Somitogenesis Signals, Adhesion Dynamics & Intramembranous Ossification

Clarifications on Neurulation & Somite-Related Quiz Questions

• MHP (Medial Hinge Point) vs. DLHP (Dorsolateral Hinge Point)
  • MHP proteins sit in the neural groove and permit fold formation only; they do not drive convergence of folds into a tube.
  • DLHP does mediate convergence; therefore, a convergence defect is DLHP-related, not MHP-related.
  • Quiz wording nuance: “DLHP allows convergence and then facilitates tube formation.”
  • Correct fill-in: DLHP, not MHP, closes the tube in absence of “TO” (typo referenced by students).
• Terminology inside a formed somite
  • After segmentation the block is called a somite (not “in the process of forming”).
  • Cells inside an established somite cease to be generic mesenchymal; they are somatocoele cells encased by a thin epithelial sheet.
  • “Mesenchymal” properly describes the loose, undifferentiated cells before epithelialization/segmentation.
• Fissure vs. Separation in Somitogenesis
  • Microscopic “cracks” look like ice fissures but do not imply true separation; blocks remain attached until full segmentation.
  • Compaction & adhesion protein expression (cadherins) provide the tensile integrity during fissure appearance.
• N-cadherin / E-cadherin usage
  • Temporary, “Post-it-note”–style adhesions; appear in neurulation, somite condensation, and many subsequent morphogenetic events.
  • Contrasted with permanent junctions like desmosomes.
• Fibronectin vs. Fibroblast
  • Fibronectin = extracellular adhesion glycoprotein, part of cytoskeletal tapestry.
  • Fibroblast = multipotent stromal cell that can become several connective-tissue derivatives.

Foundational Cell & Tissue Definitions

• Mesenchymal Cell = pluripotent, spindle/irregular shaped, motile; secretes amorphous extracellular matrix (ECM).
• Somatocoele Cell = previously mesenchymal, now residing inside an epithelialized somite.
• Osteoblast → Osteocyte lineage
  • Osteoblast: secretes osteoid (organic bone matrix + minerals).
  • Osteocyte: mature, maintenance cell, “walled-in” by self-secreted bone.
• Osteogenic Cell = stem cell that gives rise to osteoblasts.
• Periosteum / Endosteum
  • Periosteum: outer osteogenic layer of bone.
  • Endosteum: inner osteogenic lining.
• Compact vs. Spongy Bone
  • Compact: dense, non-porous.
  • Spongy (cancellous): porous network of trabeculae (arches & bridges).

Paracrine Signaling Map for Somite Derivation

Signals FROM Neural Tube

• NT_3 (Neurotrophin-3) → Central dermatome-myotome (CDM)
  • Induces disaggregation & EMT back to mesenchymal shape.
• WNT1 & WNT3 → Epaxial (dorsal) dermatomyotome
  • Kick-starts myogenesis template.

Signals FROM Surface Ectoderm

• WNT_7 → Hypaxial (lateral) dermatomyotome
  • Activates MyoD transcription; shuts off non-muscle genes.
• PAX-3 subsequently refines myogenesis within limb muscle fields.

Signals FROM Lateral Plate Mesoderm

• BMP4 & FGF5
  • Dorsally: promote dermatomal identity.
  • Ventrally (sclerotome): inhibit premature myoblast formation & act as timing “clock.”
  • Possible redundancy/synergy → faster developmental transitions.

Signals FROM Notochord (→ Sclerotome)

• SHH (Sonic Hedgehog)
  • Turns on PAX_1 → chondroblast differentiation (chondrogenesis).
  • Activates IMF (Inhibitor of MyoD Family) → blocks local myogenesis to favor cartilage/bone route.

Adhesion, Cytoskeleton & Dynamic “Hold-and-Go” Philosophy

• Cadherin-mediated adhesion is temporary, allowing iterative epithelial↔mesenchymal transitions.
• Development is “program → hold → move on” with each structure responding to transient cues before being remodelled or silenced.

Overview of Bone Formation Routes

1. Intramembranous Ossification
   • “Spontaneous,” no cartilage template.
   • Produces flat bones of skull, mandible, part of clavicle.
   • Always centered around newly vascularized mesenchyme.
2. Endochondral Ossification
   • Requires a pre-made cartilaginous template from chondroblasts.
   • Generates long bones, vertebrae, and most of post-cranial skeleton.

Intramembranous Ossification – Stepwise Detail

1. Mesenchymal Condensation Around Capillaries
   • Capillary invasion → mesenchymal cells crowd → ECM thickens (“condenses”).
2. Mesenchymal → Osteoblast Transition
   • Trigger unknown (possibly vascular signal + tissue thickness).
   • Osteoblasts secrete osteoid, forming islands (ossification centers).
   • Finger-like projections = spicules.
3. Spicule Fusion → Honeycomb Spongy Bone
   • Islands grow until spicules meet, generating porous lattice.
   • Enlarging vasculature weaves through future trabeculae.
4. Periosteal / Endosteal Compaction
   • Surface osteoblasts lay down compact bone externally & internally.
   • Center retains spongy architecture; osteoblasts locked in become osteocytes.
   • Process remains incomplete at birth to allow skull flexibility (fontanelles).

Instructor Metaphors & Visual Aids

• “Mopping yourself into a corner”: osteoblasts wall themselves in, then convert to osteocytes.
• “Spider-web / lattice” appearance of early cranial plates.
• Comparing chick vs. mouse skull stains: highlights species size differences in osteoblasts & osteocytes.

Abnormal Intramembranous Outcomes

• Cyclopia / Craniofacial defects
  • Exposed in lambs, pigs, etc.
  • Linked to maternal ingestion of specific plants at a critical gestational window.
  • Chemical toxins decrease SHH and mis-regulate WNT_{11} → impaired midline patterning, absent maxilla/nasal fusion, cleft palate variants.
• Cleft Palate in dogs: failed maxillary fusion; same pathway disruptions.

Ethical & Practical Implications

• Understanding plant teratogens can inform grazing management to prevent livestock losses.
• Clinically, knowing timing of SHH/BMP/WNT pathways aids prenatal screening & potential gene-therapy strategies.
• Highlights importance of terminological precision (e.g., DLHP vs. MHP) in diagnostic exams and research publications.

Quick Terminology Recap (Exam-Ready)

• NT_3 – disaggregation/EMT of central dermatome.
• WNT_{1,3} – epaxial myogenesis start.
• WNT_7 – activates MyoD.
• BMP4 / FGF5 – timing + dorsal/ventral modulation.
• SHH – chondrogenesis via PAX_1, also activates IMF to block muscle genes.
• Spicule → Trabecula → Spongy Bone; Periosteum (outer), Endosteum (inner).