Explain somite formation. Describe the development of

Dr. Nimir
Dr. Safaa
 Understand the development of muscles (skeletal, cardiac
and smooth).
Explain somite formation.
Describe the development of limb musculature.
Enlist the derivatives of Primaxial & Abaxial domains.
Define the relation of muscle with its nerve supply.
Understand the development of skull.
Understand the development of limbs.
Explain the mechanism of limb innervation.
Discuss the anomalies of the limbs.
Understand the development of vertebrae.
Explain the anomalies of vertebrae.
Understand the development of ribs & sternum.
Development Of Muscular System
 With the exception of some smooth
muscle tissue the muscular system
develops from the mesodermal germ
layer and consists of :
 Skeletal
 Smooth
 Cardiac muscle
 Skeletal muscle is derived from Paraxial
mesoderm,which forms somites from the
occipital to the sacral regions and
somitomeres in the head.
 Smooth muscle differentiates from
splanchnic mesoderm surrounding the gut
and its derivatives and from ectoderm
(pupillary, mammary gland, and sweat gland
 Cardiac muscle is derived from splanchnic
mesoderm surrounding the heart tube.
 Somite formation:
 Somites develop from
paraxial mesoderm as
Seven somitomeres will
form the head musculature
The rest somitomeres will
become somites:
4 occipital, 8 cervical, 12
thoracic, 5 lumbar, 5 sacral,
and 8 to 10 coccygeal pairs.
The first occipital and the
last five to seven coccygeal
somites later disappear.
Somites form the axial
 The ventral region of each
somite forms the sclerotome
(bone-forming cells for the
vertebrae and ribs).
 Cells in the upper region of
the somite form the
dermatome and two muscleforming areas at the
ventrolateral (VLL) and
dorsomedial (DML) lips (or
 Cells from these two areas
form muscle cells ventral to
the dermatome, thereby
forming the
 Some cells from VLL migrate into parietal layer of
lateral plate mesoderm. Here they form:
 Infrahyoid, abdominal wall (rectus abdominus,
internal and external oblique, and transversus
abdominus), and limb muscles.
 The remaining cells in the myotome form muscles of
the back, shoulder girdle, and intercostal muscles.
 The border between each somite
and the parietal layer of lateral
plate mesoderm called the
lateral somitic frontier.
 The lateral somitic frontier
separates two mesodermal
domains in the embryo:
 1-The primaxial domain that
comprises the region around
the neural tube and contains
only somite-derived (paraxial
mesoderm) cells.
 2- The abaxial domain that
consists of the parietal layer of
lateral plate mesoderm together
with somite cells that have
migrated across the lateral
somitic frontier.
 Muscle cells that cross the
frontier (those from the VLL
edge of the myotome) and
enter the lateral plate
mesoderm comprise the
abaxial muscle cell precursors.
 Muscle cells that remain in the
paraxial mesoderm and do not
cross the frontier (the
remaining VLL cells and all of
the DML cells) comprise the
primaxial muscle cell
Origins of Muscles from Abaxial and Primaxial Precursors
Cervical region
Thoracoabdominal region
Pectoralis major and minor
External oblique
Internal oblique
Transversus abdominus
Rectus abdominus
Pelvic diaphragm
Upper limb
Levator scapulae
Latissimus dorsi
Distal limb muscles
Origins of the Craniofacial Muscles
Mesodermal Origin
Somitomeres 1 and 2
Superior, medial,
ventral recti
Oculomotor (III)
Somitomere 3
Superior oblique
Trochlear (IV)
Somitomere 4
Jaw closing
Trigeminal (V)
Somitomere 5
Lateral rectus
Abducens (VI)
Somitomere 6
Jaw opening, other
second arch
Facial (VII)
Somitomere 7
Somites 1 and 2
Intrinsic laryngeals
Intrinsic laryngeals
Somites 2 to 5a
Hypoglossal (XII)
Somites 2 to 5 constitute the occipital
group (somite 1 degenerates for the most
 Limb musculature:
 Appears about the seventh
week of development as
mesenchyme near the base of
the limb buds.
 The mesenchyme is derived
from dorsolateral cells of the
 Connective tissue that dictates
the pattern of muscle
formation, is derived from the
parietal layer of lateral plate
mesoderm, which also gives
rise to the bones of the limb.
Innervation of skeletal muscles
 The new description of muscle
development (primaxial and
abaxial domains) differs from the
old concept (epimeres and
hypomeres ), which was based on
 The new description is based on
the actual embryological origin of
muscle cells.
 Epaxial (above the axis) muscles
(back muscles) are innervated by
dorsal primary rami, whereas
hypaxial (below the axis) muscles
(body wall and limb muscles) are
innervated by ventral primary
Congenital anomalies:
 Partial or complete absence
of one or more muscles is
common.Examples are:
 Total or partial absence of
the pectoralis major muscle
(Poland anomaly) .
 Similarly, the palmaris
longus, the serratus
anterior, and the quadratus
femoris muscles may be
partially or entirely absent.
 Partial or complete absence
of abdominal musculature
is called prune belly
syndrome .
 Poland anomaly. The left
pectoralis major muscle
is absent.
Prune belly syndrome:
 a distended abdomen
from atrophy of
abdominal wall

similar documents