#cell signaling

Morphogen Induction-l

What does the term 'competence' refer to in developmental biology?

Competence is developmental concept denoting capacity of recipient tissue to receive and interpret inductive signal at particular stage. It reflects expression of appropriate receptors, intracellular transducers like Smads, chromatin accessibility and presence of co-factors. Competence is transient, acquired then lost, explaining why same inducer evokes different responses at distinct stages. For example, ectoderm competent to form neural tissue only during gastrulation when BMP antagonists present, later refractory becoming epidermis. Indefinitely dividing, migratory ability or permanent differentiation describes stemness, motility and determination, not context-specific responsiveness defining competence window for induction.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 3: Concept of Competence in Induction.

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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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Morphogen Induction-l

Which of the following signaling types is NOT typically associated with morphogen activity?

Morphogen activity typically requires localized source and short to medium range dissemination to generate gradient positional fields, involving paracrine diffusion through extracellular matrix, juxtacrine contact-dependent relay like Notch for boundary sharpening, and autocrine amplification sustaining competence and feedback. Endocrine mode involves secretion into bloodstream for systemic distribution to distant organs, resulting in uniform circulating levels that cannot maintain steep spatial gradient or precise threshold boundaries within a single tissue field. Hormones like insulin, estrogen and thyroxine act systemically regulating metabolism, not providing fine spatial coordinates. Hence endocrine signaling is generally not considered morphogenetic gradient mechanism for patterning.

Ref: Alberts, Molecular Biology of the Cell, 6th ed., Chapter 15: Endocrine vs Morphogen Signaling Modes.

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Morphogen Induction-l

What property distinguishes instructive induction from permissive induction?

Distinction between instructive and permissive induction lies in provision of novel fate information versus supportive environment. Instructive induction actively redirects competent cells toward new fate via specific instructive ligand altering gene regulatory network, e.g., mesenchyme instructing epithelium to form salivary branching via FGF10. Without instructive signal alternative fate does not occur. Permissive induction permits execution of already determined program by providing extracellular matrix, survival factors or hormones permitting differentiation but not changing fate choice. Therefore instructive requires specific signal to induce differentiation, permissive allows differentiation without specific instructive cue, relying on tissue autonomy plus environment permitting expression.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 3: Instructive vs Permissive Induction Differences.

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Morphogen Induction-l

Which of the following is NOT a property of a morphogen?

Definitive morphogen properties include ability to act at distance via extracellular diffusion, to form concentration gradient from localized source, to evoke distinct cellular responses at different thresholds, to pattern fields without requiring additional relay signals and to act directly activating target genes. Acting only through direct cell-cell contact contradicts long-range diffusive nature essential for forming broad gradient spanning many cell diameters up to millimeter scale. Concentration-dependent gene expression, functioning as transcription or paracrine factor and establishing body axes are genuine attributes reflecting gradient interpretation. Contact-only action characterizes juxtacrine Notch signaling, not morphogens like Bicoid, Shh or BMP which spread extracellularly.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 4: Morphogen Criteria and Contact-Independent Action.

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Morphogen Induction-l

Which experimental finding supports the existence of morphogen gradients?

Existence of morphogen gradients was substantiated by visualizing Bicoid protein distribution in Drosophila early syncytial blastoderm using antibody staining. Immunostaining revealed anterior-high posterior-low exponential gradient correlating with fate map and hunchback expression boundary. Manipulating bicoid dosage by genetics or mRNA injection shifted gap gene expression boundaries anteriorly or posteriorly in predictable threshold manner, and ectopic gradient induced mirror-image anterior structures posteriorly. This quantitative correlation between protein concentration, enhancer occupancy and transcriptional outcome provided direct evidence that graded distribution of single factor could impart positional values and specify distinct segment identities along embryonic axis without additional signals.

Ref: Driever and Nusslein-Volhard, Cell 1988, Bicoid Gradient Defines Anterior Pattern in Drosophila.

