primitive streak of chick embryos results in:

In normal chick gastrula FGF8 expressed in primitive streak maintains streak cells in epithelial state expressing Brachyury and regulates directional egress of mesoderm. Precise gradient restricts streak width and guides extension via interaction with Wnt3a maintaining stem zone. Excess FGF8 saturates receptors FGFR1/3 causing prolonged MAPK activation, disrupting Wnt-PCP mediated convergent extension and altering beta-catenin localization, leading to ectopic streak formation, altered orientation and failure of node regression. Result is not simple paraxial expansion but altered Wnt signaling and primitive streak orientation reflecting feedback loop disruption between FGF and Wnt pathways.

Ref: Stern, Gastrulation in Chick, FGF signaling and primitive streak orientation, Development Journal.

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