Somitogenesis, Signaling, and Somite Differentiation

Embryonic Orientation & Early Morphology

• The embryo begins as a flattened disc with a central median axis; as the neural tube elongates, the disc morphs into a 3-D “creature.”
• Surface topography: two longitudinal “mountains” (neural folds) and a deep central “valley” (neural groove).
• Cells that sit on the left and right “mountain slopes” cannot chemically contact each other across the groove; nevertheless, they must act synchronously to preserve bilateral symmetry.
• Synchrony analogy: students standing on one leg when the instructor calls “1-2-3” → cells likewise wait for a common cue.

Where Does the Synchronizing Signal Originate?

• Contrary to older “anterior-first” models, the critical paracrine signal arises at the caudal / posterior end where the embryo is still continuous—no mid-line chasm is present.
• Mechanism nick-named “Reverse-Signal Activation Theory.”
– Posterior cells secrete a paracrine hormone in rapid succession (domino effect).
– Signal propagates cranially, triggering somite formation at equidistant intervals.
• Left & Right columns of presomitic mesoderm (PSM) respond simultaneously; any timing or size discrepancy is usually embryonic-lethal.

Foundational Cell Biology

• Epithelial cell – specialized lining cell; apical-basal polarity, tight/adhesive junctions.
• Mesenchymal cell – undifferentiated, \text{pluri- or multipotent}, spindle-shaped “squash-bug;” motile; can transition back & forth with epithelial state (EMT/MET).

Step-by-Step Somitogenesis

1. Periodicity

• Presomitic mesoderm aligns next to the closing neural tube.
• New somite “pairs” bud off anterior→posterior at fixed temporal intervals (“clock and wavefront model”).

2. Fissure Formation

• A deep fissure (think glacial crevasse) cleaves the PSM column.
• Trigger is morphological, not a literal knife cut:
– Cells reorganize cytoskeleton/adhesion.
– Key proteins: Fibronectin + N-cadherin form a transient “stitch” ring that pinches the segment.

3. Epithelialization (Somite Segmentation)

• Boundary cells undergo MES → epithelial transition to seal the segment.
• Molecular pair:
– \textbf{EphA4} (membrane-tethered ligand) on posterior PSM cells.
– \textbf{EphrinB2} (receptor) on anterior neighboring cells.
• Binding activates receptor tyrosine kinase → phosphorylation cascade:
– ↓ \text{CDC42} (acts like “skeleton glue”).
– ↑ mysterious \text{Factor X} (cytoskeletal remodeler).
• Result: cytoskeletal rearrangement, loss of N-cadherin adhesion, physical separation of the new somite.
• Once detached, surface cells revert to mesenchymal fate; epithelial state was only temporary “Velcro.”

Fate Mapping After Somite Birth

• Each somite immediately “counts” its position (Somite #1, #2…).
• Positional identity encoded by HOX genes (e.g., \text{HOX4A–D},\;\text{HOX9},\;\text{HOX11}).
– Gradient & subtype combinations specify occipital, cervical, thoracic, lumbar, sacral patterns.
• Classic graft experiment: transplanting Somite 17 (thoracic) into a cervical field → chick grew ribs in the neck, proving fate is intrinsic.

Primary Somite Subdivisions

Sclerotome Resegmentation → Vertebral Column

• Each PSM segment’s sclerotome splits into anterior & posterior halves.
• \text{Anterior}{(n)}+\text{Posterior}{(n+1)} \;\to\; \text{one vertebral body} (re-segmentation).
• Explains spinal nerve exit between vertebrae and staggered muscle attachment.

Chemical “No-Mix” Barrier

• Dermatome/myotome secrete factors that repel sclerotome cells, preventing fate confusion.
• Sclerotome remains confined until instructed by notochord & neural tube to chondrify/ossify.

Experimental & Ethical Notes

• Embryologists use laser ablation, protein blocking, gene knock-outs; many manipulations are lethal or trigger immediate apoptosis.
• Dish handling itself can induce programmed cell death, so large embryo numbers are cultured.
• Ethical caution: every cell “changes destiny” multiple times; no surplus, no wastage.

Classroom Analogies / Examples

• Standing-on-one-leg game (chemical cue vs visual cue).
• Cadherins described as “sticky-note proteins” (temporary).
• Factor X likened to “adhesive remover” freeing the cytoskeleton.
• Mountain-valley imagery for neural groove; dance-team / synchronized-swim for bilateral timing.

Key Vocabulary & Abbreviations

• PSM – Presomitic Mesoderm.
• EphA4 – membrane-bound ligand (NOT secreted).
• EphrinB2 – cognate receptor.
• CDC42 – small GTPase; maintains actin architecture.
• Factor X – unidentified effector that remodels actin.
• EMT/MET – Epithelial ⇄ Mesenchymal Transition.
• HOX code – combinatorial pattern specifying A-P identity.
• Epaxial/Hypaxial – relative to neural tube (medial vs lateral).

Clinical / Pathological Relevance

• Mis-timed segmentation or size mismatch → scoliosis, vertebral fusions, or embryonic lethality.
• Disrupted Eph/Ephrin signaling implicated in congenital vertebral/rib anomalies.
• HOX gene mutations produce homeotic transformations (e.g., lumbarization, cervical ribs).

Conceptual Flow

1. Posterior chemical clock emits signal.
2. PSM cells receive synchronized cue, create fissure via cytoskeletal band.
3. EphA4–EphrinB2 interaction triggers epithelial border → somite forms.
4. Somite immediately decodes HOX position, subdivides into dermatome, myotome, sclerotome.
5. Sclerotome halves resegment to sculpt vertebrae & ribs; dermatome/myotome spread dorsally/laterally to populate skin & muscle.
6. Continuous feedback from neural tube, notochord, and surface ectoderm refines differentiation.

End of detailed study notes.