Overview
This lecture covers the embryological development of the muscular system, detailing the origins, types, genetic regulation, and clinical correlations of skeletal, cardiac, and smooth muscles.
Types and Origins of Muscles
- Three main muscle types: skeletal, smooth, and cardiac.
- Most muscles develop from the mesoderm germ layer.
- Exceptions: muscles of the iris, sweat glands, and mammary glands develop from ectoderm.
Mesodermal Layers and Muscle Development
- Mesoderm divides into paraxial (forms somites), lateral plate (parietal and visceral layers).
- Paraxial mesoderm forms somites, which differentiate into muscle, bone, and dermis.
- Skeletal muscle originates mainly from somites (paraxial mesoderm).
- Smooth muscle forms from visceral (splanchnic) mesoderm around the gut.
- Cardiac muscle forms from visceral mesoderm surrounding the heart tube.
Somite Differentiation and Muscle Domains
- Each somite differentiates into sclerotome (bone), dermatome (dermis), and myotome (muscle).
- Myotome splits into ventrolateral (forms limb/body wall muscles) and dorsomedial (forms back muscles) regions.
- The lateral somitic frontier (boundary) separates primaxial (near neural tube) and abaxial (lateral plate mesoderm) domains.
- Primaxial muscles: near the spine from somite-only cells.
- Abaxial muscles: body wall and limb muscles, involving migrated somite cells and lateral plate mesoderm.
Muscle Innervation and Patterning
- Each myotome is innervated by the spinal nerve from its segment of origin.
- Dorsal ramus innervates back (primaxial) muscles; ventral ramus innervates body wall/limb (abaxial) muscles.
- Muscle patterning is influenced by surrounding connective tissue, derived from neural crest in the head and lateral plate mesoderm in limbs/body wall.
Genetic and Molecular Regulation
- Muscle development regulated by genes such as MyoD and Myf5 (myogenic regulatory factors).
- Signal proteins (BMP4, Wnt, Sonic Hedgehog) direct muscle gene expression.
- Connective tissue environment influences muscle differentiation and pattern formation.
Development of Cardiac and Smooth Muscle
- Cardiac muscle develops from visceral mesoderm; specialized fibers (Purkinje) form the heart conduction system.
- Smooth muscle in vessels derives from lateral plate mesoderm and neural crest; gut smooth muscle from visceral mesoderm.
- Serum Response Factor (SRF) is essential for smooth muscle differentiation.
Clinical Correlations and Anomalies
- Absence or defects in muscle (e.g., Poland sequence—missing pectoral muscles).
- Prune belly syndrome: absence or hypoplasia of abdominal muscles.
- Muscular dystrophies (Duchenne and Becker) result from mutations in the dystrophin gene, affecting muscle integrity.
Key Terms & Definitions
- Somite — block of mesoderm forming vertebrae, muscle, and dermis.
- Paraxial Mesoderm — mesoderm beside the embryo's midline, source of somites.
- Lateral Plate Mesoderm — outermost mesoderm, splits into parietal and visceral layers.
- Primaxial Domain — muscle domain near neural tube, from somite-derived cells.
- Abaxial Domain — muscle domain lateral, involving migrated somite and lateral plate mesoderm.
- Lateral Somitc Frontier — boundary between somites and lateral plate mesoderm.
- MyoD/Myf5 — genes essential for muscle cell differentiation.
- Poland Sequence — congenital absence of pectoral muscles.
- Prune Belly Syndrome — absence/hypoplasia of abdominal muscles.
- Duchenne/Becker Muscular Dystrophy — muscle diseases due to dystrophin gene mutations.
Action Items / Next Steps
- Review tables summarizing muscle origins and innervation for different body regions.
- Study diagrams illustrating mesodermal layers and muscle migration.
- Memorize key gene functions and their roles in muscle development.