Group 2 - muscular system & body cavities
Joshua Griffith
Ashley Singh
Kamini Reddi
Macela Harding
EMBRYOLOGY OF THE MUSCULAR
SYSTEM
• As with the skeltal system most of the
muscular system also develops from the
mesodermal germ layer (exceptions - some
head muscles e.g. iris derived from neural
crest cells).
• Smooth muscle develops from splanchnic
mesoderm which surrounds gut /derivatives.
• Cardiac muscle develops from splanchnic
mesoderm which surrounds the heart tube.
•
Progenitor cells for muscle tissues are derived from ventrolateral (VLL) and dorsomedial
(DML) edges of the prospective dermomyotome and myotome
•
Myoblasts fuse and form long, multinucleated muscle fibers
•
Myofibrils and cross striations appear in the cytoplasm
•
Tendons are derived from sclerotome cells adjacent to myotomes at the anterior and
posterior borders of somites
Derivatives of Precursor Muscle Cells
•
The epimere and the hypomere
•
Dorsal primary ramus for the epimere and ventral primary ramus for the hypomere
•
Epimere gives rise to extensor muscles of the vertebral column
•
Hypomere gives rise to limb and body wall muscles
•
Cervical hypomeres form the scalene, geniohyoid muscle, prevertebral muscles and
infrahyoid muscles
Derivatives of Precursor Muscle Cells
•
Thoracic hypomere include intercostal muscles (external, internal, innermost or
transversus thoracis)
•
External oblique, internal oblique, transversus abdominis, and rectus abdominis (sternalis
in thorax) in the ventrolateral abdominal wall
•
Lumbar segments (lumborum muscle)
•
Sacral and coccygeal region (pelvic diaphragm and striated muscles of the anus)
Limb Musculature
•
Condensation of mesenchyme near the base of limb buds (7th week)
•
Mesenchyme is derived from dorsolateral cells of somites
•
Migrate into limb bud to form the muscles
•
Connective tissue dictates the pattern of muscle formation
•
Upper limb buds lie opposite the lower five cervical and upper two thoracic segments
•
There is a 180° medial rotation of the lower limb compared to developing upper limb
(angle of flexion differs)
Smooth Muscle
•
Dorsal aorta and large arteries  lateral plate mesoderm and neural crest cells
•
Coronary arteries  proepicardial and neural crest cells
•
Wall of the gut and gut derivatives  lateral plate mesoderm (splanchnic layer)
•
Sphincter and dilator (pupil) muscles, muscle tissue in the mammary and sweat glands 
ectoderm
•
Clinical Correlations
BODY CAVITIES, MESENTERIES & DIAPHRAGM
INTRAEMBRYONIC COELOM
INTRAEMBRYONIC COELOM
•
Appears as isolated spaces in the lateral mesoderm
•
In the 4th week, the spaces fuse to form a single horseshoe-shaped (U-shaped) cavity
•
The coelom divides the lateral mesoderm into:
1. Somatic (parietal) layer: under ectoderm
2. Splanchnic (visceral) layer: over endoderm
•
Somatopleure = somatic mesoderm + overlying ectoderm
•
Splanchnopleure = splanchnic mesoderm + underlying endoderm
•
DERIVATIVES: It gives rise to three body cavities:
1. A pericardial cavity: the curve of U
2. Two pericardioperitoneal canals (future pleural cavities): the proximal parts of the
limbs of U
3. Two peritoneal cavities: the distal parts of the limbs of U
•
Each cavity has a parietal layer (derived from somatic mesoderm) & a visceral layer
(derived from visceral mesoderm)
•
FUNCTION: It provides space for the organs to develop & move
DEVELOPMENT OF PERITONEAL CAVITY
•
Major part of intraembryonic coelom
•
Develop from the distal parts of the limbs of the U-shaped cavity
•
Originally, it is connected with extraembryonic coelom (midgut herniates to the
outside through this connection)
•
At 10th week, it looses its connection with extraembryonic ceolom (when midgut
returns to abdomen)
MESENTERIES
•
A MESENTERY is a double layer of peritoneum that begins as an extension of the
visceral peritoneum covering an organ
•
The mesentery connects the organ to the body wall and transmits vessels and nerves
to it
•
Transiently, the dorsal & ventral mesenteries divide the peritoneal cavity into right
& left halves
•
The ventral mesentery disappears EXCEPT where stomach develops
PERICARDIAL CAVITY
•
Develops from the curve of the U-shaped cavity
•
During formation of head fold, the heart & pericardial cavity move ventrocaudally
& become anterior to the foregut (esophagus)
•
It is bounded by an outer somatic & an inner visceral layer, forming the serous
pericardium
PERICARDIAL CAVITY
•
Originally, it is connected with the 2 pericardioperitoneal canals
•
Later on, it become separated from the 2 pericardioperitoneal canals
PERICARDIAL CAVITY
•
Originally, the bronchial buds are small relative to the heart
•
Bronchial buds grow laterally into pericardioperitoneal canals (future pleural
cavities)
•
Pleural cavities expand ventrally around heart & splits mesoderm into:
1. Outer layer: forms thoracic wall
2. Inner layer: pleuropericardial membrane
PLEUROPERICARDIAL MEMBRANES
•
THE PARTS SURROUNDING THE SEROUS PERICARDIUM: form the fibrous
pericardium
•
THE PARTS BEHIND THE HEART: fuse with the ventral mesentery of the
esophagus (at 7th week), forming the mediastinum & separating pericardial from
pleural cavities
•
N.B.: The right pleural cavity separates from pericardial cavity earlier than left
PLEURAL CAVITIES
•
Develop from the 2 pericardiperitoneal canals
•
Originally, they are connected with pericardial & peritoneal cavities
•
Later on, they become separated from:
1. Pericardial cavity
2.Peritoneal cavity
PLEUROPERITONEAL MEMBRANES•
Produced when developing lungs & pleural cavities expand into the body wall
•
During 6th week, they fuse with dorsal mesentery of esophagus & septum
transversum, separating pleural cavities from peritoneal cavity
•
N.B.: The right pleural cavity separates from peritoneal cavity earlier than left