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Witzmann F, Fröbisch N. Morphology and ontogeny of carpus and tarsus in stereospondylomorph temnospondyls. PeerJ 2023; 11:e16182. [PMID: 37904842 PMCID: PMC10613440 DOI: 10.7717/peerj.16182] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/13/2023] [Accepted: 09/05/2023] [Indexed: 11/01/2023] Open
Abstract
Skeletal development is well known in temnospondyls, the most diverse group of Paleozoic and Mesozoic amphibians. However, the elements of carpus and tarsus (i.e., the mesopodium) were always the last bones to ossify relative to the other limb bones and with regard to the rest of the skeleton, and are preserved only in rare cases. Thus, in contrast to the other parts of the limb skeleton, little is known about the ontogeny and sequence of ossification of the temnospondyl carpus and tarsus. We intended to close this gap by studying the ontogenies of a number of Permo/Carboniferous stereospondylomorphs, the only temnospondyls with preserved growth series in which the successive ossification of carpals and tarsals can be traced. Studying the degree of mesopodial ossification within the same species show that it is not necessarily correlated with body size. This indicates that individual age rather than size determined the degree of mesopodial ossification in stereospondylomorphs and that the largest individuals are not necessarily the oldest ones. In the stereospondylomorph tarsus, the distal tarsals show preaxial development in accordance with most early tetrapods and salamanders. However, the more proximal mesopodials exhibit postaxial dominance, i.e., the preaxial column (tibiale, centrale 1) consistently started to ossify after the central column (centralia 2-4, intermedium) and the postaxial column (fibulare). Likewise, we observed preaxial development of the distal carpals in the stereospondylomorph carpus, as in most early tetrapods for which a statement can be made. However, in contrast to the tarsus, the more proximal carpals were formed by preaxial development, i.e., the preaxial column (radiale, centrale 1) ossified after the central column (centralia 2-4, intermedium) and before the postaxial column (ulnare). This pattern is unique among known early tetrapods and occurs only in certain extant salamanders. Furthermore, ossification proceeded from distal to proximal in the central column of the stereospondylomorph carpus, whereas the ossification advanced from proximal to distal in the central column of the tarsus. Despite these differences, a general ossification pattern that started from proximolateral (intermedium or centrale 4) to mediodistal (distal tarsal and carpal 1) roughly in a diagonal line is common to all stereospondylomorph mesopodials investigated. This pattern might basically reflect the alignment of stress within the mesopodium during locomotion. Our observations might point to a greater variability in the development of the mesopodium in stereospondylomorphs and probably other early tetrapods than in most extant tetrapods, possibly mirroring a similar variation as seen in the early phases of skeletogenesis in salamander carpus and tarsus.
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Affiliation(s)
- Florian Witzmann
- Museum für Naturkunde Berlin, Leibniz Institute for Evolution and Biodiversity Science, Berlin, Germany
| | - Nadia Fröbisch
- Museum für Naturkunde Berlin, Leibniz Institute for Evolution and Biodiversity Science, Berlin, Germany
- Department of Biology, Humboldt Universität Berlin, Berlin, Germany
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Jia J, Anderson JS, Jiang JP, Wu W, Shubin NH, Gao KQ. Ossification patterns of the carpus and tarsus in salamanders and impacts of preaxial dominance on the fin-to-limb transition. Sci Adv 2022; 8:eabq7669. [PMID: 36240271 PMCID: PMC9565805 DOI: 10.1126/sciadv.abq7669] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Grants] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 04/29/2022] [Accepted: 08/26/2022] [Indexed: 06/16/2023]
Abstract
Early limb skeletogenesis in salamanders is characterized by preaxial elements, digits I and II forming earlier than their postaxial counterparts (digits III to V), a phenomenon known as preaxial dominance, whereas in amniotes and anurans, these developmental sequences are reversed. This pattern characterizes the late skeletogenesis of digits and zeugopodium of anamniote tetrapods but remains unknown in carpals/tarsals. To correct this gap in knowledge, we investigate the ossification patterns of the carpals/tarsals in six salamander families/clades based on micro-computed tomography scans. We found that preaxial dominance is seen in the distal carpals/tarsals of several salamander clades and diverse early tetrapods, such as temnospondyls and amniotes. This distribution suggests that preaxial dominance is a primitive developmental pattern in tetrapods. Our results demonstrate that the distal carpals/tarsals are developmentally and evolutionarily independent in the autopodium, and preaxial dominance facilitates stabilization of the number of distal carpals/tarsals during fin-to-limb transition and digit reduction in early tetrapods.
