Embryology: Musculo-Skeletal

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Epithelial like cells of somite (medial & ventral walls):

Differentiate into connective tissue (cartilage and bone)

Epithelial like cells of somite (dorsal & lateral walls):

Dorso-medial & dorso-lateral border of dermomyotomes called Myotome differentiates to skeletal muscle

Central region of dermomyotome called Dermatome differentiates to the dermis of the skin but the major contribution is from somatic mesoderm.

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Skeletal System

3 distinct lineages generate the skeleton: 1. Somites provide axial (vertebral) skeleton, 2. Lateral plate mesoderm provided limb skeleton, 3. Cranial neural crest give rise to branchial arches and craniofacial bones and cartilage.

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Axial Skeleton

Axial skeleton (Vertebral column, ribs and sternum) Derived from sclerotomes of the somites and induced by the notochord.

Vertebral column:

Transverse migration of sclerotome cells to form vertebra around the notochord

Vertebral arch forms from migrating cells of left & right somites.

Vertebral body forms from TWO adjacent somitic sclerotomes (2 segments) & notochord: Allows segmental spinal nerves to emerge through foramen between vertebrae

Scleroterms to vertebrae:

longitudinal fusion of sclerotomes

condensing into dense & diffuse zones

Separation into vertebrae

Junction into disc tissues (around notochord)

Nerve out through junction

Sclerotomes migrate to form vertebrae

Myotomes migrate to form muscle

Notochord regresses & combines with 2 sclerotomes to form intervertebral disc

Neural crest cells migrate to form ganglions & nerves to follow muscles

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Ribs:

Develop from Sclerotome - mesenchymal cells of costal processes of thoracic vertebrae

The distal end will extend toward the ventral midline depending on species, the number of ribs articulate with the sternum in order to form costal arch

Sternum:

Develop from paired sternal mesenchymal cells (Somatic mesoderm) to form cartilage & ossified to be sternum (Manubrium (cranial) and Xiphisternum or Xiphoid (caudal)

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Appendicular skeleton (limbs)

Axial skeleton forms a protective casing around soft internal tissues. In contrast, appendicular skeleton forms a central supportive core of the limbs.

Therefor, apical ectodermal ridge of the limb bud is required for the limb formation.

Limb bud development begins towards the end of the third-fourth week of gestation in domestic species.

Process:

Limb bud begins activation of mesenchymal cells of the somatic mesoderm in multiple areas.

Limb bones, connective tissues, blood vessels form from lateral plate mesoderm

Limb skin forms from ectoderm

Limb muscles form from myotomes of somites

Limb nerve forms from neural crest

The limb bud elongates and becomes flattened in dorsoventral plane of the embryo. Then, the limbs rotate approximately 90 degrees along proximodistal axis.

Development of digit is the same for all domestic mammals but become modified by species specific changes

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Joints:

Fibrous joint: development of mesenchymal cells differentiate into fibrous connective tissue in inter-zonal region & attach at the end of opposing bones to each other

Cartilaginous joint: mesenchymal cells in inter-zonal region differentiate into hyaline or fibrocartilage to join bones together

Synovial joint: mesenchymal cells in the centre of inter-zonal region undergo apoptosis leaving a central space (synovial cavity) & outer cells form ligament and double layered joint capsule. The inner layer forms a secretory epithelial tissue to produce synovial fluid in the joint. – For flexion, extension, rotation, adduction & abduction

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Skull:

Development of head skeleton is divided into two parts:

Neurocranium builds a protective case around the brain

Viscerocranium forms the skeleton of the face, oral cavity, pharynx and upper respiratory passages.

The bones of skull originate from several different sources such as somites and neural crest.

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Muscle Systems

Muscles: Skeletal, cardiac & smooth muscles develop from mesoderm. – Splanchnic mesoderm forms cardiac m. & smooth m. of the gut and respiratory tracts while Myotome of somite forms skeletal muscles of the trunk, neck and limbs. Other smooth muscles originate from local mesoderm.

Myotome proliferate and rise to progenitor muscle cells called myoblasts

Myoblasts from dorso-medial region of myotome forms a structure called epimere while dorso-lateral forms hypomere

Epimere is located dorsal to transverse process of vertebae while hypomere is located ventral to the process

Epimere forms extensor muscles of the vertebral column

Hypomere forms limb and three abdominal muscles (external & internal oblique abdominal & transverse abdominal muscles) including muscles of lumbo-sacral region