At the End of This Session the Students Should Be Able To

LEARNING OBJECTIVES

Development of Muscular System

At the end of this session the students should be able to:

·  Describe the various types of muscles

·  Describe the embryological development of different types of muscles

·  Know the different embryological components involved in the development of muscles

LECTURE OUTLINE

Development of Muscular System

The Muscular System develops from mesoderm except for muscles of iris which are derived from neurectoderm

There are three types of muscles;

n  Skeletal

n  Smooth

n  Cardiac

•  The skeletal muscles are derived from paraxial mesoderm Somitomeres & somites i.e. the Myotome part forms the axial skeleton of body wall limbs and head.

Development of Muscles:

•  The first indication of myogenesis is the elongation of nuclei cell bodies of the mesenchymal cells and are differentiated into Myoblast

•  These myoblasts fuse and form large elongated, multinucleated tubes the Myotubes the growth of muscle depends of the rate of fusion of Myotubes

•  Myobfilament & organelles develop in the cytoplasm of Myotubes

•  The muscle cells are long & narrow that’s why they are called Muscle fibers

•  These muscle fibers are wrapped in connective tissue sheath known as External lamina, Epimysium & Perimysium layers of fibrous sheaths are produced by fibroblasts

•  Endomysium is formed by external lamina which is derived from muscle fibers & reticular fibers

•  The muscles increase in length & diameter in order to grow with the skeleton of the body.

MYOTOMES

•  Each typical myotome divides into dorsal Epiaxial or Epimere & a ventral hypoaxial or hypomere

•  The typical spinal nerve divides into dorsal & ventral division i.e. the dorsal primary ramus supplies the Epimere & the ventral primary ramus supplies the hypomere

•  Some of the muscles like the intercostal muscles remain segmentally arranged like somites, but most muscles migrate away from myotomes to form unsegmented muscles.

DERIVATIES OF PARAXIAL DIVISION

The muscles derived from these Myoblast are

•  Extensor muscle of neck & vertebral column

•  The embryonic muscles derived from the sacral & coccygeal myotomes degenerateand form dorsal sacro-coccygeal ligaments in adults

Derivatives of Hypomere

•  Myoblasts from these divisions of cervical myotome form the Scalene, Prevertebral, Geniohyoid Infrahyoid muscle

•  The thoracic myotome forms lateral &ventral flexor muscles of vertebral column

•  The lumbar Myotomes form the Quadratus Lumborum muscle

•  The Sacro-coccygeal myotome forms the muscles of Pelvic Diaphragm striated muscles of Anus & sex organs

PHARYNGEAL ARCH MUSCLES

The migration of myoblasts from the pharyngeal arches forms the muscles of mastication, facial expression, pharynx, and larynx. These muscles are innervated by pharyngeal arch nerves.

OCULAR MUSCLES

•  The origin of the extrinsic eye muscles is unclear, but it is thought that they may be derived from mesenchymal cells near the prechordial plate.

•  The mesoderm in this area is thought to give rise to them preotic myotomes.

•  Myoblasts differntiatiate from mesenchymal cells derived from these myotomes.

•  Groups of myoblasts, each supplied by its own nerve (CNIII, CNIV, and CNVI) form the extrinsic muscles of the eye.

LIMB MUSCLES

•  The musculature of the limbs develops from myogenic cells (myoblasts) surrounding the developing bones.

•  These cells are first located in the ventral part of the dermo-myotome and are epithelial in nature.

DEVELOPMENT SMOOTH MUSCLES

•  Smooth muscle fibers differentiate from splanchnic mesenchyme surrounding the endoderm of the primordial gut and its derivates.

•  The smooth muscles in the walls of many blood and lymphantic vessels arise from somatic mesoderm.

•  The muscles of the iris (sphincter and dilator pupillae) and the myoepithelial cells in mammary and sweat glands are thought to be derived from mesenchymal cells that originate from ectoderm.

•  The first sign of differentiation of smooth muscle is the development of elongated nuclei in spindle-shaped myoblasts.

•  During early development new myoblasts continue to differentiate from mesenchymal cells but do not fuse, they remain mononucleated.

•  During later development, division of existing myoblasts gradually replaces the differentiation of new myoblasts in the production of new smooth muscle tissue.

•  As smooth muscle fibers develop into sheets or bundles, they receive autonomic innervations.

•  Fibroblasts and muscle cells synthesize and lay down collagenous elastic and reticular fibers.

DEVELOPMENT OF CARDIAC MUSCLE

•  Cardiac muscle develops from the lateral splanchnic mesoderm, which gives rise to the mesenchyme surrounding the development heart tube.

•  Cardiac myoblasts differentiate from the primordial myocardium.

•  Heart muscle is recognizable in the fourth week and likely develops through expression of cardio- specific genes.

•  Cardiac muscle fibers arise by differentiation and growth of single cells, unlike striated skeletal muscle fibers, which develop by fusion of cells.

•  Growth of cardiac muscle fibers results from the formation of new myofilaments.

•  The myoblasts adhere to each other as in developing skeletal muscle, but the intervening cell membranes do not disintegrate; these areas of adhesion give rise to intercalated discs.

•  Late in embryonic period special bundles of muscle cells develops with relatively few myofibrils and relatively larger diameters then typical cardiac muscle fibers.

•  These atypical cardiac muscle cells, purkinje fibers, form the conducting system of the heart.