Questions: Morphogen Gradients

Question 1

Sonic Hedgehog (Shh) is secreted from the notochord and floor plate of the developing neural tube, forming a ventral-to-dorsal gradient. How do cells at different positions along this gradient adopt different fates?

• A. All cells respond identically to Shh regardless of concentration
• B. Cells closest to the Shh source receive the highest concentration and activate transcription factors (like Nkx2.2) that specify ventral neural fates; cells farther away receive lower concentrations and activate different factors (like Pax6) that specify progressively more dorsal fates
• C. Shh only acts as an on/off switch with a single threshold
• D. Cells respond to Shh duration only, not concentration

Answer: (B)

Shh acts as a classic morphogen in the neural tube. Ventral cells (closest to the notochord) receive the highest Shh concentration and express ventral-specific transcription factors (Nkx2.2, Olig2) that specify motor neurons and ventral interneurons. Cells at intermediate distances express intermediate factors (Nkx6.1, Dbx1/2), and dorsal cells far from the Shh source express dorsal factors (Pax3, Pax7) under the influence of BMP signals from the roof plate. This creates a precise stripe pattern of different neuron types along the DV axis of the neural tube, all specified by cells reading their position in the Shh gradient.

Question 2

Morphogen gradients are inherently fragile — small changes in morphogen production rate dramatically shift all cell fate boundaries.

• T. True
• F. False

Answer: False

Morphogen gradients are remarkably robust to fluctuations in production rate. Multiple mechanisms ensure this: negative feedback (morphogen signaling induces its own inhibitors), self-enhanced degradation (morphogen-receptor binding triggers receptor internalization and morphogen destruction), and downstream transcriptional network interactions that sharpen boundaries and make them less sensitive to absolute concentration. Additionally, opposing gradients (e.g., BMP from dorsal and Shh from ventral in the neural tube) create a more robust coordinate system than a single gradient. The robustness of morphogen-based patterning despite molecular noise is a major area of research in developmental biology.

Question 3

What is the difference between a morphogen acting through a concentration gradient versus acting through a relay mechanism?

Model answer: In a concentration gradient mechanism, the morphogen itself diffuses directly from source to target cells, and each cell reads the local concentration to determine its fate. In a relay mechanism, the morphogen acts only on nearby cells, which then produce a secondary signal that acts on the next set of cells, creating a sequential cascade of short-range interactions. The gradient model predicts direct, long-range action of a single molecule; the relay model predicts sequential activation by different signals. Experimental tests distinguish them: in a true gradient, blocking the morphogen's receptor at distant cells eliminates their response, while in a relay, blocking the morphogen receptor only affects the first step. Most developmental morphogens (Shh, BMP, Wnt) act through direct long-range gradients, though relay mechanisms also exist.

The question of how morphogens actually traverse tissue — by free diffusion through extracellular space, by cytoneme-mediated direct cell-cell transfer, or by transcytosis through cells — remains actively debated. Each transport mechanism would produce different gradient shapes and noise properties.