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Muscular System
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With the exception of some smooth muscle tissue, the
muscular system develops from the mesodermal germ layer
and consists of skeletal, smooth, and 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 muscles).
Cardiac muscle is derived from splanchnic mesoderm
surrounding the heart tube.
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Striated Skeletal Musculature
Somites and somitomeres form the musculature of
the axial skeleton, body wall, limbs, and head.
From the occipital region caudally, somites form and
differentiate into the sclerotome, dermatome, and
two muscle-forming regions.
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Head Musculature
All voluntary muscles of the head region are derived
from paraxial mesoderm (somitomeres and somites),
including musculature of the tongue, eye (except
that of the iris, which is derived from optic cupectoderm), and that associated with the pharyngeal
arches.
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Limb Musculature
The first indication of limb musculature is observed in theseventh week of development as a condensation of
mesenchyme near the base of the limb buds.
The mesenchyme is derived from dorsolateral cells of the
somites that migrate into the limb bud to form themuscles.
As in other regions, connective tissue dictates the pattern
of muscle formation, and this tissue is derived from
somatic mesoderm, which also gives rise to the bones ofthe limb.
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With elongation of the limb buds, the muscle tissue splits
into flexor and extensor components.
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Although muscles of the limbs are segmental initially,
with time they fuse and are then composed of tissue
derived from several segments.
The upper limb buds lie opposite the lower five
cervical and upper two thoracic segments, and thelower limb buds lie opposite the lower four lumbar
and upper two sacral segments.
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As soon as the buds form, ventral primary rami from the
appropriate spinal nerves penetrate into the
mesenchyme.
At first each ventral ramus enters with isolated dorsal
and ventral branches, but soon these branches unite to
form large dorsal and ventral nerves.
Thus the radial nerve, which supplies the extensor
musculature, is formed by a combination of the dorsal
segmental branches, whereas the ulnar and median
nerves, which supply the flexor musculature, are formed
by a combination of the ventral branches.
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Immediately after the nerves have entered the limb
buds, they establish an intimate contact with the
differentiating mesodermal condensations.
Spinal nerves not only play an important role in
differentiation and motor innervation of the limb
musculature, but also provide sensory innervationfor the dermatomes.
Although the original dermatomal pattern changes
with growth of the extremities, an orderly sequence
can still be recognized in the adult.
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Cardiac Muscle
Cardiac muscle develops from splanchnic mesoderm
surrounding the endothelial heart tube.
Myoblasts adhere to one another by special attachments that
later develop into intercalated discs.
Myofibrils develop as in skeletal muscle, but myoblasts do not
fuse.
During later development, a few special bundles of muscle
cells with irregularly distributed myofibrils become visible.
These bundles, the Purkinje fibers, form the conducting
system of the heart.
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Smooth Muscle
Smooth muscle in the wall of the gut and gut
derivatives is derived from splanchnic mesoderm
surrounding the endoderm of these structures.
Vascular smooth muscle differentiates frommesoderm adjacent to vascular endothelium.
Sphincter and dilator muscles of the pupil and
muscle tissue in the mammary gland and sweat
glands originate from ectoderm.
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Body Cavities
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Formation of the Intraembryonic
Cavity
At the end of the third week, intraembryonic
mesoderm on each side of the midline differentiates
into a paraxial portion, an intermediate portion, and a
lateral plate. When intercellular clefts appear in the lateral
mesoderm, the plates are divided into two layers: the
somatic mesoderm layer and the splanchnic
mesoderm layer. The space bordered by these layers forms the
intraembryonic cavity (body cavity).
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At first the right and left sides of the intraembryonic
cavity are in open connection with the
extraembryonic cavity, but when the body of the
embryo folds cephalocaudally and laterally, this
connection is lost.
In this manner a large intraembryonic cavity
extending from the thoracic to the pelvic region
forms.
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Body Wall Defects
Ventral body wall defects in the thorax or abdomen may
involve the heart, abdominal viscera, and urogenital organs.
They may be due to a failure of body folding, in which case
one or more of the four folds(cephalic, caudal, and two
lateral) responsible for closing the ventral body wall at the
umbilicus fail to progress to that region.
Another cause of these defects is incomplete development of
body wall structures, including muscle, bone, and skin.
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Omphalocele is herniation of abdominal viscera through an
enlarged umbilical ring. The viscera, which may include liver, small and large
intestines, stomach, spleen, or bladder, are covered by
amnion.
The origin of omphalocele is a failure of the bowel to return tothe body cavity from its physiological herniation during the
6th to 10th weeks.
Omphalocele, which occurs in 2.5/10,000 births, is associated
with a high rate of mortality and severe malformations. Chromosomal abnormalities are present in approximately
50%.
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Gastroschisis is a herniation of abdominal contents throughthe body wall directly into the amniotic cavity.
It occurs lateral to the umbilicus, usually on the right, through
a region weakened by regression of the right umbilical vein,
which normally disappears.
Viscera are not covered by peritoneum or amnion, and the
bowel may be damaged by exposure to amniotic fluid.
Gastroschisis occurs in 1/10,000 births but is increasing in
frequency, especially among young women, and this increase
may be related to cocaine use.
Unlike omphalocele, gastroschisis is not associated with
chromosome abnormalities or other severe defects.
The survival rate is excellent.
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