Muscular system
• Most muscles arise from the mesoderm, skeletal
muscles derived from
paraxial mesoderm
,
including :
(1)- somites, which give rise to muscles of the axial
skeleton, body wall, and limbs.
(2)- somitomeres, which give rise to muscles of the
head.
The innervation of axial skeletal muscles based
on the embryological origin of muscle cells from 2
different populations of muscle cell precursors: the
apaxial & primaxial cells.
• The apaxial (above the axis) muscles (back
muscles) are innervated by dorsal primary
rami.
• The hypaxial (below the axis) muscles (body
wall &limb muscles) are innervated by ventral
primary rami.
• The tendons for the attachment of the
muscles to bones are derived from sclerotome
cells lying adjacent to myotomes at the
anterior and posterior borders of somites.
Patterning of muscles
• Patterns of muscle formation are controlled by
connective tissue into which myoblasts migrate.
• In the head region, these connective tissues are
derived from neural crest cells; in cervical &
occipital regions, they differentiate from somitic
mesoderm
; and in the body wall and limbs, they
originate from the parietal layer of lateral plate
mesoderm.
• Head musculature :
All voluntary muscles of the head region are
derived from paraxial mesoderm, including of
the tongue, eye (except that of the iris, which is
derived optic cup ectoderm), and that
associated with the pharyngeal arches .
• Limb musculature:
The 1
st
indication of limb musculature is
observed in the 7
th
week of development as a
condensation of mesenchyme near the base of
the limb buds which is derived from the somites.
• The upper limb buds lie opposite the lower 5
cervical and upper 2 thoracic segments, & the
lower limb buds lie opposite to the lower 4
lumber & upper 2 sacral segments.
• With elongation of the limb buds, the muscle
tissue splits into flexor & extensor
components
• Cardiac muscle is derived from
visceral
splanchnic
mesoderm surrounding the heart
tube.
Myoblasts adhere to one another by special
attachments that later develop to
intercalated
discs
. During later development, a few special
bundles of muscle cells become visible, are
called
purkinje fibers
, form the conducting
system of the heart.
• Smooth muscles are derived from
visceral
splanchnic mesoderm
surrounding gut tube
and its derivatives, and from
ectoderm
(pupillary, mammary gland, and sweat gland
muscles).