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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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