#anterior-posterior axis

C-Elegans Development-l

Which of the following determines the anterior-posterior axis in C. elegans?

Fertilization entry point defines posterior in C. elegans through centrosome-derived polarity cue. Sperm centrosome enters at one pole, initiating cytoplasmic streaming and actomyosin cortical flows that push PAR-3, PAR-6, PKC-3 and CDC-42 anteriorly, while PAR-1 kinase and PAR-2 RING protein accumulate posteriorly. Microtubule organizing center reorganizes cortex, establishing mutually exclusive domains via phosphorylation feedback. This sperm-position cue creates anterior-posterior axis before first mitosis, dictating unequal cleavage, spindle positioning, and segregation of P-granules and maternal mRNAs essential for asymmetric patterning and early developmental symmetry breaking.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 8: Axis specification in C. elegans - sperm entry defines posterior and PAR proteins.

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Limb Development -lll

Which molecule plays a key role in anterior-posterior axis formation?

Anterior-posterior polarity and digit identity are dictated by graded Sonic Hedgehog morphogen from posterior margin ZPA. SHH diffuses anteriorly, creating high posterior low anterior gradient, regulating Gli3 processing: high SHH preserves full-length Gli activator posteriorly, anterior Gli3 repressor predominates restricting posterior genes. This gradient specifies digit five posterior to digit one anterior and controls number through proliferative expansion via Cyclin D1. Wnt3a induces AER formation, FGF8 drives PD outgrowth, RA proximalizes stylopod. SHH therefore serves as instructive AP morphogen integrating growth and pattern specification.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 20: SHH as AP morphogen in limb.

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Limb Development -lll

Which experiment demonstrated the role of the ZPA in anterior-posterior axis formation?

John Saunders and Mary Gasseling established ZPA function by transplanting posterior margin mesenchyme to anterior border of chick wing bud. Recipient buds developed mirror-image digit duplications with pattern 65432 alongside normal 23456, revealing a diffusible morphogen controlling anterior-posterior polarity and concentration-dependent digit specification. Later work identified morphogen as SHH acting through Gli3 processing and Hoxd activation. AER removal truncates proximo-distally, retinoic acid inhibition proximalizes, FGF8 overexpression extends outgrowth but does not duplicate AP pattern. Only anterior ZPA graft reproduces posterior digits.

Ref: Saunders & Gasseling 1968, Gilbert Developmental Biology Chapter 20: ZPA graft experiment.

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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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