Learning Objectives:
§ By the end of this 55 min session, you should be able to:
ú Identify the sources of muscular tissue
ú Describe the histogenesis of skeletal, cardiac & smooth
muscles.
ú Describe the process of somitogenesis.
ú Identify different regions of early somite and further
differentiation & contribution of each region.
Identify the sources of muscular tissue
Describe the histogenesis of skeletal, cardiac & smooth
muscles.
Describe the process of somitogenesis.
Identify different regions of early somite and further
differentiation & contribution of each region.
Enumerate the muscles which are derived from Primaxial &
Abaxial precursors.
Define lateral somitic frontier. Enlist the structures whose
borders are defined by lateral somitic frontier.
Describe the innervation of axial skeletal muscles.
Describe that how patterns of muscle formation & final shape of
muscle are determined
Identify the name and location of each division of myotome &
Enlist the derivatives of each division
Correlate developmental origin of muscles with their nerve
supply
Explain the embryological basis of developmental anomalies
related with muscular system.
Enlist the developmental source of Head muscles, Limb muscles,
Cardiac muscles, & Smooth muscles
SOURCES OF MUSCULAR TISSUE
Mesoderm
Skeletal muscles:
Paraxial mesoderm (somites & somitomeres)
Somatic mesoderm of body wall
Mesenchyme of pharyngeal arches
Smooth muscles:
Visceral splanchnic mesoderm surrounding gut & its
derivatives
Ectoderm (pupillary, myoepithelial cells of mammary
gland & sweat gland muscles)
Cardiac muscles
Visceral splanchnic mesoderm surrounding heart tube
HISTOGENESIS/ MYOGENESIS OF SKELETAL MUSCLES
Develop from mesenchyme derived from one of
the following 3 sources:
• Somites
• Somatic mesoderm of body wall
•Mesenchyme of pharyngeal arches
Whatever the source, development of skeletal
muscles is preceded by differentiation of
myoblasts from mesenchyme
HISTOGENESIS OF CARDIAC MUSCLES
HISTOGENESIS OF SMOOTH MUSCLES
Visceral splanchnic mesoderm
surrounding gut & its derivatives.
Ectoderm (pupillary, mammary
gland & sweat gland muscles)
Lateral plate mesoderm (Somatic
mesoderm) and neural crest cells (For dorsal
aorta & large arteries).
Proepicardial cells and neural crest cells (For
coronory arteries, proximal segments).
Serum response factor (SRF):
DEVELOPMENT OF STRIATED SKELETAL MUSCULATURE
•A similar process occurs in the seven somitomeres in the
head región rostral to the occipital somites. However,
somitomeres never segregate into recognizable regions of
sclerotome and dermomyotome segments prior to
differentiation.
•Tendons for the attachment of muscles to bones are
derived from sclerotome cells lying adjacent to myotomes
at the anterior and posterior borders of somites.
•The transcription factor SCLERAXIS regulates development
of tendons.
STAGES OF SOMITOGENESIS
MUSCLES OF NECK & TRUNK
Derived from myotome regions of dermomyotome
which are derived from dorsolateral wall of somites.
Myoblasts derived from dorsomedial lip:
Primaxial domain: primaxial/ epaxialmusculature
Myoblasts derived from ventrolateral lip:
Migrate into somatic layer of lateral plate
mesoderm. At the same time, mesodermal cells in
somatic layer of LPM differentiate into myoblasts
Abaxial domain: abaxial/ hypaxialmusculature
Lateral somitic frontier
Initially, there is a
well defined border
b/w each somite &
parietal layer of lat
plate mesoderm
Functions of Lateral somitic frontier:
Separates two mesodermal domains in the embryo.
Defines the border between dermis :
derived from dermatomes in the back and
dermis derived from lateral píate mesoderm in the body wall.
Defines a border for rib development::
such that the bony components of each rib are derived from
primaxial sclerotome cells and
the cartilaginous parts of those ribs that attach to the
sternum are derived from sclerotome cells that migrate
across the lateral somitic frontier (abaxial cells).
Developing transverse process divides the myotome into:
Abaxial/ Hypaxial division
(supplied by
ventral primary rami)
Primaxial/ Epaxial division
(supplied by dorsal
primary rami)
MUSCLES OF HEAD:
Most muscles of head are derived from mesenchyme
of pharyngeal arches.
Myoblasts derived from mesenchyme of pharyngeal arches
migrate to form muscles of mastication, muscles of facial
expression, muscles of pharynx & larynx.
These muscles are supplied by pharyngeal arch nerves
Muscles of tongue
Myotomes of occipital somites (innervated by CN-XII)
Extrinsic muscles of eyeball
Masses of condensed mesenchyme located over optic cup.
These masses are derived from mesenchyme that is formed
in relation to prechordal plate. Supplied by CN-III, IV, VI)
Amyotome is the group of muscles that a single spinal
nerve innervates.
PATTERNING OF MUSCLES
Patterns of muscle formation & final shape of muscles
are determined/ controlled by C.T. FRAMEWORK (in
which myoblasts migrate) & not by myoblasts
themselves.
In head region:
C.T. is derived from neural crest cells
In cervical & occipital regions
C.T. is derived from somitic mesoderm
In body wall & limbs:
C.T. originate from parietal layer of lateral plate
mesoderm
PRUNE BELLY SYNDROME
DUCHENNE MUSCULAR DYSTROPHY,
BECKER MUSCULAR DYSTROPHY
