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DEVELOPMENT SMOOTH MUSCLES

• Smooth muscle fbers dierentiate rom splanchnic mesenchyme surrounding

the endoderm o the primordial gut and its derivates.

•  The smooth muscles in the walls o many blood and lymphantic vessels arise

rom somatic mesoderm.

•  The muscles o the iris (sphincter and dilator pupillae) and the myoepithelial

cells and sweat glands are thought to be derived rom mesenchymal cells

that originate rom ectoderm.

•  The frst sign o dierentiation o smooth muscle is the development o

elongated nuclei in spindle-shaped myoblasts.

• During early development new myoblasts continue to dierentiate rom

mesenchymal cells but do not use they remain mononucleated.• During later development division o e!isting myoblasts gradually replaces

the dierentiation o new myoblasts in the production o new smooth muscle

tissue.

• "s smooth muscle fbers develop into sheets or bundles they receive

autonomic innervations.

• #ibroblasts and muscle cells synthesi$e and lay down collagenous elastic and

reticular fbers .

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•  The appendicular s1eleton consists o the pectoral and pelvic girdles and the limb

bones.

• *esenchymal bones orm during the fth wee1 as condensations o mesenchyme

appear in the limb buds.

• During the si!th wee1 the mesenchymal bone models in the limbs undergo

chondrifcation to orm hyaline cartilage bone models .

•  The clavicle initially develops by membranous ossifcation and it later orms growthcartilages at both ends.

   The models o the pectoral girdle and upper limb bones appear slightly beore those

o the pelvic girdle and lower limb bones.

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