Somitogenesis is a crucial process in early development, forming segmented blocks of mesoderm along the embryo's axis. These somites later differentiate into vertebrae, ribs, and muscles. It's a perfect example of how complex structures arise from simpler precursors.
The molecular clock and wavefront model explains how somites form at regular intervals. Oscillating gene expression acts as a clock, while signaling gradients create a moving wavefront. This interplay ensures precise timing and positioning of somite boundaries as the embryo grows.
Paraxial Mesoderm and Somite Development
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Somitogenesis forms paired blocks of mesoderm (somites) along the anterior-posterior axis of the embryo
Paraxial mesoderm located on either side of the neural tube and notochord generates somites
Somites bud off from the presomitic mesoderm (PSM) in a rostral-to-caudal direction at regular intervals
PSM divides into more mature anterior region and less differentiated posterior region
Somite formation involves epithelialization of mesenchymal cells in anterior PSM creating spherical structure
Outer epithelial layer surrounds mesenchymal core
Species-specific somite formation varies in number and timing
Humans form 42-44 somite pairs over ~4 weeks
Mesenchymal cells in anterior PSM undergo epithelialization
Cells reorganize and polarize to form epithelial outer layer
Extracellular matrix deposition occurs between forming somites
Somite boundaries established through differential cell adhesion
Newly formed somites separate from PSM through a "budding off" process
Each somite develops distinct anterior and posterior compartments
Somite maturation continues as new somites form caudally
Molecular Clock and Wavefront Model
Clock Component
Oscillating gene expression in PSM creates temporal periodicity for somite formation
Key oscillating genes belong to Notch, Wnt, and FGF signaling pathways
Gene expression cycles match somite formation periodicity
Hes genes act as transcriptional repressors regulating clock gene oscillations
Cyclic gene expression propagates as waves through the PSM
Oscillations synchronized between neighboring cells through cell-cell communication
Wavefront Component
Moving front of gene expression progresses caudally defining somite competence region
Opposing gradients establish wavefront:
FGF/Wnt (high posterior to low anterior)
Retinoic acid (high anterior to low posterior)
Gradients create "determination front" where cells become competent to form somites
FGF and Wnt maintain PSM in undifferentiated state
Retinoic acid promotes somite differentiation
Intersection of clock oscillations and wavefront determines somite boundary formation
Model explains regular interval formation and consistent somite size despite embryonic growth
Notch and Wnt Signaling in Segmentation
Notch Pathway in Molecular Clock
Notch signaling establishes molecular clock oscillations within PSM
Cyclic expression of Notch pathway components (receptor, ligands like Delta)
Notch activation induces Hes gene expression
Hes proteins repress own transcription creating negative feedback loop
Oscillations propagate through PSM via synchronized Notch signaling
Disruption of Notch signaling leads to somite formation defects (irregular boundaries)
Wnt Signaling in Clock and Wavefront
Wnt pathway contributes to both molecular clock and wavefront components
Maintains undifferentiated state of PSM cells
Regulates expression of key segmentation genes (T-box transcription factors)
Wnt signaling gradually declines from posterior to anterior PSM
Helps establish determination front for somite competence
Cyclic Wnt target genes (Axin2) contribute to molecular clock mechanism
Wnt signaling interacts with FGF pathway to regulate PSM maturation
Pathway Interactions
Cross-talk between Notch, Wnt, and FGF pathways coordinates somite formation
Notch and Wnt oscillations coupled through shared target genes
FGF signaling modulates Notch and Wnt activity in PSM
Retinoic acid antagonizes FGF/Wnt signaling to promote somite differentiation
Integration of multiple signaling inputs ensures robust segmentation process
Mutations in pathway components lead to vertebral abnormalities (scoliosis)
Somite Differentiation into Sclerotome, Myotome, and Dermatome
Sclerotome forms from ventromedial portion of somite
Gives rise to vertebrae and ribs
Induced by signals from notochord and floor plate
Primary signal: Sonic hedgehog (Shh)
Shh activates Pax1 expression in sclerotome
Sclerotome cells undergo epithelial-to-mesenchymal transition
Migrates around notochord and neural tube to form vertebral bodies
Sclerotome patterning establishes vertebrae segmentation (resegmentation)
Myotome Development
Myotome develops from dorsolateral portion of somite
Gives rise to skeletal muscles of trunk and limbs
Influenced by signals from dorsal neural tube and surface ectoderm
Key signals: Wnt proteins and bone morphogenetic proteins (BMPs)
Myogenic regulatory factors (MRFs) drive muscle cell differentiation
MyoD, Myf5, myogenin, MRF4
Myotome cells form early muscle fibers and muscle progenitor cells
Progenitors migrate to form limb and body wall muscles
Dermatome Specification
Dermatome forms from dorsal-most region of somite
Gives rise to dermis of the back
Regulated by BMP signaling from dorsal neural tube and surface ectoderm
Dermatome maintains epithelial characteristics longer than other somite regions
Cells eventually undergo EMT and migrate to form dermis
Dermatome patterning influenced by positional cues along the body axis
Contributes to regional specialization of skin (scales, feathers, hair follicles)