Morphogen Induction-l

Morphogen Induction-l

Which experiment demonstrated that morphogen gradients determine cell fate?

Proof that graded morphogen concentration determines fate came from Xenopus animal cap experiments where defined doses of recombinant Activin elicited distinct mesodermal and endodermal programs. Low Activin induced ventral mesoderm and pan-mesodermal marker Xbra, intermediate induced muscle and MyoD, high induced organizer genes goosecoid, chordin and notochord plus endoderm markers Sox17. Dose-response mirrored embryonic distribution of endogenous Activins and Nodal-related signals from vegetal hemisphere, demonstrating quantitative threshold responses and differential enhancer affinity. Griffith, Hershey-Chase, Avery-MacLeod-McCarty experiments concerned DNA as heredity material. Activin titration remains classic vertebrate morphogen demonstration linking gradient to discrete fate specification.

Ref: Green and Smith, Nature 1990, Activin Concentration Determines Xenopus Cell Fate.

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Morphogen Induction-l

Which morphogen plays a key role in left-right axis determination?

Left-right asymmetry originates near embryonic node where motile cilia generate leftward fluid flow concentrating Nodal around left lateral plate mesoderm. Nodal, TGF-beta member, activates left-specific cascade including Pitx2 specifying left identity of heart looping and gut situs. Nodal antagonists Lefty2 and Cerberus restrict signal to left, while BMP and FGF modulate intensity. Mutations in Nodal or ciliary dynein cause situs inversus. Fibronectin matrix, Sox2 pluripotency factor and actin cytoskeleton lack instructive laterality information. Nodal gradient thus acts as critical left-right determinant linking ciliary flow to organ laterality and visceral patterning.

Ref: Shiratori and Hamada, Development 2006, Nodal and Left-Right Axis Determination.

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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 morphogen regulates digit formation in vertebrate limb development?

Vertebrate limb anterior-posterior patterning and digit identity depend on Sonic hedgehog secreted from zone of polarizing activity in posterior mesenchyme under Hand2 and Hoxd13 control. Shh gradient high posterior specifies digit 5, intermediate specifies digits 4 and 3, low specifies digit 2, absence leads to digit 1. Shh induces nested Bmp2 antagonism and Gli3 repressor gradient establishing digit primordia via Gremlin loop. Ectopic Shh bead anteriorly induces mirror duplication and polydactyly. Myosin provides contractile force for shape changes, not positional code. Therefore Shh is key digit patterning morphogen via concentration-dependent patterning.

Ref: Tickle and Towers, Limb Development, Chapter 4: Shh Regulates Digit Patterning.

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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 factor prevents differentiation in Drosophila germline stem cells (GSCs)?

Drosophila germarium contains germline stem cells anchored to somatic cap cells forming stem cell niche at anterior tip. Cap cells secrete Decapentaplegic, fly BMP2/4 ortholog, activating TGF-beta receptors thickveins and punt on adjacent germline stem cell. Ligand binding phosphorylates Mad, homolog of Smad1, which partners with Medea to repress transcription of differentiation factor bag-of-marbles called bam. Repression maintains self-renewal and prevents cystoblast formation and meiosis entry. When daughter cell moves away from source, BMP signal declines, bam derepressed triggering differentiation into cystocyte. JAK-STAT maintains somatic niche cells, Notch and FGF not primary differentiation inhibitors here for germline.

Ref: Xie and Spradling, Science 1998, BMP Maintains Drosophila Germline Stem Cells.

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Morphogen Induction-l

Which of the following morphogens is involved in neural crest migration?

Neural crest specification and migration rely on coordinated Wnt and BMP signaling at neural plate border. Canonical Wnt6 from non-neural ectoderm plus intermediate BMP activate border factors Snail2, FoxD3, Sox9 defining premigratory crest. Wnt signaling activates beta-catenin transcription of Slug promoting epithelial-mesenchymal transition, delamination and acquisition of migratory machinery including integrins and metalloproteinases. Post-delamination, Wnt guides directional migration and proliferation of crest derivatives. BMP4 contributes to delamination but primary motility driver is Wnt, while Myosin motor and fibronectin matrix provide mechanical execution rather than morphogen instruction.

Ref: Mayor and Theveneau, Development 2013, Wnt Signaling in Neural Crest Migration.

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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 morphogen is required for neural tube ventralization?

Ventral identities in neural tube are induced by Sonic hedgehog derived from notochord then floor plate reinforcing gradient ventrally. Ventral progenitors experience highest Shh leading to Gli activator induction of Nkx2.2 floor plate, Olig2 motor neuron progenitors and Nkx6.1 V3 interneurons, while dorsal BMP counter-gradient promotes Pax6. Adding Shh to dorsal explants ventralizes them generating motor neurons, while Shh blockade dorsalizes. BMP4 promotes dorsal fates, retinoic acid posteriorizes, Wnt3a dorsalizes. Thus Shh functions as ventralizing morphogen interpreting concentration into distinct neuronal progenitor domains essential for spinal cord.

Ref: Briscoe et al., Nature 2000, Shh Gradient Specifies Ventral Neural Tube Fates.

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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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