MUSCULAR SYSTEM DEVELOPMENT (Human Embryology)

The formation of the muscular system begins about 4th week of embryonic development.

1. DEVELOPMENT OF THE MUSCULAR SYSTEM HUMAN EMBRYOLOGY (ZOO 404) Presented by : Fasama H. Kollie Antoinette H. Wright STELLA MARIS POLYTECHNIC MOTHER PATERN COLLEGE OF HEALTH SCIENCES APRIL 4, 2017

2. OVERVIEW The formation of the muscular system begins about 4 Th 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. OVERVIEW The muscular system consist of; Skeletal musculature Cardiac musculature Smooth musculature Skeletal muscles are derived from paraxial mesoderm This forms; Somites from the occipital to the sacral regions Somitomeres in the head

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

5. STRIATED SKELETAL MUSCULATURE 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 T wo 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)

6. P recursor 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

7. PATTERNING OF MUSCLES 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

8. DERIVATIVES OF PRECURSOR MUSCLE CELLS By the end of the 5 th 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

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

10. 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.

11. 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

12. HEAD MUSCULATURE 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 )

13. LIMB MUSCULATURE 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

14. 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

15. 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.

16. CARDIAC MUSCLE 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.

17. SMOOTH MUSCLE D erived 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

18. SUMMARY Most muscles arise from the mesoderm in the 3 rd 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 5 th 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

19. REFERENCE PRIMARY REFERENCE : T.W. SADLER – LANGMAN’S MEDICAL EMBRYOLOGY - 9 TH AND 12 TH EDI. Human Embryology and Developmental Biology – 4 th Ed Bruce M. Carlson, MD, PhD Copyright © 2009, 2004, 1999, 1994 by

20. THE END Thanks for your attention

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