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DEVELOPMENT
OF
MUSCLES
LEARNING OBJECTIVES
• At the end of the lecture, students should be able to:
• Mention the development of skeletal muscle.
• Discuss the development of Myotomes and derivatives of epaxial
divisions of myotomes and derivatives of hypaxial divisions of
myotomes.
•
The Muscular System develops from mesoderm except the muscles of
the iris which are derived from neurectoderm
There are three types of muscles ;
 Skeletal
 Smooth
 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 and cell bodies of the
mesenchymal cells, and they are differentiated into Myoblast
• These myoblasts fuse and form large elongated ,multinucleated tubes the Myotubes. The
growth of the muscle depends of the rate of fusion of Myotubes
• Myobfilament develop in the cytoplasm of Myotubes
• The muscle cells are long & narrow that’s why they are also 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
part hypaxial or hypomere.
• The typical spinal nerve divides into dorsal & ventral division. 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.
DERIVATIVES 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
degenerate and 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 to form 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 differntiate from mesenchymal cells derived from these
myotomes.
• Groups of myoblasts, each supplied by its own nerve ( cniii, cniv, or cnvi ),
form the extrinsic muscles of the eye.
LIMB MUSCLES
• The musculature of the limbs develop from myogenic cells (myoblasts)
surrounding the developing bones.
• These cells are first located in the ventral part of the dermato-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
arises from somatic mesoderm.
• The muscles of the iris (sphincter and dilator pupillae) and the myoepithilial
cells in mammary and sweat glands are thought to be derived from
mesenchymal cells that originate from neural crest cells.
• 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 developing
heart tube.
• Heart muscle is recognizable in the fourth week and likely develops
through expression of cardia- specific genes.
• Cardiac muscle fibers arise by differentiation and growth of single
cells, unlike striaited skeletal muscle fibers, which develop by fusion
of cells.
• Growth of cardiac muscle fibers results from the formation of new
myofilaments.
DEVELOPMENT OF CARDIAC MUSCLE
• 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.
REFERENCES
• KLM Embryology – 8th Edition.
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