The Wnt to Twist1 to Sox9 Brake, How a Face Cell Avoids Cartilage
Developmental domain · Lesson 12 of 20 · Principles of Biomedical Science (PBS)
Today's goal: Explain that Wnt switches on Twist1 and Twist1 holds down Sox9, then predict the fate switch when Wnt or Twist1 is lost.
This is a model of the work you should turn in. Use it to check your own: match the structure and the level of detail, do not copy it. Your wording should be your own.
Decision tree (one cranial mesenchyme cell at ~E11.5):
- Wnt/beta-catenin ON: Wnt switches on Twist1. Twist1 holds down Sox9, so the cell can become bone (osteoblast) or skin (dermal fibroblast).
- Wnt/beta-catenin OFF: Twist1 decreases, Sox9 is released, and the cell follows its cartilage default even where bone belonged.
Caption: Wnt switches on Twist1, and Twist1 holds down Sox9. Twist1 is a major part of the cartilage brake, but a parallel ERK arm and partial reversion show it is not the whole story (Goodnough et al. 2012; Ibarra, Machen, and Atit 2021).
This model shows the level of evidence and organization needed to complete: A two-branch fate diagram that correctly traces Wnt to Twist1 to Sox9 and assigns the default fate.
- Date and label the entry.
- Record the procedure, observation, or design decision clearly.
- End with what the evidence means and the next step.
Keep the structure. Replace the question, facts, measurements, and evidence. Then recheck units, vocabulary, and whether the conclusion goes beyond the evidence.
The finished product above did not appear all at once. Here is the path from the question to the turned-in work, so you can follow the same steps.
- 1Start from today's question: Why did springy support appear where hard bone should have formed when one message was removed?
- 2Work the Model and the Explore questions to reason it out before writing anything.
- 3Pull the specific evidence the product needs from the reading and any database you used.
- 4Write it up in the required format: Draw a decision tree for one cranial cell with two branches. Label the Wnt to Twist1 to Sox9 chain when Wnt is ON, then show what happens to Twist1, Sox9, and cell fate when Wnt is OFF.
- 5Check it against the rubric, then submit.
| Criterion | Proficient | Developing | Beginning |
|---|---|---|---|
| Complete | Every required part of the artifact is present and filled in. | Most parts are present, but one is missing or left blank. | Several parts are missing. |
| Accurate | The science and data are correct and match the evidence. | Mostly correct, with a small factual slip. | Key science or data is wrong. |
| Scientific reasoning (CER) | States a claim, backs it with specific evidence, and explains the reasoning. | Has a claim and evidence, but the reasoning is thin or missing. | Gives an answer with no evidence or reasoning. |
| Professional communication | Clear, organized, and labeled the way a clinician or scientist would write it. | Readable but disorganized or missing labels. | Hard to follow. |
| Submitted | Turned in through the route named under Submit here and confirmed. | Turned in, but in the wrong place or unconfirmed. | Not turned in. |
- CompleteProficient: Nothing is left blank: the model fills every part of "Draw a decision tree for one cranial cell with two branches. Label the Wnt to Twist1 to Sox9 chain when Wnt is ON, then show what happens to Twist1, Sox9, and cell fate when Wnt is OFF.".
- AccurateProficient: Every number and claim matches the case evidence.
- Scientific reasoning (CER)Proficient: It names a claim, cites the specific evidence, and explains the reasoning, not just the answer.
- Professional communicationProficient: It is organized and labeled like a real chart note.
- SubmittedProficient: It would be attached to your class form or handed in, and confirmed.
One exam-style question that uses exactly what you practiced today. Try it before you reveal the answer, then read why each choice is right or wrong.
Tap an answer to see the full explanation. Nothing is recorded or graded.
It builds this reusable test skill: Interpreting a loss-of-signal result as release of an intermediate brake on a default fate.
- Name the concept or data pattern being tested.
- Cross out choices that violate that rule or the evidence.
- Justify the best remaining choice before checking the answer.