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Cellular Processes in Segmentation
Section I: The Diversity of Segmentation
: Segmentation: A Zoological Concept of Seriality
Segments and Segmentation
Segments and Series
Ontogeny of Seriality and Segmentation
Theories on the Evolution of Seriality and Segmentation
How Often Did Segments Evolve?
Problems with Structural Definitions of a Segment
Problems with Ontogenetic Definitions of a Segment
Structural and Ontogenetic Segment Definitions Lead to Paradoxes
Segments Do Not Form Spatial and Differential Units
Simple Anomalies Disturb the Pattern of Segmentation
Criticism of the Term “Segment”
Acknowledgments
References
: Diversity in Segmentation Mechanisms
Introduction
The Sequence of Segment Formation
Embryonic or Post-Embryonic Segmentation
Simultaneous or Sequential Segmentation
Cyclical Processes in Segmentation
Proliferation versus Cell Rearrangement As Drivers of Segment Formation
Types of Cells in Segmentation Processes
Commonalities in Segmentation Processes: Are There Any?
References
Section II: Cellular Mechanisms of Segmentation
: Cell Division, Movement, and Synchronization in Arthropod Segmentation
Introduction
Segmentation and Elongation: The Evolving Roles of Cell Division and Cell Rearrangement
The Role of Cell Division
Case Studies Support a New Model for the Role of Cell Division in the Posterior
Thamnocephalus
Oncopeltus
Tribolium
What Have These Case Studies Revealed?
The Growth Zone Typically Requires Mitosis But Only at Low Rates
The Anterior Growth Zone Functions As a Transition Zone in Segmental Specification
Cell Division Is Highly Regulated and Regionalized in Both the Growth Zone and Trunk
The Role of Cell Rearrangement
Cell Rearrangements from Diverse Taxa Have Some Common Features
Convergent Extension Drives Elongation in Drosophila
Elongation in Drosophila Occurs Primarily by Junctional Remodeling
Intra- and Intercellular Effectors of Cell Movements Are Polarized in Drosophila
Pair-Rule Genes Drive Periodic Expression of the Toll Receptors Required for Convergent Extension
Cell Rearrangements in Sequentially Segmenting Arthropods: The Tribolium Model
Live Imaging Shows Clear Convergent Extension in the Tribolium Germband
How Do Posterior Cells in Tribolium Converge and Extend?
Role of Pair-Rule Genes in Elongation in Tribolium
Role of Toll Receptors in Tribolium
Polarized Effectors of Cell Movement Have Not Been Documented in Tribolium
Possible Hypotheses for Mechanisms of Convergent Extension in Tribolium
Synchronizing Cell Division, Cell Rearrangements, and Cell Fate
Summary
Acknowledgments
References
: Cellular and Molecular Mechanisms of Segmentation in Annelida
Introduction to the Annelida
Segmentation in Annelids
An Overview of Annelid Development
Early Embryonic Development
Cell Fate Maps: Trochophore Larvae and Direct Development
Segmentation of the Trunk
Segment Development in Owenia
Segment Development in Platynereis dumerilii
Segment Development in Capitella teleta
Segment Development in Tubifex tubifex
Molecular Basis of Annelid Segmentation
Evolutionary Remarks
References
: Progenitor Cells in Vertebrate Segmentation
Segmentation in Vertebrates
The Clock and Wavefront Model of Somitogenesis
Neuromesodermal Progenitors As the Cellular Source of Vertebrate Segments
Do All Somites along the Anterior–Posterior Axis Come from NMPs?
Are NMPs Stem Cells?
The Transcription Factor Brachyury Links NMP Maintenance with Segmentation
Signaling Pathways Coordinating Mesoderm Induction and Segmentation
Induction of Paraxial/Somite Fate in NMP-Derived Mesoderm
Termination of Somitogenesis
Progenitor Cell Behaviors That Influence the Synchronization of Cycling Gene Expression
NMPs As a Development Module Affecting Evolutionary Change in Segment Number and Body Length
Do NMPs Exist Outside the Vertebrate Clade?
