The extraocular muscles arise from mesoderm in somitomeres (preotic mesodermal segments), with the primitive muscle cone first appearing at 5 to 7 weeks gestation (Fig. 1-13). The oculomotor innervated muscles originate from the first and second somitomeres, the superior oblique muscle from the third somit-omere, and the lateral rectus muscle from the fifth somito-mere.77 The motor nerves to the extraocular muscles grow from the brain to the muscle and innervate the mesodermal condensation at approximately 1 month.
The extraocular muscles develop from local mesenchyme (mesodermal origin) in situ within the orbit, rather than from anterior growth of surrounding mesoderm, as had been an earlier hypothesis. Additionally, muscles do not grow from the orbital apex anteriorly; rather, the insertion, belly, and origin develop simultaneously.93,94 The orbital axes rotate from the early stages of optic cup development to adulthood.
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