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Affiliation(s)
- Jia Jia
- School of Earth and Space Sciences, Peking University, 5 Yiheyuan Road, Beijing 100871, China
- Department of Comparative Biology and Experimental Medicine, University of Calgary, 3330 Hospital Drive, Calgary, Alberta T2N 1N4, Canada
- State Key Laboratory of Palaeobiology and Stratigraphy (Nanjing Institute of Geology and Palaeontology, CAS), 39 East Beijing Road, Nanjing, Jiangsu Province 210008, China
| | - Jason S. Anderson
- Department of Comparative Biology and Experimental Medicine, University of Calgary, 3330 Hospital Drive, Calgary, Alberta T2N 1N4, Canada
| | - Jian-Ping Jiang
- Chengdu Institute of Biology, Chinese Academy of Sciences, Chengdu, Sichuan Province 610041, China
| | - Wenhao Wu
- Research Center of Palaeontology and Stratigraphy, College of Earth Sciences, Jilin University, Changchun, Jilin Province 130061, China
| | - Neil H. Shubin
- Department of Organismal Biology and Anatomy, Biological Sciences, The University of Chicago, Chicago, IL 60637, USA
| | - Ke-Qin Gao
- School of Earth and Space Sciences, Peking University, 5 Yiheyuan Road, Beijing 100871, China
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Lewis ZR, Kerney R, Hanken J. Developmental basis of evolutionary lung loss in plethodontid salamanders. Sci Adv 2022; 8:eabo6108. [PMID: 35977024 PMCID: PMC9385146 DOI: 10.1126/sciadv.abo6108] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 02/14/2022] [Accepted: 07/01/2022] [Indexed: 06/15/2023]
Abstract
One or more members of four living amphibian clades have independently dispensed with pulmonary respiration and lack lungs, but little is known of the developmental basis of lung loss in any taxon. We use morphological, molecular, and experimental approaches to examine the Plethodontidae, a dominant family of salamanders, all of which are lungless as adults. We confirm an early anecdotal report that plethodontids complete early stages of lung morphogenesis: Transient embryonic lung primordia form but regress by apoptosis before hatching. Initiation of pulmonary development coincides with expression of the lung-specification gene Wnt2b in adjacent mesoderm, and the lung rudiment expresses pulmonary markers Nkx2.1 and Sox9. Lung developmental-genetic pathways are at least partially conserved despite the absence of functional adult lungs for at least 25 and possibly exceeding 60 million years. Adult lung loss appears associated with altered expression of signaling molecules that mediate later stages of tracheal and pulmonary development.
