#embryonic axis

amphibian Development-l

The region opposite sperm entry forms the:

Cortical rotation moves determinants opposite sperm entry, forming gray crescent with high nuclear beta-catenin. This region becomes Nieuwkoop center and later Spemann organizer, establishing dorsal side of embryo that will form notochord, somites, and neural tube. Sperm entry side maintains high BMP and ventralizing signals, becoming ventral side producing blood, lateral plate mesoderm, and epidermis. Manipulations that reverse rotation reverse dorsal-ventral polarity, demonstrating opposite side rule. Anterior-posterior axis forms later via Wnt and retinoic acid gradients, not directly from sperm entry point positioning alone during development.

Ref: Wolpert, Principles of Development, Chapter 6: Cortical rotation establishes dorsal side opposite sperm.

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chick development-l

The dorsal-ventral axis in chick embryos is determined by:

Early chick axis establishment involves subtle bioelectric differences rather than simple genetic prepattern alone. Measurements show posterior marginal cells exhibit distinct intracellular pH elevation and depolarized membrane potential regulating beta-catenin nuclear transport and Vg1 exocytosis. These electrochemical gradients influenced by gravity during egg rotation in albumen and shell create biased Wnt8c and Nodal expression posteriorly. Subsequent feedback through Wnt, FGF and TGF-beta reinforces dorsoventral and anteroposterior polarity. While genes elaborate pattern, initial bias linked experimentally to pH and membrane potential asymmetry across blastoderm, not solely gravity or genetic factors alone.

Ref: Gilbert, Developmental Biology, 11th ed., Chapter 12: Chick axis specification - pH, membrane potential and gravity influences.

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chick development-l

The primitive streak defines which axis in chick embryos?

Primitive streak appears as posterior-to-anterior linear thickening where epiblast cells converge, undergo epithelial-mesenchymal transition mediated by Snail2 and E-cadherin loss, and ingress. Anterior tip defines future head and Hensen's node, posterior end tail bud, thereby visibly establishing anterior-posterior polarity long before somites appear. Molecularly Brachyury, Wnt3a and Nodal show graded posterior-high expression. Left-right asymmetric genes Shh, FGF8, Nodal later expressed around node use streak as midline reference. Regression of streak after ingression deposits notochord and paraxial mesoderm along AP axis, coordinating body elongation and axis patterning.

Ref: Wolpert, Principles of Development, 5th ed., Chapter 5: Primitive streak defining anterior-posterior axis in chick.

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Drosophila -ll

Gurken mRNA localization specifies:

Gurken mRNA localizes to oocyte nucleus corner near anterodorsal follicle cells translating into EGF-like ligand activating Torpedo receptor dorsally, initiating dorsal follicle fate via repression of Pipe sulfotransferase. Ventral follicle cells retain Pipe modifying vitelline membrane to permit ventral Spatzle processing activating Toll and Dorsal gradient. Cascade specifies DV axis of eggshell with dorsal appendages and embryo with ventral mesoderm. Gurken transport driven by dynein and localized translation is symmetry-breaking event upstream of eggshell patterning, not specifying anteroposterior, left-right, or segmentation patterns dependent on bicoid, nodal, or pair-rule systems.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 9: Gurken mRNA localization specifies DV axis via EGFR Torpedo signaling.

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