- In an experiment, activin-secreting beads were placed on unspecified cells from an early Xenopus embryo. The activin then diffused from the beads. If the beads contained 1nM of activin, it elicited expression of Xbra gene in cells near to the beads.
If the beads contained 4 nM activin, the expression of Xbra was elicited in cells, but only at a distance of several cell diameters away from the beads. In the latter case, expression of goosecoid gene was observed near the source bead. Beads with no activin did not elicit the expression of the two genes.
Following statements were made regarding the observations and the role of activin in determining cell fate.
A. High concentration of activin activates goosecoid, whereas lower concentrations activate Xbra.
B. Lower concentrations of activin help specify the dorsal-most structures of the frog’s embryo
C. Concentrations of activin that do not lead to expression of the two genes specifies the cell to
become blood vessels and heart
Which of the above statement(s) are correct?
(1) A only (2) C only
(3) A and C (4) B and C
The correct statements in the experiment involving activin-secreting beads and gene expression in Xenopus embryos are (3) A and C:
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Statement A is correct: Beads without activin do not elicit expression of Xbra or goosecoid genes.
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Statement C is correct: Cells farthest from the activin source, receiving negligible activin, do not activate Xbra or goosecoid and instead follow the default pathway to become blood vessels and heart.
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Statement B is incorrect because high, not low, activin concentration induces goosecoid, which specifies dorsal-most structures.
Introduction
Activin, a member of the TGF-β family, acts as a morphogen during early Xenopus embryogenesis to pattern the mesoderm by inducing specific gene expression dictated by its local concentration. Experimental placement of activin-secreting beads near Xenopus embryonic cells reveals that gene activation and subsequent cell fate decisions depend on the gradient of activin signaling.
Activin Concentration-Dependent Gene Expression
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High concentrations of activin near the source induce expression of goosecoid, a transcription factor specifying dorsal-most mesodermal structures like the Spemann organizer.
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Intermediate concentrations of activin induce expression of Xbra, a T-box transcription factor important for ventrolateral mesodermal fates such as muscle formation.
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At very high activin levels, goosecoid can repress Xbra expression, contributing to spatial pattern refinement.
Default Cell Fate at Low Activin Concentrations
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Cells receiving negligible or no activin signal fail to express Xbra or goosecoid and differentiate into ventral mesoderm derivatives including blood vessels and the heart.
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This highlights how the absence of morphogen signaling can specify alternative cell fates by default developmental programs.
Experimental Observations from Activin Bead Implantation
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Beads lacking activin do not activate either Xbra or goosecoid, confirming activin’s necessity for gene induction (Statement A).
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At 4 nM activin concentration, goosecoid is expressed near the beads while Xbra expression is induced several cell diameters away, consistent with concentration thresholds (Statement B, however, incorrectly attributes dorsal structures to low activin).
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Cells distant from activin source express neither gene and form blood and heart cells, confirming Statement C.
Summary Table: Activin Concentration Effect on Gene Expression and Cell Fate
| Activin Concentration | Gene Induced | Cell Fate Indicated |
|---|---|---|
| High (close to bead) | Goosecoid | Dorsal-most mesoderm (organizer) |
| Intermediate (at distance) | Xbra | Ventrolateral mesoderm (muscle, somites) |
| Negligible/None (far away) | None | Ventral mesoderm (blood vessels, heart) |
Conclusion
Statements A and C correctly reflect the effects of activin gradients on gene expression and cell fate in Xenopus embryos. High activin induces goosecoid, intermediate induces Xbra, and absence of activin leads to ventral mesoderm fates like blood and heart. Statement B is incorrect, as dorsal-most structures require high, not low, activin concentrations.
FAQ
Q: What gene does high activin concentration induce?
High activin induces goosecoid for dorsal mesoderm specification.
Q: What gene does intermediate activin concentration induce?
Intermediate levels induce Xbra expressed in ventrolateral mesoderm.
Q: What happens to cells with no activin exposure?
They follow default programs to form blood vessels and heart.
This detailed understanding of activin gradient effects on Xenopus embryonic development offers insights into morphogen concentration-dependent gene regulation and patterning essential for developmental biology studies and competitive exams.
