Embryology L8 PDF

Embryology L8 MUSCULAR SYSTEM  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. Smooth muscle differentiates from splanch...

Embryology L8 MUSCULAR SYSTEM  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. Smooth muscle differentiates from splanchnic mesoderm. Cardiac muscle is derived from splanchnic mesoderm.  STRIATED SKELETAL MUSCULATURE Head musculature is derived from seven somitomeres, which are derived from paraxial mesoderm. Musculature of the axial skeleton, body wall, and limbs is derived from somites, the ventral region of each somite forms the sclerotome, the bodyforming cells for the vertebrae and ribs. Cells in the upper region of the somite form the dermatome and two muscle-forming area at the ventrolateral (VLL) and dorsomedial (DNL) lips. Celle from these two areas migrate and proliferate to form progenitor muscle cells than forming the dermomyotome. Some cells from the ventrolateral region also migrate into the adjacent parietal layer of the lateral plate mesoderm. Here they form infrahyoid, abdominal wall, and limb muscle. The remaining cells in the myotome form muscles of the back, shoulder girder, and intercostal muscles.  STRIATED SKELETAL MUSCULATURE There is a border between each somite and the parietal layer of lateral plate mesoderm called the lateral somitic frontier. This frontier separates two mesodermal domains in the embryo: (1) the primaxial domain and (2) the abaxial domain. Muscle cells that cross this frontier and enter the lateral plate mesoderm comprise the abaxial muscle cell precursors and receive many of their signals for differentiation from lateral plate mesoderm; those that remain in the paraxial mesoderm and do not cross the frontier comprise the primaxial muscle cell precursors and receive many of their developmental signals from the neural tube and notochord. Each myotome receives its innervation from spinal nerves derived from the same segment as the muscle cells. INNERVATION OF AXIAL SKELETAL MUSCLES The new description of muscle development characterized by primaxial and abaxial domains differs from the old concept of epimeres (back muscles) and hypomeres (limb and body wall muscles), which was based on a functional definition of innervation: epimeric muscles were innervation by dorsal primary rami; hypomeric muscles by ventral primary rami. The new description is based on the actual embryological origin of muscle cells from two different populations of muscle cell precursors, the abaxial and primaxial cells, and not their innervation. The description does not preclude the fact that epaxial (above the axial) muscles (back muscles ) are innervation by dorsal primary rami, whereas hypaxial (below the axial ) muscles (body wall and limb muscles) are innervation by ventral primary rami.  PATTERNING OF MUSCLES Patterns of muscles 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 somitic mesoderm. in the body wall and limbs, they originate from the parietal layer of lateral plate mesoderm.  HEAD MUSCULATURE All voluntary muscles of the head region are derived from paraxial mesoderm (somitomeres and somites ), including musculature of the tongue, eye, and that associated with the pharyngeal (visceral ) arches. Patterns of muscle formation in the head are directed by connective tissue elements derived from neural crest cells.  LIMB MUSCULATURE The first indication of limb musculature is observed in the seventh week of development as a condensation of mesenchyme near the base of the limb bods. The mesenchyme is derived from dorsolateral cells of the somites that migrate into the limb bud to form the muscles. As in other regions, connective tissue dictates the pattern of muscles formation, and this tissue is derived from the parietal layer of lateral plat mesoderm, which also gives rise to the bones of the limb. With elongation of the limb buds, the muscles tissue splits into flexor and extensor components. Although muscles of the limb are segmental initially, with time, they fuse of the are then composed of tissue derived from several segments.  LIMB MUSCULATURE 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 lumber and upper two sacral segments. 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 ton 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 medial nerves, which supply the flexor musculature, 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 innervation for the dermatomes.  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.  SMOOTH MUSCLE Smooth muscle for the dorsal aorta and large arteries is derived from lateral plate mesoderm and neural crest cells, in the coronary arteries, smooth muscle originates from proepicardial cells and neural crest cells (proximal segments). Smooth muscle in the wall of the gut and gut derivatives is derived from the splanchnic layer of lateral plate mesoderm that surrounds these structures. Only the sphincter and dilator muscles of the pupil and muscle tissue in the mammary and sweat glands are derived from ectoderm. SUMMARY THANK YOU！