Origin of the smooth musculature
The smooth musculature of the viscera, trachea and bronchia, the blood vessels and of glandular tissue, stems from mesoderm cells that immigrate via the primitive streak directly into the vicinity where these organs arise. Initially an animated proliferation phase of the myoblasts begins. Then the cells lengthen; the typical muscle filaments actin and myosin are synthesized and elongated, tapering muscle fibers arise.
In contrast to skeletal muscles, each smooth muscle cell has its own cell nucleus, located in the middle of the cell. The contractile elements are not ordered geometrically.
The internal eye muscles
The internal eye muscles (sphincter and dilator muscles of the pupil) are derivatives of the external layer of the optic cup in the region of the pars iridica retinae. They thus stem from the neural ectoderm. The ciliary muscle possibly stems from material of the neural crest (12th week).