Download LEARNING OBJECTIVES

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;
•

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 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.