(3.1) In the figure below, the top row of figures are from a typically developing tadpole, the bottom row of figures is of a tadpole which was expose to a chemical signal along its side while it was a gastrula. Which chemical signal were they most likely exposed to?

Question

BlackTom AI · Accepted Answer

Noggin is a secreted BMP antagonist produced by the Spemann organizer that dorsalizes ectoderm and induces neural tissue; exposure of ectoderm to Noggin during gastrulation can create ectopic neural axis and secondary body structures.

1) Relevant knowledge points and concepts
- During gastrulation, gradients of BMP signaling specify epidermal (high BMP) versus neural (low BMP) fates in ectoderm.  
- Organizer-derived secreted inhibitors (Noggin, Chordin, Follistatin) bind BMPs and reduce BMP signaling, promoting neural induction and dorsal structure formation.  
- Ectopic application of organizer signals at the gastrula stage can induce a secondary axis (neural tube, somites, tail structures).  
- SHH patterns ventral neural tube (motor neuron vs interneuron identities) but does not induce a complete secondary dorsal axis. Estrogen is an endocrine hormone not responsible for early dorsal-ventral axis induction. “Permissive cue” is not a specific morphogen known to generate ectopic axes.

2) Step-by-step reasoning applied to the images
- The top row shows a normally developing tadpole with a single midline neural axis.  
- The bottom row (exposed along one side at gastrula) shows ectopic outgrowth and additional neural tissue (visible by fluorescent neural marker) lateral to the normal axis — consistent with induction of neural tissue and axis formation at the site of exposure.  
- Only a molecule that antagonizes BMP (i.e., an organizer signal) applied laterally would convert epidermis to neural tissue and induce secondary axis structures.

3) Calculations
- No numerical calculations are required for this developmental patterning interpretation.

4) Why the correct answer is correct
- Noggin specifically mimics organizer activity by inhibiting BMPs, inducing neural fate and secondary axis formation when applied ectopically at gastrulation—exactly the phenotype shown.

5) Why the other options are incorrect
- SHH: patterns ventral identities within the neural tube but does not induce a full ectopic dorsal neural axis.  
- Estrogen: affects later sexual differentiation and physiology, not organizer-like axis induction at gastrulation.  
- Permissive Cue: too vague and not a known organizer morphogen that creates ectopic axes.

6) Conclusion
The phenotype (ectopic neural/axis induction when exposed at gastrulation) is characteristic of Noggin activity. Therefore the correct answer is Noggin, matching the provided answer.

(3.1) In the figure below, the top row of figures are from a typically developing tadpole, the bottom row of figures is of a tadpole which was expose to a chemical signal along its side while it was a gastrula. Which chemical signal were they most likely exposed to?Single choice

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