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The key life stages of Xenopus. Stages of Xenopus development as illustrated by Nieuwkoop and Faber (1994). Eggs are fertilised externally and embryos begin to develop, undergoing gastrulation, neurulation, organogenesis and tailbud extension. They then hatch as larvae within 3-4 days (NF22-27), dependent on temperature. After 5-7 days of development, the larvae/tadpoles begin feeding (from NF45 onwards). From NF48 onwards, during a ‘pre-metamorphic’ phase, animals develop limb buds, starting with the hindlimbs, which form joints and digits during metamorphosis. Metamorphosis ends with resorption of the tail at NF62-66. Following tail resorption, froglets become juveniles in around 4-6 months and grow until they reach sexual maturity by 6-18 months, depending on sex and species.
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Phylogenetic tree of taxons and common species used as models of regeneration. Coloured dashed lines denote the general regenerative capacity of each taxon, from high (green) to low (dark red). However, it should be noted that this capacity might vary amongst species within the same taxon (e.g. salamanders, teleost fish) and with age within the same species (e.g. fetal versus adult in mammals, or tadpoles versus adults in Xenopus). Lines are not to scale with regards to time.
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Tail regeneration. (A) The intact tadpole tail contains somitic muscle (orange), a notochord (light blue), vasculature (red) and epidermal cells, including the recently identified re-organisation cells (ROCs, purple) and melanophores (black). The tail also contains a spinal cord (dark blue), from which axons exit through spinal ganglia to innervate the fin. Motor neurons extend axons along the intersomitic boundaries. Regeneration of the tail can be studied following amputation, which is commonly made at a position halfway to two-thirds of the way along the total tail length (shown here by a dotted line). (B) Key stages of regeneration of the Xenopus tail are shown in bold. Injury induces wound healing and the initiation of regeneration (0-6 h post-amputation, hpa), which requires bioelectric changes and the upregulation of signalling molecules (shown in inset), including reactive oxygen species (ROS) and transforming growth factor β (TGFβ). TGFβ signalling is required for wound healing by p63+ epidermal cells. Lef1+ ROCs, which can also express p63, contribute to the wound epithelium and to the initiation of regeneration. From 6-48 hpa, the blastema forms, containing proliferating cells and the regenerative structures of the spinal cord and notochord. From 2 dpa, regenerative outgrowth begins, coinciding with innervation of the regenerate by neuronal axons from the rostral spinal cord and outgrowth of the spinal cord, notochord and vasculature. Myofibres degenerate and are replaced with new myofibres, which originate from Pax7+ satellite cells (not shown). Melanophores arise within the regenerate from melanophore precursors. Some of the key signalling pathways implicated in regeneration are shown alongside the regenerating tail. EF, electric field; JI, electric current density; TEP, transepithelial potential.
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Spinal cord regeneration. (A) Spinal cord regeneration following spinal cord transection. The rostral spinal cord contains a ventricular zone (pink) containing Sox2/3+ progenitor cells and neuronal cell bodies, and white matter (green) containing axonal tracts. In pre-metamorphosis regenerative stages (R), the cut stumps close, Sox2/3+ cells transiently proliferate and axonal tracts bridge the cut tissue by 10 days post-transection (dpt). Cells migrate to reform the central canal by 20 dpt. In non-regenerative (NR) stages, proliferation is predominantly observed in Sox2/3− cells outside of the spinal cord stumps. Fibrillary material (blue) is observed within the transection gap by 20 dpt, and injury persists up to 40 dpt. (B) Spinal cord regeneration following tail amputation. The regenerate spinal cord originates from cells rostral to the amputation site (blue). The injured spinal cord first forms a bulbous neural ampulla. This structure (shown in inset) contains Sox2/3+ cells (red) that begin to proliferate (yellow) at 2-3 dpa, correlating with the onset of spinal cord outgrowth. Axons do not exit the regenerate spinal cord; instead, axons from the rostral tail grow into the regenerate to innervate the new tail. This model can be used to identify the molecular mechanisms required for epimorphic regeneration, and to assess interactions with other regenerating tissues. As an example, the notochord (light blue, shown in inset) has previously been shown to express sonic hedgehog (Shh), which promotes proliferation of cells in the spinal cord. This model can also be used to study nerve dependency, as without the spinal cord the tail does not regenerate. Dashed lines indicate amputation site.
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Cardiac regeneration. (A) In 6-month-old juvenile and >5-year-old adult X. laevis frogs, heart regeneration is unsuccessful following apical resection using either surgical scissors (in the case of juveniles) or endoscopy-based resection (E, in the case of adults). Amputation results in hypertrophy of cardiomyocytes, with limited wound closure and fibrotic scarring from 1 month post-amputation (mpa) to 3.3 years post-amputation (ypa). (B) By contrast, heart regeneration is successful in X. laevis tadpoles (NF57) and X. tropicalis adults (1 year old) following apical resection using surgical scissors. Amputation of the ventricular apex (dashed line) leads to wound healing, cardiomyocyte proliferation (yellow nuclei) and deposition of connective tissue (fibrosis, pink), which gradually reduces as regeneration continues until little to no scar is present. The time required for heart regeneration in the X. laevis tadpole (top time line; dpa, days post-amputation) is longer than that required in the X. tropicalis adult (bottom timeline; mpa, months post-amputation).
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