This is a citation list accompanying my thread on the planarian worm.
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A clarification: there are actually hundreds of species of planarians, and despite being closely related, some of them are capable of the full-body regeneration described in the thread, while others have limited regenerative capacity, and still others show no regeneration at all. This divergence is itself a fascinating area of inquiry, as described in Evolutionary dynamics of whole-body regeneration across planarian flatworms. There is also some debate about whether regeneration is an ancestral trait or a novelty in specific lines of evolution, see Animal regeneration: ancestral character or evolutionary novelty?.
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Seconding the above point, not all planarian species show an immunity to aging, but some of them do, specifically the Schmidtea mediterranea. See Telomere maintenance and telomerase activity are differentially regulated in asexual and sexual worms.
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The picture on the top-level tweet is from Wikimedia.
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Several of the diagrams are from Collective intelligence: A unifying concept for integrating biology across scales and substrates, which I wrote a thread about earlier.
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The picture showing the bioelectric network of the worm (hyperpolarized vs depolarized) came from Bioelectricity: the collective intelligence of cells by Shamay Agaron, and it originally came from Bioelectric signaling: Reprogrammable circuits underlying embryogenesis, regeneration, and cancer.
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The diagram explaining morphogen gradients came from this video.
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The image depicting the polarity of the worm, and how cells on either side of a cut have the same local positional information, came from Modeling Planarian Regeneration: A Primer for Reverse-Engineering the Worm.
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For the diagram on planarian regeneration based on cuts of different angles, as well as the diagram showing the details of the planarian’s complex anatomy, see Model systems for regeneration: planarians.
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For the diagram on planarians growing/shrinking their body size, see Not your father’s planarian: a classic model enters the era of functional genomics. This article is where the stat about being cut into over 200 pieces came from (“a planarian can regenerate from a piece of tissue that represents less than 1/279 of the adult organism.”), as well as the note about shrinking below birth size: “during periods of prolonged starvation, planarians can shrink; during the course of many months, a full-grown adult (~20 mm in length) can shrink to a size that is smaller than when it hatched from the egg capsule (~1 mm).”
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The stat about shrinking/growing by 40x came from Stem Cells, Patterning and Regeneration in Planarians: Self-Organization at the Organismal Scale: “they do not have a fixed body size and grow when fed but shrink when starved. Yet all throughout a ~40-fold range in body length or a >800-fold range in total cell numbers (A. Thommen, unpublished), they maintain their species-specific shape.”
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For the diagram on inducing the planarians to generate heads from different species, see Gap Junctional Blockade Stochastically Induces Different Species-Specific Head Anatomies in Genetically Wild-Type Girardia dorotocephala Flatworms. See also Physiological inputs regulate species-specific anatomy during embryogenesis and regeneration .
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The image showing the one-headed, no-headed, and two-headed worm is credited to Michael Levin and was found in Mike Levin on electrifying insights into how bodies form.
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The quote on the very last tweet about aging is from Aging as a loss of morphostatic information: A developmental bioelectricity perspective.