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Affiliation(s)
- Zachary R. Lewis
- Department of Organismic and Evolutionary Biology and Museum of Comparative Zoology, Harvard University, Cambridge, MA, USA
| | - Ryan Kerney
- Department of Biology, Gettysburg College, Gettysburg, PA, USA
| | - James Hanken
- Department of Organismic and Evolutionary Biology and Museum of Comparative Zoology, Harvard University, Cambridge, MA, USA
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Trofka A, Huang BL, Zhu J, Heinz WF, Magidson V, Shibata Y, Shi YB, Tarchini B, Stadler HS, Kabangu M, Al Haj Baddar NW, Voss SR, Mackem S. Genetic basis for an evolutionary shift from ancestral preaxial to postaxial limb polarity in non-urodele vertebrates. Curr Biol 2021; 31:4923-4934.e5. [PMID: 34610275 DOI: 10.1016/j.cub.2021.09.010] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/12/2019] [Revised: 06/30/2021] [Accepted: 09/03/2021] [Indexed: 11/24/2022]
Abstract
In most tetrapod vertebrates, limb skeletal progenitors condense with postaxial dominance. Posterior elements (such as ulna and fibula) appear prior to their anterior counterparts (radius and tibia), followed by digit-appearance order with continuing postaxial polarity. The only exceptions are urodele amphibians (salamanders), whose limb elements develop with preaxial polarity and who are also notable for their unique ability to regenerate complete limbs as adults. The mechanistic basis for this preaxial dominance has remained an enigma and has even been proposed to relate to the acquisition of novel genes involved in regeneration. However, recent fossil evidence suggests that preaxial polarity represents an ancestral rather than derived state. Here, we report that 5'Hoxd (Hoxd11-d13) gene deletion in mouse is atavistic and uncovers an underlying preaxial polarity in mammalian limb formation. We demonstrate this shift from postaxial to preaxial dominance in mouse results from excess Gli3 repressor (Gli3R) activity due to the loss of 5'Hoxd-Gli3 antagonism and is associated with cell-cycle changes promoting precocious cell-cycle exit in the anterior limb bud. We further show that Gli3 knockdown in axolotl results in a shift to postaxial dominant limb skeleton formation, as well as expanded paddle-shaped limb-bud morphology and ensuing polydactyly. Evolutionary changes in Gli3R activity level, which also played a key role in the fin-to-limb transition, appear to be fundamental to the shift from preaxial to postaxial polarity in formation of the tetrapod limb skeleton.
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Affiliation(s)
- Anna Trofka
- Cancer and Developmental Biology Laboratory, Center for Cancer Research, NCI, Frederick, MD, USA
| | - Bau-Lin Huang
- Cancer and Developmental Biology Laboratory, Center for Cancer Research, NCI, Frederick, MD, USA
| | - Jianjian Zhu
- Cancer and Developmental Biology Laboratory, Center for Cancer Research, NCI, Frederick, MD, USA
| | - William F Heinz
- Optical Microscopy and Analysis Laboratory, Cancer Research Technology Program, Frederick National Laboratory for Cancer Research, Frederick, MD, USA
| | - Valentin Magidson
- Optical Microscopy and Analysis Laboratory, Cancer Research Technology Program, Frederick National Laboratory for Cancer Research, Frederick, MD, USA
| | - Yuki Shibata
- Laboratory of Gene Regulation and Development, Eunice Kennedy Shriver NICHD, Bethesda, MD, USA
| | - Yun-Bo Shi
- Laboratory of Gene Regulation and Development, Eunice Kennedy Shriver NICHD, Bethesda, MD, USA
| | | | - H Scott Stadler
- Division of Skeletal Biology, Shriners Hospitals for Children, Portland, OR, USA; Department of Orthopaedics and Rehabilitation, Oregon Health & Science University, Portland, OR, USA
| | - Mirindi Kabangu
- Department of Neuroscience, Spinal Cord and Brain Injury Research Center, and Ambystoma Genetic Stock Center, University of Kentucky, Lexington, KY, USA
| | - Nour W Al Haj Baddar
- Department of Neuroscience, Spinal Cord and Brain Injury Research Center, and Ambystoma Genetic Stock Center, University of Kentucky, Lexington, KY, USA
| | - S Randal Voss
- Department of Neuroscience, Spinal Cord and Brain Injury Research Center, and Ambystoma Genetic Stock Center, University of Kentucky, Lexington, KY, USA.
| | - Susan Mackem
- Cancer and Developmental Biology Laboratory, Center for Cancer Research, NCI, Frederick, MD, USA.
