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#axis formation

3 public questions tagged with this topic.

chick development-l

The anterior-posterior axis formation in chick embryos depends on:

Avian egg undergoes rotation within oviduct during albumen deposition and within shell due to chalazae twisting, generating fluid shear and gravitational cues. Experimental tilting of eggs before primitive streak formation shifts site of Koller's sickle activation and streak origin, indicating gravity-sensitive ionic and pH gradients bias Wnt and Vg1 localization posteriorly. Albumen pressure and blastoderm tension amplify asymmetry. Unlike amphibian sperm entry point, chick anterior-posterior specification relies on extraembryonic mechanical cues and gravity during rotation, later reinforced by TGF-beta signaling gradients establishing AP polarity.

Ref: Wolpert, Principles of Development, Chapter 4: Gravity and egg rotation in avian anterior-posterior axis formation.

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frog development

Organizer genes include:

Spemann organizer transcriptome characterized by secreted antagonists and homeodomain transcription factors. Chordin and noggin are defining BMP antagonists co-expressed specifically in dorsal lip cells and required for dorsalization and neuralization. Goosecoid, FoxA4a and Lim1 are organizer transcription factors, Cerberus is additional antagonist. VegT and Vg1 are maternal vegetal determinants upstream of organizer induction, BMP and Activin are ventral signals antagonized by organizer, Xbra marks general mesoderm. Therefore classic organizer genes include chordin and noggin representing BMP inhibition module essential for axis formation. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.

Ref: Balinsky, Introduction to Embryology: Organizer specific genes - chordin, noggin, goosecoid expression.

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frog development

Initial dorsal-ventral axis in amphibians is set by:

Initial breaking of radial symmetry in amphibians occurs at fertilization when sperm aster forms in vegetal cortex. Astral microtubules guide cortical rotation moving dorsal determinants 30 degrees away from sperm entry point. Sperm entry site therefore becomes ventral pole while opposite side accumulates Wnt11, Dishevelled and GBP, stabilizing beta-catenin dorsally. Gravity can bias but not determine axis; genetic differences and uterine orientation irrelevant in oviparous amphibians. Thus point of sperm entry sets dorso-ventral axis by directing cortical rotation direction and dorsal determinant transport. This illustrates conserved developmental logic of morphogen gradients patterning embryonic axes through Wnt and BMP antagonism.

Ref: Wolpert, Principles of Development, Chapter 4: Sperm entry and amphibian axis specification.

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