Conclusions
References
: Teloblasts in Crustaceans
Introduction
Formation of the Germ Disc
Germ Band
Growth
Evolution of the Stereotyped Division Pattern
Anomaly of Cell Shapes and Behavior
Hierarchy of Germ Layers
Segmentation
Short and Long Germ Development
Segment Morphogenesis
Cell Lineage and Cell Fate
Homology Issues
Perspectives
Acknowledgments
References
: Segmentation in Leeches
Introduction
Segmentation in the Helobdella Body Plan
Boundary-Driven versus Lineage-Driven Segmentation
An Axial Posterior Growth Zone (PGZ) Originates from the D Quadrant in Helobdella and Other Clitellate Annelids
The Leech PGZ Provides a Highly Simplified and Experimentally Accessible Example of Axial Growth and Patterning
Evolutionary Antiquity of the M Teloblasts
Kinship Groups Are Well Conserved within Each Lineage, But Heterogeneous in Terms of Cell-Type Composition and Spatial Distribution
Kinship Groups Are Not Clones
The O–P Equivalence Group
Grandparental Stem Cell Lineages
Expression Patterns of Drosophila Segmentation Gene Homologs
Spatiotemporal Registration of Blast Cell Clones
Segment Identities and the Paradoxical Specification of Regional Differences along the Anterior–Posterior Axis
The “Leech Perspective” on the Counting Problem in Segmentation
Genesis of Endoderm from a Syncytial Yolk Cell
Mesoderm As a Primary Driver of Segmental Patterning in Leeches
Summary and Conclusions
References
: Segmentation in Motion
Introduction
Live Imaging to Study Sequential Segmentation in Vertebrates
Cell Dynamics during Somite Border Formation
Real-Time Imaging of the Segmentation Clock
The Migratory Behavior of the Presomitic Mesoderm Cells
Live Imaging during Annelid Sequential Segmentation
Live Imaging during Arthropod Sequential Segmentation
Conclusions
References
Section III: Beyond Segmentation
: Segmental Traits in Non-Segmented Bilaterians
Background
What Is Segmental?
Xenacoelomorpha
Deuterostomia
Hemichordata (Acorn Worms)
Echinodermata (Sea Urchins, Sea Stars, and Others)
Cephalochordata (Lancelets or Amphioxi)
Tunicata (Sea Squirts and Salps)
Ecdysozoa
Nematoda (Round Worms)
Nematomorpha (Horsehair Worms)
Priapulida (Penis Worms)
Kinorhyncha (Mud Dragons)
Loricifera (Girdle Wearers)
Tardigrada (Water Bears)
Onychophora (Velvet Worms)
Spiralia
Chaetognatha (Arrow Worms)
Rotifera (Wheel Worms)
Micrognathozoa (Jaw Animals)
Gastrotricha (Hairybacks)
Platyhelminthes (Flatworms)
Nemertea (Ribbon Worms)
Mollusca (Snails and Squids)
Brachiopoda (Lamp Shells)
Phoronida (Horseshoe Worms)
Bryozoa (Moss Animals)
Discussion
Acknowledgments
References
: Axial Regeneration in Segmented Animals: A Post-Embryonic Reboot of the Segmentation Process
Introduction
The Phylogenetic Distribution of Regeneration
Common Aspects of Animal Regeneration
Wound Healing
Cell Reorganization and Blastema Formation
Cell Differentiation and Morphogenesis
Regeneration in Annelids
Stages of Axial Regeneration
Wound Healing
Cell Migration
Cell Proliferation
Cellular Sources and Development of the Blastema
Neural Regeneration
Muscle Regeneration
Segmentation
Segmentation during Posterior Regeneration
Segmentation during Anterior Regeneration
Morphallactic Processes
Concluding Remarks
Acknowledgments
References
Index
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