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Hincapie Agudelo M, Carbonell Medina BA, Arenas Gómez CM, Delgado JP. Ambystoma mexicanum, a model organism in developmental biology and regeneration: a colombian experience. Acta biol Colomb 2021. [DOI: 10.15446/abc.v27n1.88309] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [What about the content of this article? (0)] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/09/2022] Open
Abstract
Ambystoma mexicanum is a urodele amphibian endemic to Xochimilco Lake in Mexico, it belongs to the salamander family Ambystomatidae. This species has frequently been used as model organism in developmental biology and regeneration laboratories around the world due to its broad regenerative capacities and adaptability to laboratory conditions. In this review we describe the establishment of the first colony of axolotls in Colombia to study tissue regeneration and our perspectives on the use A. mexicanum as a model organism in Colombia are discussed emphasizing its possible uses in regeneration and developmental biology
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Dwaraka VB, Voss SR. Towards comparative analyses of salamander limb regeneration. J Exp Zool B Mol Dev Evol 2021; 336:129-144. [PMID: 31584252 PMCID: PMC8908358 DOI: 10.1002/jez.b.22902] [Citation(s) in RCA: 19] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 05/03/2019] [Revised: 08/13/2019] [Accepted: 08/31/2019] [Indexed: 08/29/2023]
Abstract
Among tetrapods, only salamanders can regenerate their limbs and tails throughout life. This amazing regenerative ability has attracted the attention of scientists for hundreds of years. Now that large, salamander genomes are beginning to be sequenced for the first time, omics tools and approaches can be used to integrate new perspectives into the study of tissue regeneration. Here we argue the need to move beyond the primary salamander models to investigate regeneration in other species. Salamanders at first glance come across as a phylogenetically conservative group that has not diverged greatly from their ancestors. While salamanders do present ancestral characteristics of basal tetrapods, including the ability to regenerate limbs, data from fossils and data from studies that have tested for species differences suggest there may be considerable variation in how salamanders develop and regenerate their limbs. We review the case for expanded studies of salamander tissue regeneration and identify questions and approaches that are most likely to reveal commonalities and differences in regeneration among species. We also address challenges that confront such an initiative, some of which are regulatory and not scientific. The time is right to gain evolutionary perspective about mechanisms of tissue regeneration from comparative studies of salamander species.
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Affiliation(s)
- Varun B. Dwaraka
- Department of Neuroscience, Spinal Cord and Brain Injury Research Center, and Ambystoma Genetic Stock Center, University of Kentucky, Lexington, Kentucky
- Department of Biology, University of Kentucky, Lexington, Kentucky
| | - S. Randal Voss
- Department of Neuroscience, Spinal Cord and Brain Injury Research Center, and Ambystoma Genetic Stock Center, University of Kentucky, Lexington, Kentucky
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Abstract
An integrative definition of sesamoid bones has been recently proposed, highlighting their relationship with tendons and ligaments, their genetic origin, the influence of epigenetic stimuli on their development, and their variable tissue composition. Sesamoid bones occur mainly associated with a large number of mobile joints in vertebrates, most commonly in the postcranium. Here, we present a survey of the distribution pattern of sesamoids in 256 taxa of Caudata and Gymnophiona and 24 taxa of temnospondyls and lepospondyls, based on dissections, high-resolution X-ray computed tomography from digital databases and literature data. These groups have a pivotal role in the interpretation of the evolution of sesamoids in Lissamphibia and tetrapods in general. Our main goals were: (1) to contribute to the knowledge of the comparative anatomy of sesamoids in Lissamphibia; (2) to assess the evolutionary history of selected sesamoids. We formally studied the evolution of the observed sesamoids by optimizing them in the most accepted phylogeny of the group. We identified only three bony or cartilaginous sesamoids in Caudata: the mandibular sesamoid, which is adjacent to the jaw articulation; one located on the mandibular symphysis; and one located in the posterior end of the maxilla. We did not observe any cartilaginous or osseous sesamoid in Gymnophiona. Mapping analyses of the sesamoid dataset of urodeles onto the phylogeny revealed that the very conspicuous sesamoid in the mandibular symphysis of Necturus beyeri and Amphiuma tridactylum is an independent acquisition of these taxa. On the contrary, the sesamoid located between the maxilla and the lower jaw is a new synapomorphy that supports the node of Hydromantes platycephalus and Karsenia coreana. The absence of a mandibular sesamoid is plesiomorphic to Caudata, whereas it is convergent in seven different families. The absence of postcranial sesamoids in salamanders might reveal a paedomorphic pattern that would be visible in their limb joints.
