A Turing reaction-diffusion system requires two components: a short-range activator and a long-range inhibitor. If both components diffuse at the same rate, what happens?
AA perfectly periodic pattern forms
BNo pattern forms — equal diffusion rates mean any local activation is matched by equal local inhibition, preventing the amplification of spatial differences
DRandom noise determines the pattern, which changes every time
The Turing instability requires differential diffusion: the inhibitor must diffuse faster than the activator. When the activator creates a local peak, the inhibitor spreads farther, suppressing activation at a distance while the activator remains concentrated locally. This creates peaks of activation surrounded by zones of inhibition, generating a periodic pattern. If both diffuse equally, local activation and inhibition are balanced everywhere, and spatial perturbations are not amplified — the system remains uniform. The ratio of diffusion rates determines whether patterns form and controls their wavelength.
Question 2 True / False
Lateral inhibition through Notch-Delta signaling creates a 'salt-and-pepper' pattern of alternating cell types.
TTrue
FFalse
Answer: True
Notch-Delta lateral inhibition is a local self-organizing mechanism. When a cell expresses Delta (ligand), it activates Notch (receptor) in its immediate neighbors. Notch activation suppresses Delta expression in the receiving cell, creating a feedback loop: a cell that expresses more Delta inhibits its neighbors from doing the same, and those inhibited neighbors further reinforce the first cell's high Delta state. The result is a fine-grained alternating pattern — a 'salt-and-pepper' arrangement where high-Delta cells are surrounded by low-Delta (Notch-active) cells. This mechanism patterns neural precursors in Drosophila, hair cells in the inner ear, and many other tissues where spacing between specialized cells is critical.
Question 3 Short Answer
How do positional information (morphogen gradients) and self-organization (Turing patterns) work together in real developmental systems?
Think about your answer, then reveal below.
Model answer: Morphogen gradients provide coarse, large-scale positional information — defining broad domains (e.g., the limb bud region versus flank). Within these domains, self-organizing mechanisms generate fine-grained periodic patterns (e.g., the spacing of digits within the limb). The morphogen gradient constrains and modulates the self-organizing process: it may set the wavelength of the Turing pattern (by influencing activator/inhibitor production rates), restrict pattern formation to a specific region, or determine the number of repeating units. The combination produces patterns that are both precisely positioned (by the gradient) and internally organized with regular spacing (by self-organization).
Digit patterning in the limb illustrates this beautifully: Shh signaling from the zone of polarizing activity sets up a posterior-to-anterior gradient that specifies digit identity, while a BMP/Wnt Turing-type mechanism self-organizes the periodic spacing of digit condensations within the limb bud. Neither mechanism alone explains the full pattern.