Embryology of the Muscular System Prof. Abdulameer Al-Nuaimi E-mail: a.al-nuaimi@sheffield.ac.uk E. mail: abdulameerh@yahoo.com

Development of Skeletal muscle Muscles of the head are derived from seven somitomeres. Somitomeres: are partially segmented spirals of mesenchymal cells derived from paraxial mesoderm. Muscles of the axial skeleton, body wall and limbs are derived from somites (derived also from paraxial mesoderm). Somites extend from the occipital region to the tail bud. In humans there are 42-44 somite pairs

Parts of the mesoderm Somatic (parietal) mesod Parietal Mesoderm Transverse Section Splanch nopleur Meso (visceral) Lateral plate Somatic (parietal) Mesoder Second. Y. S Endodearm Splanchnopleuric Mesoderm (visceral) www.google.co.uk/search?

Cells in the upper regions of somites and somitomeres give rise to dermatome and two muscle-forming areas at the ventrolateral (VLL) and dorsomedial (DML)  lips. ventral to the dermatome, cells from these two areas (VLL and DML) migrate under the dermatome and proliferate to form progenitor muscle cells, the myotome.

Differentiation of somite Dorsomedial muscle cells Ventrolateral muscle cells Notochord Myotome Sclerotome

There is a well-defined border between each somite and parietal layer of lateral plate mesoderm called the lateral somitic frontier. Some muscle cells come from the Ventrolateral edge of the myotome, cross the lateral somitic frontier, and enter the lateral plate mesoderm forming the abaxial muscle cell precursors. Cells that remain in the paraxial mesoderm and do not cross the frontier (the remaining Ventrolateral edge of the myotome cells and all of the Dorsomedial edge of myotome cells) comprise the primaxial muscle cell precursors. The lateral somitic frontier separates two mesodermal domains: 1-The primaxial domain  2-The abaxial domain 

abaxial muscle cell precursors: form infrahyoid, abdominal wall (rectus abdominis, internal and external oblique and transversus abdominis), and limb muscles. primaxial muscle cell precursors: form muscles of the back, shoulder girdle and intercostal muscles.

Parietal mesoderm Visceral Mesoderm

Limb musculature Limb musculature appears in the seventh week of fetal life. It is shown as a condensation of mesenchymal tissue near the base of the limb. The mesenchyme is derived from abaxial muscle cell precursors which has migrated into the limb bud to form the muscles. Mesenchyme of the limb muscles is derived from the same parietal layer of lateral plate mesoderm, which also gives rise to the bones of the limb. As the limb elongates muscles are grouped into flexor and extensor components. Muscular tissue of the limb is derived from many segments; as these muscle fibres fuse, they give rise to muscles, therefor each muscle is derived from several segments.

Upper limb bud appears opposite the lower four cervical and the first thoracic spinal cord segments The lower limb bud lies opposite the lower four lumbar and upper three sacral spinal cord segments When the limb bud is formed, ventral primary rami from the opposite spinal nerves penetrate into the mesenchyme of the limb, they unit to form the brachial and lumbosacral plexuses. Tendons for the attachment of muscles to the bones are derived from sclerotome cells.

BRANCHIAL ARCH MUSCLES: myoblasts from the arches migrate to form the muscles of mastication, of facial expression, and muscles of the pharynx and larynx. They are innervated by branchial arch nerves V, VII, IX, and X, respectively OCULAR MUSCLES are probably derived from mesenchymal cells around the prechordal plate (mesodermal cells) which gives rise to 3 preoptic myotomes. Groups of myoblasts with cranial nerves III, IV, and VI form the extrinsic muscles of the eyeball TONGUE MUSCLES: 4 pairs occipital myotomes are seen first, but the first pair disappears. The last 3 pairs form the tongue muscles, innervated by cranial nerve XII LIMB MUSCLES develop in situ from mesenchyme around the developing limb bones coming from the Ventrolateral edge of the myotome and lateral plate mesoderm

During differentiation of the myotome, the precurser cells (myoblasts) elongate, combine to form parallel bundles, and fuse to form multinucleated cells. The central nuclei move to the periphery, and during fetal life myofibrils are seen in the cytoplasm. By the end of 3rd month, cross-striations can be seen Skeletal muscle .

سورة المؤمنون الآية 14 فى خلق الإنسان ::قال الله تعالى (وَلَقَدْ خَلَقْنَا الإنسان مِن سلالة مِّن طِينٍ* ثُمَّ جَعَلْنَاهُ نُطْفَةً في قَرَارٍ مَّكِينٍ* ثُمَّ خَلَقْنَا النُّطْفَةَ عَلَقَةً فَخَلَقْنَا الْعَلَقَةَ مُضْغَةً فَخَلَقْنَا الْمُضْغَةَ عِظَاماً فَكَسَوْنَا الْعِظَامَ لَحْماً ثُمَّ أَنشَأْنَاهُ خَلْقاً آخَرَ فَتَبَارَكَ اللَّهُ أَحْسَنُ الخالقين) "خلال الأسبوع السادس يبدأ الهيكل العظمي بالانتشار في الجسم و تأخذ العظام شكلها المألوف ، وفي نهاية الأسبوع السابع و خلال الأسبوع الثامن تأخذ العضلات وضعيتها حول أشكال العظام الأبحاث الميكروسكوبية أثبتت أن تطور الجنين داخل رحم الأم يتم كما وصفته آيات القرآن، فأولاً تتكون الأنسجة الغضروفية التي تتحول إلى عظام الجنين ، ثم تكون بعدها خلايا العضلات ثم تتجمع مع بعضها و تتكون لتلتف حول العظام . 

Innervation Epimeric muscles (above the axis) - back muscles - are innervated by dorsal primary rami Hypomeric muscles (below the axis) - body wall and limb muscles - are innervated by ventral primary rami Innervation to developing musculature

Development of the Cardiac Muscle Cardiac muscle develops from splanchnic mesoderm surrounding the endothelial heart tube. Myoblasts adhere to one another by special attachments that later develop into intercalated discs. Myofibrils develop as in skeletal muscle, but myoblasts do not fuse. During later development, a few special bundles of muscle cells with irregularly distributed myofibrils become visible. These bundles, Purkinje's fibers, form the conducting system of the heart

Development of the Smooth Muscle Smooth muscle for the dorsal aorta and large arteries is derived from lateral plate mesoderm and neural crest cells. Coronary arteries smooth muscle originates from proepicardial cells and neural crest cells. Smooth muscle in the wall of the gut derivatives is derived form the splanchnic layer of lateral plate mesoderm that surrounds these structures. The sphincter and dilator muscles of the pupil and muscle tissue in mammary and sweat glands are derived from ectoderm

Congenital anomalies of muscular system Pyloric stenosis

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