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Affiliation(s)
- María Laura Ponssa
- Área Herpetología, Unidad Ejecutora Lillo (UEL), CONICET-Fundación Miguel Lillo, San Miguel de Tucumán, Tucumán, Argentina
| | - Virginia Abdala
- Instituto de Biodiversidad Neotropical (IBN), UNT-CONICET. Cátedra de Biología General, Facultad de Ciencias Naturales e IML, UNT, Yerba Buena, Tucuman, Argentina
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Stynoski JL, Womack MC, Trama FA, Coloma LA, Hoke KL. Whispers from vestigial nubbins: Arrested development provokes trait loss in toads. Evol Dev 2020; 23:5-18. [PMID: 33107688 DOI: 10.1111/ede.12356] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/01/2019] [Revised: 09/21/2020] [Accepted: 09/28/2020] [Indexed: 12/29/2022]
Abstract
Despite the use of acoustic communication, many species of toads (family Bufonidae) have lost parts of the tympanic middle ear, representing at least 12 independent evolutionary occurrences of trait loss. The comparative development of the tympanic middle ear in toads is poorly understood. Here, we compared middle ear development among two pairs of closely related toad species in the genera Atelopus and Rhinella that have (eared) or lack (earless) middle ear structures. We bred toads in Peru and Ecuador, preserved developmental series from tadpoles to juveniles, and examined ontogenetic timing and volume of the otic capsule, oval window, operculum, opercularis muscle, columella (stapes), and extracolumella in three-dimensional histological reconstructions. All species had similar ontogenesis of the otic capsule, oval window, operculum, and opercularis muscle. Moreover, cell clusters of primordial columella in the oval window appeared just before metamorphosis in both eared and earless lineages. However, in earless lineages, the cell clusters either remained as small nubbins or cell buds in the location of the columella footplate within the oval window or disappeared by juvenile and adult stages. Thus, columella growth began around metamorphosis in all species but was truncated and/or degenerated after metamorphosis in earless species, leaving earless adults with morphology typical of metamorphic anurans. Shifts in the timing or expression of biochemical pathways that regulate the extension or differentiation of the columella after metamorphosis may be the developmental mechanism underlying convergent trait loss among toad lineages.
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Affiliation(s)
- Jennifer L Stynoski
- Department of Biology, Colorado State University, Fort Collins, Colorado, USA.,Instituto Clodomiro Picado, Universidad de Costa Rica, Coronado, Costa Rica
| | - Molly C Womack
- Department of Biology, Colorado State University, Fort Collins, Colorado, USA.,Department of Vertebrate Zoology, National Museum of Natural History, Smithsonian Institution, Washington, DC, USA
| | - Florencia A Trama
- Grupo de Investigación en Entomología y Medio Ambiente, Universidad San Ignacio de Loyola, Lima, Peru.,Centro de Capacitación en Conservación y Desarrollo Sostenible (CDS/CNEH-Perú), Oxapampa, Peru
| | - Luis A Coloma
- Centro Jambatu de Investigación y Conservación de Anfibios, Fundación Jambatu, Quito, Ecuador
| | - Kim L Hoke
- Department of Biology, Colorado State University, Fort Collins, Colorado, USA
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Kavanagh KD, Bailey CS, Sears KE. Evidence of five digits in embryonic horses and developmental stabilization of tetrapod digit number. Proc Biol Sci 2020; 287:20192756. [PMID: 32019446 DOI: 10.1098/rspb.2019.2756] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
Abstract
Previous work comparing the developmental mechanisms involved in digit reduction in horses with other mammals reported that horses have only a 'single digit', with two flanking metapodials identified as remnants of digit II and IV. Here we show that early Equus embryos go through a stage with five digit condensations, and that the flanking splint metapodials result from fusions of the two anterior digits I and II and the two posterior digits IV and V, in a striking parallel between ontogeny and phylogeny. Given that even this most extreme case of digit reduction exhibits primary pentadactyly, we re-examined the initial stages of digit condensation of all digit-reduced tetrapods where data are available and found that in all cases, five or four digits initiate (four with digit I missing). The persistent pentadactyl initiation in the horse and other digit-reduced modern taxa underscores a durable developmental stability at the initiation of digits. The digit evodevo model may help illuminate the biological circumstances under which organ systems become highly stabilized versus highly plastic.
