Most muscles arise from paraxial mesoderm in the 3rd week of development. Skeletal muscles derive from paraxial mesoderm, with progenitor cells coming from the dorsolateral and dorsomedial portions of somites. By the 5th week, muscle precursor cells are divided into a small dorsal epimere and larger ventral hypomere. Cardiac muscle develops from splanchnic mesoderm surrounding the heart tube, while smooth muscle arises from surrounding splanchnic mesoderm of the gut and vasculature.

2.  The formation of the muscular system begins
about 4Th week of embryonic development.
 The beginning cells are called Myoblasts.
 Most of our muscles develops from the
mesodermal germ layer
 Except some smooth muscle tissues (pupil,
sweat glands and mammary gland
differentiate from ectoderm)

3.  The muscular system consist of;
1. Skeletal musculature
2. Cardiac musculature
3. Smooth musculature
 Skeletal muscles are derived from paraxial
mesoderm
 This forms;
Somites from the occipital to the
sacral regions
Somitomeres in the head

4.  Smooth muscles differentiate from
splanchnic mesoderm surrounding the gut
and its derivatives.
 Cardiac muscles are derived from
splanchnic mesoderm surrounding the
heart tube

5.  Musculature of the head, axial skeleton and
body wall are formed by Somites and
somitomeres
 From the occipital region caudally, somites form and
differentiate into;
►Sclerotome
►Dermatome
►Two muscle-forming regions
One in the dorsolateral region of the somite
provides progenitor cells for limb and body
wall musculature (hypomeric)
The other in the dorsalmedial region forms the
myotome (epimeric musculature)

7.  Precursor cells, the myoblasts, fuse and
form long, multinucleated muscle fibers
 Myofibrils soon appear in the cytoplasm, and
by the end of the third month, cross-
striations appear in skeletal muscle
 A similar process occurs in the seven
somitomeres in the head region rostral to the
occipital somites

8.  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 and occipital regions they
differentiate from somatic mesoderm; and
 In the body wall and limbs they originate from
somatic mesoderm

9.  By the end of the 5th week prospective muscle
cells are collected into two parts:
 Epimere (small dorsal portion) – innervated by the
dorsal primary ramus
 Hypomere (larger ventral part) – innervated by the
ventral primary ramus
 Myoblasts of the epimeres form the extensor
muscles of the vertebral column, and those of
the hypomeres give rise to muscles of the
limbs and body wall

10. Transverse section through the thoracic
region of a 5-week embryo

11.  Myoblasts from cervical hypomeres form the
scalene, geniohyoid, and prevertebral muscles.
 Those from thoracic segments split into three
layers, which in the thorax are represented by;
 External Intercostal
 Internal Intercostal
 Innermost Intercostal
 In the abdominal wall these three muscle layers
consist of the external oblique, the internal
oblique, and the transversus abdominis
muscles.

12.  Myoblasts from the hypoblast of lumbar
segments form the quadrates lumborum
muscle
 Those from sacral and coccygeal regions form
the pelvic diaphragm and striated muscles of
the anus.
 A ventral longitudinal column arises at the
ventral tip of the hypomeres.
 This column is represented by the rectus
abdominis muscle and the infrahyoid
musculature

13.  All voluntary muscles of the head region are
derived from paraxial mesoderm (somitomeres
and somites);
 Including muscle of the tongue, eye (except that
of the iris, which is derived from optic cup
ectoderm), and that associated with the
pharyngeal (visceral) arches.
 Patterns of muscle formation in the head are
directed by connective tissue elements (Neural
crest cells)

14.  Connective tissue dictates the pattern of
muscle formation in the limb
 Derived from the somatic mesoderm
 The mesenchyme is derived from
dorsolateral cells of the somites that migrate
into the limb bud to form the muscles
 With elongation of the limb buds, the muscle
tissue splits into flexor and extensor
components

15.  The upper limb buds lie
opposite the lower five
cervical and upper two
thoracic segments, and
 the lower limb buds lie
opposite the lower four
lumbar and upper two
sacral segments

16.  As soon as the buds form, ventral primary rami 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
 The radial nerve is formed by a combination of the dorsal
segmental branches
 The ulnar and median nerves are formed by a
combination of the ventral branches.
 Spinal nerves not only play an important role in
differentiation and motor innervation of the limb
musculature, but also provide sensory innervations for the
dermatomes.

17.  Develops from splanchnic mesoderm surrounding the
endothelia 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.

18.  Derived from splanchnic mesoderm
surrounding the endoderm of the gut and its
derivatives
 ciliary muscle and sphincter papillae muscles
of the eye are derived from neural crest
ectoderm
 Vascular smooth muscle differentiates from
local mesoderm adjacent to vascular
endothelium

19.  Most muscles arise from the mesoderm in the 3rd week of embryonic
development
 Skeletal muscles are derived from paraxial mesoderm,
 Progenitor cells for muscle tissues are derived from the dorsolateral and
dorsomedial portions of the somites.
 Cells in the dorsolateral portion migrate to form hypomeric muscle; cells
in the dorsomedial portion migrate ventral to the dermatome to form
the myotome, and ultimately form epimeric musculature.
 By the 5th week muscle precursor cells are divided into a small dorsal
portion, the epimere, innervated by a dorsal primary ramus, and a larger
ventral portion, the hypomere, innervated by a ventral primary ramus

20.  PRIMARY REFERENCE : T.W. SADLER –
LANGMAN’S MEDICAL EMBRYOLOGY - 9TH
AND 12TH EDI.
 Human Embryology and Developmental Biology
– 4th Ed
Bruce M. Carlson, MD, PhD
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