Origin and Development of Muscles

Origin

● Smooth muscle is derived from: splanchnic (visceral) mesoderm: GIT smooth muscle, ectoderm: sweat glands, mammillary gland and pupillary muscles

● Cardiac muscle is derived from: splanchnic (visceral) mesoderm surrounding the heart tube.

● Skeletal muscle is derived from: paraxial mesoderm – somites. ⮚ Formed similar to skeletal muscle without the fusion of myoblasts

Skeletal muscles

The somites undergo a process of epithelization, forming a a donut shaped structure of epithelial cells. Each somite differentiate to:

● Ventral part: sclerotome 🡪 bone-forming cells of the vertebrae and ribs.

● Upper part: dermatomes - On each side of these dermatomes are muscle-forming areas. There are 2 areas: 1.Ventrolateral (VLL) muscle forming area. Some cells migrate to the parietal mesoderm 2.Dorsomedial (DLL) muscle forming area. Cells from these 2 areas undergo epithelial to mesenchyme transition and migrate and proliferate ventral to the dermatome, forming the myotome (together dermomyotomes)

Each myotome receives innervation from a spinal nerve according to its original segment. Corresponds with dermatomes as well.

Lateral somitic frontier:

a distinct border (at least in the beginning) between each somite and the parietal mesoderm, separate 2 mesodermal domains:

● Primaxial domain- is the area around the neural tube, containing the somites and their derivatives (back and intercostal muscles, shoulder girdle)

● Abaxial domain- parietal mesoderm and migrated somatic cells (infrahyoid, abdominal and limb muscles)

The lateral somitic frontier also separates:

• The cartilagenous parts of the ribs (parietal mesoderm) from the bony part (somite)
• The dermis formed from the dermatome (somite) and the dermis from the parietal mesoderm.

Molecular mechanism

BMP4 (Bone morphogenic proteins) together with WNT causes VLL cells of dermomyotomes to express MyoD gene. BMP4 is produced in lateral plate mesoderm and ectoderm (it induce WNT proteins production in dorsal neural tube). Simultaneously, low concentration of sonic hedgehog protein (SHH) secreted by notochord and neural tube floor plate – together with WNT cause DLL cells of dermomyotomes to express MYF5 and MyoD. MFR’s (myogenic regulatory factors) both MYF5 and MyoD – cause muscle development. Serum response factor (SRF) causes smooth muscle cell differentiation.