Development of the Muscular System

Introduction

• Focus on the skeletal muscle system.
• Follow the development path of the skeletal system.
• Structure: Head/Neck muscles, Trunk muscles, Limb muscles.

Head and Neck Muscles Development

• Embryo Anatomy:
  • Buccopharyngeal membrane (cranial end)
  • Cloacal membrane (caudal end)
  • Sagittal section view.
• Primitive Pharynx:
  • Formed by ectoderm, mesoderm, and endoderm.
  • Vesiculation process leads to pharyngeal apparatus formation.
• Pharyngeal Arches:
  • 1st, 2nd, 3rd, 4th, and 6th arches; 5th generally disappears.
  • Mesodermal cores form muscles; specific cranial nerves supply them:
    • 1st Arch: Muscles of mastication (Cranial Nerve V - Trigeminal)
    • 2nd Arch: Muscles of facial expression (Cranial Nerve VII - Facial)
    • 3rd Arch: Stylopharyngeus (Cranial Nerve IX - Glossopharyngeal)
    • 4th & 6th Arches: Muscles of larynx, pharynx (Cranial Nerve X - Vagus)
• Muscles Derived from Arches:
  • 1st Arch: Temporalis, Masseter, Pterygoids, Mylohyoid, Digastric (anterior belly), Tensor Veli Palatini.
  • 2nd Arch: Muscles of facial expression, Stylohyoid, Digastric (posterior belly), Stapedius.
  • 3rd Arch: Stylopharyngeus.
  • 4th & 6th Arches: Levator Veli Palatini, Pharyngeal constrictors, Laryngeal muscles.

Trunk Muscles Development

• Embryo Anatomy:
  • Ectoderm, paraxial mesoderm, intermediate mesoderm, lateral plate mesoderm.
• Somite Formation:
  • Paraxial mesoderm forms somites (44-45 by week 5).
  • Somatocoel divides somites into:
    • Dermatomyotome (dorsal): Dermatome, Myotome
    • Sclerotome (ventral)
• Muscle Layers:
  • Sclerotome: Forms vertebrae, intervertebral discs, ribs.
  • Syndetome: Forms tendons.
  • Myotome: Divides into epaxial (dorsal) and hypaxial (ventral) muscles.
• Muscle Functions:
  • Epaxial Muscles: Back muscles (erector spinae, multifidi).
  • Hypaxial Muscles: Trunk and limb muscles (diaphragm, intercostals, serratus, abdominal muscles).

Limb Muscles Development

• Gene Influence:
  • Hox genes, TBX4/5, fibroblast growth factors.
• Limb Bud Formation:
  • From C5-T1 and L2-S3 regions.
  • Lateral plate mesoderm drives bone formation.
• Myotome Migration:
  • Forms anterior/posterior condensations.
  • Upper Limb:
    • Anterior: Flexors, Pronators.
    • Posterior: Extensors, Supinators.
  • Lower Limb:
    • Anterior: Extensors, Adductors, Dorsiflexors.
    • Posterior: Flexors, Plantarflexors, Abductors.

Muscle Fiber Differentiation

• Mesoderm to Muscle Transition:
  • Mesenchymal cells differentiate into myoblasts, then muscle fibers.
• Gene Activation:
  • Pax genes, MyoD, Myf, Myogenin.

Conclusion

• Comprehensive overview of skeletal muscle development from embryonic structures to functional adult muscles.
• Importance of gene activation in muscle differentiation.