Development of Muscular System

Muscular system develops from the mesoderm germ layer. Mesoderm is formed by cells invaginate through the Primitive streak, Which appears on the surface of Epiblast during the end of second week.

Mesoderm germ layer

When the mesoderm is formed it consists of: Notochord, at the midline Paraxial mesoderm on each side Intermediate mesoderm Lateral plate.

Paraxial mesoderm:

Forms pairs of somites, which are separated blocks of condensed mesenchymal tissues, except in head region, where they are called somitomeres.

Differentiation of Somites The cells of ventro - medial part of the

somite lose their compact appearance and become polymorphous.This part is called sclerotome. Then they surround the notochord and spinal cord, to form the vertebral column.

The dorso-lateral part is called Dermomyotome

Cells from dorsomedial and ventrolateral regions migrate ventral to dermotome to form momyotome.

Cells from ventrolateral region migrate into parietal layer of lateral plate . In combination , somatic cells and lateral plate mesoderm cells form the abaxial mesodermal domain.

Primaxial domain (around the neural tube) is formed from somatic cells only.

Dermomyotome

Dermomyotome consists of two parts: Myotome , and dermotome. The cells of myotome form precursors of skeletal

muscle cells. These cells express a specific genes: Myogenic

determining factors: Myf-5, Myogenin, MyoD, Myf-6.

The expression of these specific genes is under the influence of factors produced from surrounding tissues. FGFs, BMP4, and WNT proteins.

Myotome cell mass: At the end of fifth week divides into: Epimere (epaxial), dorsal; and Hypomere (hypaxial), ventrally.

Epimere is innervated by dorsal ramus . Hypomere is innervated by ventral ramus.

Epimere and Hypomere Epimere forms the extensor muscle of the

vertebral column. Hypomeres form the muscles of the body

and limbs. Hypomeres of the body wall form 3 layers

and a ventral longitudinal column on each side.

Muscles of the limbs Cells from somites migrate to the

developing limbs. Cells of myotomes of several segments

might fuse together. Mesenchymal masses that to form

muscles, divide into: flexor groups, and extensor groups.

Nerves: Upper limb buds lie opposite the lower five

cervical and upper two thoracic segments Lower limb buds lie opposite the lower

lumbar and upper two sacral segments.

Nerves: Limbs muscles are innervated by ventral

rami from adjacent spinal nerves The upper limb buds lie opposite the lower

five cervical and upper two thoracic segments.

Lower limb buds lie opposite the lower 4 lumbar and upper two sacral segments.

Nerves play important role in differentiation of limbs muscles.

Differentiation of Muscle fibers Myotome cells form myoblasts Myoblasts aggregate and fuse to form

cylindrical:myotubes, with nuclei aligned in a central chain.

Synthesis of contractile filaments ( actin &myosin), and formation of z-discs.

7-9 weeks: formation of primary and secondary myotubes. Myofibrils are added constantly and muscle fibers are lengthen by adding sarcomeres to their ends.

9 week: beginning of formation of neuromuscular junctions in the primary tubes.

10 week: Nerve-muscle contacts become functional, nuclei move to the periphery, cross striations are visible.

14-15 week: primary and secondary myotubes are still the majority.

16-17 week: Appearance of tertiary myotubes, small and adherent to the secondary myotubes. They become independent during 18-23 weeks.

Later in fetal life : The remaining population of myoblasts become satellite cells.