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Affiliation(s)
- Kathryn D Kavanagh
- Department of Biology, University of Massachusetts Dartmouth, Dartmouth, MA, USA
| | - C Scott Bailey
- Department of Clinical Sciences, College of Veterinary Medicine, North Carolina State University, Raleigh, NC 27607, USA
| | - Karen E Sears
- Department of Ecology and Evolutionary Biology, University of California Los Angeles, Los Angeles, CA, USA
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Purushothaman S, Elewa A, Seifert AW. Fgf-signaling is compartmentalized within the mesenchyme and controls proliferation during salamander limb development. eLife 2019; 8:48507. [PMID: 31538936 PMCID: PMC6754229 DOI: 10.7554/elife.48507] [Citation(s) in RCA: 27] [Impact Index Per Article: 5.4] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/16/2019] [Accepted: 08/19/2019] [Indexed: 12/27/2022] Open
Abstract
Although decades of studies have produced a generalized model for tetrapod limb development, urodeles deviate from anurans and amniotes in at least two key respects: their limbs exhibit preaxial skeletal differentiation and do not develop an apical ectodermal ridge (AER). Here, we investigated how Sonic hedgehog (Shh) and Fibroblast growth factor (Fgf) signaling regulate limb development in the axolotl. We found that Shh-expressing cells contributed to the most posterior digit, and that inhibiting Shh-signaling inhibited Fgf8 expression, anteroposterior patterning, and distal cell proliferation. In addition to lack of a morphological AER, we found that salamander limbs also lack a molecular AER. We found that amniote and anuran AER-specific Fgfs and their cognate receptors were expressed entirely in the mesenchyme. Broad inhibition of Fgf-signaling demonstrated that this pathway regulates cell proliferation across all three limb axes, in contrast to anurans and amniotes where Fgf-signaling regulates cell survival and proximodistal patterning. Salamanders are a group of amphibians that are well-known for their ability to regenerate lost limbs and other body parts. At the turn of the twentieth century, researchers used salamander embryos as models to understand the basic concepts of how limbs develop in other four-limbed animals, including amphibians, mammals and birds, which are collectively known as “tetrapods”. However, the salamander’s amazing powers of regeneration made it difficult to carry out certain experiments, so researchers switched to using the embryos of other tetrapods – namely chickens and mice – instead. Studies in chickens, later confirmed in mice and frogs, established that there are two major signaling centers that control how the limbs of tetrapod embryos form and grow: a small group of cells known as the “zone of polarizing activity” within a structure called the “limb bud mesenchyme”; and an overlying, thin ridge of cells called the “apical ectodermal ridge”. Both of these centers release potent signaling molecules that act on cells in the limbs. The cells in the zone of polarizing activity produce a molecule often called Sonic hedgehog, or Shh for short. The apical ectodermal ridge produces another group of signals commonly known as fibroblast growth factors, or simply Fgfs. Several older studies reported that salamander embryos do not have an apical ectodermal ridge suggesting that these amphibian’s limbs may form differently to other tetrapods. Yet, contemporary models in developmental biology treated salamander limbs like those of chicks and mice. To address this apparent discrepancy, Purushothaman et al. studied how the forelimbs develop in a salamander known as the axolotl. The experiments showed that, along with lacking an apical ectodermal ridge, axolotls did not produce fibroblast growth factors normally found in this tissue. Instead, these factors were only found in the limb bud mesenchyme. Purushothaman et al. also found that fibroblast growth factors played a different role in axolotls than previously reported in chick, frog and mouse embryos. On the other hand, the pattern and function of Shh activity in the axolotl limb bud was similar to that previously observed in chicks and mice. These findings show that not all limbs develop in the same way and open up questions for evolutionary biologists regarding the evolution of limbs. Future studies that examine limb development in other animals that regenerate tissues, such as other amphibians and lungfish, will help answer these questions.
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Affiliation(s)
| | - Ahmed Elewa
- Cold Spring Harbor Laboratory, Cold Spring Harbor, United States
| | - Ashley W Seifert
- Department of Biology, University of Kentucky, Lexington, United States
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