Muscular System As with the skeltal system most of the muscular system also develops from the mesodermal germ layer Smooth muscle develops from splanchnic mesoderm which surrounds gut /derivatives. Cardiac muscle develops from splanchnic mesoderm which surrounds the heart tube.

TRUNK MUSCULATURE Skeletal muscle of the trunk develops from paraxial mesoderm (which forms somites & somitomeres) Somites differentiate

into:- 1. Sclerotome → Axial skeleton & 2. Dermomyotome → a.Dermatome  dermis and subcut tissues & b. Myotome  segmental muscles (& takes with it its own segmental nerve).

FATE OF MYOTOME CELLS Myotome cells split off, move to their definitive locations, & become elongated & spindle shaped (called myoblasts) Many myoblasts fuse to become Multinucleated muscle fibres.

Myofibrils appear in cytoplasm. By 12/52 cross striations typical for skeletal muscle appear.

Somites: form body wall musculature. It forms a dorsal epimere & a ventral hypomere. The epimere forms the vertebral extensors; while the hypomere forms the muscles of the body wall and limbs. Note the different innervation viz. dorsal and ventral primary rami

Somitomeres same process in head and neck region i.e. form myoblasts which will form extra ocular eye muscles, face, larynx, tongue etc

Head Musculature

Limb Musculature Condensation of mesenchyme near the base of limb buds (7 th week) Mesenchyme is derived from dorsolateral cells of somites Migrate into limb bud to form the muscles Connective tissue dictates the pattern of muscle formation Upper limb buds lie opposite the lower five cervical and upper two thoracic segments

Limb Musculature Lower limb buds lie opposite lower foua lumbar and upper two sacral segments There is a 180° medial rotation of the lower limb compared to developing upper limb (angle of flexion differs)

Cardiac Muscle Develops from splanchnic mesoderm surrounding the endothelial heart tube Myoblasts adhere to one another by intercalated discs Myofibrils develop as in skeletal muscle but do not fuse Few special bundles become visible (Purkinje fibers)

Smooth Muscle

Clinical Correlations

INTRAEMBRYONIC COELOM

Appears as isolated spaces in the lateral mesoderm In the 4 th week, the spaces fuse to form a single horseshoe-shaped (U-shaped) cavity The coelom divides the lateral mesoderm into: 1.Somatic (parietal) layer: under ectoderm 2.Splanchnic (visceral) layer: over endoderm

Somatopleure = somatic mesoderm + overlying ectoderm Splanchnopleure = splanchnic mesoderm + underlying endoderm

INTRAEMBRYONIC COELOM

DERIVATIVES: It gives rise to three body cavities: 1.A pericardial cavity: the curve of U 2.Two pericardioperitoneal canals (future pleural cavities): the proximal parts of the limbs of U 3.Two peritoneal cavities: the distal parts of the limbs of U Each cavity has a parietal layer (derived from somatic mesoderm) & a visceral layer (derived from visceral mesoderm) FUNCTION: It provides space for the organs to develop & move

DEVELOPMENT OF PERITONEAL CAVITY Major part of intraembryonic coelom Develop from the distal parts of the limbs of the U-shaped cavity Originally, it is connected with extraembryonic coelom (midgut herniates to the outside through this connection) At 10 th week, it looses its connection with extraembryonic ceolom (when midgut returns to abdomen)

DEVELOPMENT OF PERITONEAL CAVITY Originally, there were 2 peritoneal cavities After lateral folding of embryo, the peritoneum becomes a single cavity HOW?

MESENTERIES A MESENTERY is a double layer of peritoneum that begins as an extension of the visceral peritoneum covering an organ The mesentery connects the organ to the body wall and transmits vessels and nerves to it Transiently, the dorsal & ventral mesenteries divide the peritoneal cavity into right & left halves The ventral mesentery disappears EXCEPT where stomach develops (WHY?)

