Musculature Development in Anatomy

Musculature Development

• The muscular system, with the exception of some smooth muscle, develops from the mesodermal germ layer.
• Skeletal muscle originates from paraxial mesoderm.
• Smooth muscle differentiates from splanchnic mesoderm.
• Cardiac muscle also originates from splanchnic mesoderm.

Striated Skeletal Musculature

• Head musculature is derived from seven somitomeres, which are paraxial mesoderm-derived.
• Axial skeleton, body wall, and limb musculature develop from somites, with the ventral region forming the sclerotome (vertebrae and ribs).
• The ventrolateral and dorsomedial lips of the somite form progenitor muscle cells that develop into dermomyotomes.
• Some cells from the ventrolateral region migrate to lateral plate mesoderm, contributing to infrahyoid, abdominal wall, and limb muscles.
• The remaining cells in the myotome develop into back, shoulder, and intercostal muscles.

Striated Skeletal Musculature (Continued)

• A border, the lateral somitic frontier, separates primaxial and abaxial mesodermal domains within the embryo.
• Muscle cells that cross the frontier into lateral plate mesoderm develop into abaxial muscle cells, receiving signals for differentiation from lateral plate mesoderm.
• Muscle cells remaining in the paraxial mesoderm (primaxial) receive developmental signals from the neural tube and notochord.
• Each myotome receives innervation from spinal nerves within the same segment.

Innervation of Axial Skeletal Muscles

• The new description of muscle development (primaxial and abaxial domains) replaces the older concept of epimeres and hypomeres (based on function).
• The new description is based on embryological origin rather than innervation.
• Epaxial muscles (e.g., back) receive innervation from dorsal primary rami.
• Hypaxial muscles (e.g., body wall, limbs) are innervated by ventral primary rami.

Patterning of Muscles

• Connective tissue guides muscle formation and myoblast migration.
• Neural crest cells contribute to head region connective tissue.
• Somatic mesoderm forms connective tissue in cervical and occipital regions.
• Parietal layer of lateral plate mesoderm contributes to connective tissue of body wall and limbs.

Head Musculature

• All voluntary head muscles and associated musculature of the tongue, eyes, and pharynx are derived from paraxial mesoderm (somitomeres and somites).
• Head muscle patterning is controlled by neural crest cells.

Limb Musculature

• Limb musculature develops in the seventh week as mesenchyme condensation near limb buds from dorsolateral somite cells.
• Connective tissue (from parietal layer of lateral plate mesoderm) guides formation and is essential for limb skeletal development.
• Limb muscles initially have segmental organization (flexors/extensors).
• Ultimately, limb muscles fuse and consist of tissue from multiple segments.

Limb Musculature (Continued)

• Ventral primary rami from the appropriate spinal nerves penetrate limb bud mesenchyme, with initial separate dorsal and ventral branches.
• These unify to form larger dorsal and ventral nerves, ultimately contributing to nerves like the radial (extensor musculature) and ulnar/median (flexor musculature).
• Spinal nerves also innervate limb musculature and supply sensory input to dermatomes.

Cardiac Muscle

• Cardiac muscle develops from splanchnic mesoderm adjacent to the heart tube.
• Myoblasts connect via intercalated discs, resembling skeletal muscle myofibrils.
• Specialized bundles (Purkinje fibers) emerge for conducting the heart's electrical system.

Smooth Muscle

• Smooth muscle for the dorsal aorta and large arteries originates from lateral plate mesoderm and neural crest cells.
• Coronary arteries derive smooth muscle from proepicardial cells (proximal segments).
• Smooth muscle within the digestive system arises from the splanchnic layer of lateral plate mesoderm.
• Muscles of the pupil and mammary/sweat glands develop from ectoderm.