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Morphogen Induction-l

What does the 'sink' refer to in the source-sink model of morphogen distribution?

Source-sink model describes formation of stable gradient via localized production and distributed removal that together shape distribution. Source is synthesizing center like floor plate producing Shh or zone of polarizing activity in limb bud. Sink encompasses mechanisms clearing morphogen: receptor-mediated endocytosis and lysosomal degradation, binding to heparan sulfate decoys, cleavage by extracellular proteases like Tolloid. Sink prevents accumulation, maintains steepness and sets length scale lambda equals sqrt(D/k). Without degradation, gradient collapses to uniform distribution losing positional information needed for threshold-based patterning in limb bud and neural tube.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 4: Sink, Degradation and Gradient Stability.

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Morphogen Induction-l

Which type of cell signaling involves molecules secreted into the extracellular space and acting on nearby cells?

Paracrine factors are synthesized, modified in secretory pathway by glycosylation and lipidation, released into interstitial fluid and act within hundred micron range via diffusion, receptor-mediated uptake and endocytic degradation. Ligands such as FGFs, Hedgehogs and TGF-beta superfamily bind receptor tyrosine kinases or serine-threonine kinase receptors activating downstream MAPK, PI3K or Smad cascades in adjacent cells, creating short-range patterning and mitogenic effects. Juxtacrine requires membrane contact, autocrine targets same producing cell establishing positive feedback, endocrine travels via circulation to distant targets. Paracrine mode enables local induction and gradient patterning essential for embryonic field organization and tissue homeostasis.

Ref: Alberts, Molecular Biology of the Cell, 6th ed., Chapter 15: Paracrine Signal Diffusion and Gradient.

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Morphogen Induction-l

Which of the following statements is true regarding permissive induction?

Permissive induction describes interaction where responder is already determined and inductor merely provides permissive environment allowing execution of intrinsic program. Mammary epithelium predetermined for milk protein synthesis requires laminin-rich basement membrane to activate prolactin signaling and STAT5. Without matrix genes remain silent but potential persists, demonstrating matrix dependence. This contrasts with instructive where new genes induced de novo. Matrix acts as scaffold enabling survival, polarity and transcription, reflecting supportive rather than instructive role during late differentiation and functional maturation stages.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 3: Permissive Induction and Basement Membrane.

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Morphogen Induction-l

Which of the following is an example of instructive induction?

Instructive induction changes fate of responding tissue by delivering novel positional cues that redirect gene regulatory network. In recombination, epithelium that would form non-specific bud differentiates into salivary gland with branched lumens when combined with salivary mesenchyme, into mammary gland with milk production when combined with mammary mesenchyme. Mesenchymal FGF10, BMP2, Wnt signals activate specific enhancers altering epithelium transcription program via Pea3, Sox9 and GATA3. Differentiation after migration or release of morphogens describes phenomena but not fate instruction. Therefore mesenchyme instructing epithelium to acquire organ-specific architecture is canonical example of instructive induction directing pattern and cytodifferentiation during organogenesis.

Ref: Wessells, Tissue Interactions in Development, Chapter 4: Epithelial-Mesenchymal Instructive Induction.

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Morphogen Induction-l

Which of the following best describes the ability of cells to respond to a specific inducing signal?

Competence defines temporal window during which responding tissue possesses molecular apparatus to perceive and interpret an inductive cue. It involves expression of appropriate receptors, signal transducers such as Smads or beta-catenin and chromatin accessibility of target genes regulated by pioneer factors. For example, ectoderm competent to respond to BMP inhibition and adopt neural fate only before gastrulation; later same signal fails due to loss of competence. Potency is capacity to form multiple lineages, equivalence means cells equally potent, specification indicates reversible commitment. Competence ensures induction occurs only at correct stage and place limiting ectopic differentiation and maintaining developmental timing.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 3: Competence and Temporal Responsiveness in Induction.

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