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Muscular System
Ass. Prof. Malak A. Al-yawer
The muscular system develops from
mesoderm, except for the muscles of the iris, which develop from neuroectoderm, and the muscles of the esophagus, which are believed to develop by transdifferentiation from smooth muscle.
Striated Skeletal Musculature
Somites and somitomeres
form the musculature of the axial skeleton, body wall, limbs, and head.
From the occipital region caudally
somites form and differentiate into the sclerotome, dermatome, and two muscle-forming regions.
Muscle-forming regions
One appears at the ventrolateral edge or lip (VLL) of the prospective dermomyotome. contributes to formation of the myotome and also provides progenitor cells for limb and body wall (hypomeric, hypaxial) musculature.
Myogenesis
Myoblasts (embryonic muscle cells) are derived from (mesenchyme (embryonic connective tissue.
Myogenesis
Somitomeres
A similar process occurs in the seven somitomeres in the head region rostral to the occipital somites. Somitomeres remain loosely organized structures and never segregate into sclerotome and dermomyotome segments.
Tendons
are derived from sclerotome cells lying adjacent to myotomes at the anterior and posterior borders of somites.
Patterning of Muscles
Derivatives of Precursor Muscle Cells
Nerves innervating segmental muscles are also divided into
a Dorsal primary ramus for the epimere and a Ventral primary ramus for the hypomere these nerves will remain with their original muscle segment throughout its migration.
Myoblasts of the epimeres form the extensor muscles of the vertebral column, Myoblasts of the hypomeres give rise to muscles of the limbs and body wall
Myoblasts from hypomeres
ventral tip of the hypomeres gives rise to a ventral longitudinal column
Head Musculature
Limb Musculature
The first indication of limb musculature is observed in the 7th week of development as a condensation of mesenchyme near the base of the limb buds..
Limb Musculature
With elongation of the limb buds, the muscle tissue splits into flexor and extensor components. Although muscles of the limbs are segmental initially, with time they fuse and are then composed of tissue derived from several segments.
Limb buds
The upper limb buds lie opposite the lower five cervical and upper two thoracic segments and the lower limb buds lie opposite the lower four lumbar and upper two sacral segments
Innervation of the limb musculature
Spinal nerves play an important role in differentiation and motor innervation of the limb musculature and provide sensory innervation for the dermatomes.
Clinical Correlates
Partial or complete absence of one or more muscles is rather common. The pectoralis major muscle (Poland anomaly), the palmaris longus, the serratus anterior, and the quadratus femoris muscles may be partially or entirely absent.
Prune belly syndrome
Congenital Torticollis
Cardiac Muscle
Smooth Muscle
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