#pattern formation

Morphogen Induction-l

Which property defines a morphogen gradient?

Morphogen gradient property that confers patterning capacity is ability to influence discrete cell fate decisions proportionally to local ligand concentration via threshold decoding. Unlike uniform growth factors causing graded proliferation, morphogens exhibit threshold responses where target enhancers possess different affinities for effectors, activating sequentially as concentration rises. This converts analog concentration into digital transcriptional outputs generating distinct spatial domains. Uniform diffusion alone insufficient; must couple to differential activation and cross-repression. Short-range restriction or apoptotic activation not defining. Therefore concentration-dependent fate specification constitutes core defining attribute of morphogen interpretation across embryos.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 4: Morphogen Gradient Threshold Interpretation.

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Morphogen Induction-l

Which of the following is an example of lateral inhibition?

Lateral inhibition describes feedback where cell adopting primary fate inhibits neighbors through contact-mediated signal preventing same fate adoption. In Drosophila neuroectoderm, prospective neuroblast expresses high Delta activating Notch in surrounding cells. Notch induces Enhancer of split complex repressing proneural genes achaete-scute, forcing neighbors toward epidermal fate and limiting neuroblast density. This generates spaced array of neuroblasts separated by epidermis. BMP gradient in limb bud specifies digit identity via thresholds, not inhibitory cell-cell choice. Notch-Delta neuroblast singling out is textbook lateral inhibition model also seen in inner ear patterning.

Ref: Artavanis-Tsakonas et al., Science 1999, Notch-Delta Lateral Inhibition in Neurogenesis.

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Induction Morphogen -ll

Which of the following defines 'lateral inhibition'?

Lateral inhibition generates fine-grained mosaic patterns via feedback where differentiating cell actively inhibits adjacent cells from adopting identical fate. Classic mechanism involves Notch-Delta: prospective neuron expresses Delta, activating Notch in neighbors, which upregulates Hes repressors that suppress neural differentiation genes like achaete-scute, forcing neighbors into epidermal fate. This amplifies small initial stochastic differences, producing spaced pattern of bristles, hair cells and neural precursors. It contrasts with lateral induction where signal promotes same fate. Mathematical modeling shows lateral inhibition creates alternating checkerboard patterns essential for sensory organ spacing, angiogenic tip-stalk selection and intestinal secretory patterning.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 4: Lateral Inhibition and Notch-Delta Patterning.

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Specification Determination-ll

Mosaic developmental pattern is always:

Mosaic development represents strictly autonomous specification where each blastomere inherits different maternal cytoplasmic determinants during cleavage. These localized mRNAs and proteins activate lineage-specific transcription programs independently of neighboring cells. When isolated, a mosaic blastomere forms only its predetermined partial structure, never a complete embryo, demonstrating internally programmed fate. Classic examples include tunicates and ascidians, where ablation of a blastomere permanently eliminates its derived tissues without compensation. This contrasts regulative development where cell communication rescues missing parts through inductive signaling and community effects restoring normal pattern.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 3: Cell-Cell Communication in Development, Autonomous Specification.

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