PERICARDIAL CAVITY Develops from the curve of the U- shaped cavity During formation of head fold, the heart & pericardial cavity move ventrocaudally & become anterior to the foregut (esophagus) It is bounded by an outer somatic & an inner visceral layer, forming the serous pericardium

PERICARDIAL CAVITY Originally, it is connected with the 2 pericardioperitoneal canals Later on, it become separated from the 2 pericardioperitoneal canals HOW?

PERICARDIAL CAVITY Originally, the bronchial buds are small relative to the heart Bronchial buds grow laterally into pericardioperitoneal canals (future pleural cavities) Pleural cavities expand ventrally around heart & splits mesoderm into: 1.Outer layer: forms thoracic wall 2.Inner layer: pleuropericardial membrane

PLEUROPERICARDIAL MEMBRANES THE PARTS SURROUNDING THE SEROUS PERICARDIUM: form the fibrous pericardium THE PARTS BEHIND THE HEART: fuse with the ventral mesentery of the esophagus (at 7 th week), forming the mediastinum & separating pericardial from pleural cavities N.B.: The right pleural cavity separates from pericardial cavity earlier than left

PLEURAL CAVITIES Develop from the 2 pericardiperitoneal canals Originally, they are connected with pericardial & peritoneal cavities Later on, they become separated from: 1.Pericardial cavity 2.Peritoneal cavity (HOW?)

PLEUROPERITONEAL MEMBRANES Produced when developing lungs & pleural cavities expand into the body wall During 6 th week, they fuse with dorsal mesentery of esophagus & septum transversum, separating pleural cavities from peritoneal cavity N.B.: The right pleural cavity separates from peritoneal cavity earlier than left

DEVELOPMENT OF DIAPHRAGM

The diaphragm develops from: 1.Septum transversum: forms the central tendon 2.Dorsal mesentery of esophagus: forms the right & left crus 3.Muscular ingrowth from lateral body wall: posterolateral part (costal part) 4.Pleuroperitoneal membranes: small portion of diaphragm

SEPTUM TRANSVERSUM At 3 rd week, it is in the form of mass of mesodermal tissue in the cranial part of embryo (opposite the 3 rd, 4 th & 5 th cervical somites) At 4 th week (during formation of head fold), it moves ventrocaudally forming a thick incomplete partition between thoracic & abdominal cavities At 6 th week, it expands & fuse with dorsal mesentery of esophagus & pleuroperitoneal membranes to form the diaphragm

INNERVATION OF DIAPHRAGM Myoblasts from 3 rd, 4 th & 5 th cervical somites migrate into diaphragm & bring their nerve fibers from them Nerve fibers derived from ventral rami of 3 rd, 4 th & 5 th cervical nerves fuse to form phrenic nerve that elongate to follow the descent of diaphragm 1.Both motor & sensory supply of the diaphragm is derived from phrenic nerve 2.The part of diaphragm derived from lateral body wall receives sensory fibers from lower intercostal nerves

ANOMALIES OF DIAPHRAGM 1.CONGENITAL DIAPHRAGMATIC HERNIA 2.EVENTRATION OF DIAPHRAGM 3.CONGENITAL HIATAL HERNIA

CONGENITAL DIAPHRAGMATIC HERNIA

A posterolateral defect of diaphragm Cause: defective formation and/or fusion of pleuroperitoneal membrane with other parts of diaphragm Effects: 1.Herniation of abdominal contents into thoracic cavity 2.Peritoneal & pleural cavities are connected with one another The defect usually occurs in the left side (WHY?)

EVENTRATION OF DIAPHRAGM

Cause: failure of muscular tissue from body wall to extend into pleuroperitoneal membrane on one side Effects: superior displacement of abdominal viscera (surrounded by a part of diaphragm forming a pocket)

CONGENITAL HIATAL HERNIA Herniation of part of the stomach through a large esophageal hiatus (opening)

Reference clanatomy.ukzn.ac.za/...Embryology/EMB RYOLOGY_OF_MUSCULARclanatomy.ukzn.ac.za/...Embryology/EMB RYOLOGY_OF_MUSCULAR HENRY Gray anatomy of the human body. piggl

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