A reappraisal of the origin of human extraocular muscles.

It has previously been suggested that extraocular muscles develop from mesoderm surrounding the head cavities and grow anteriorly into the orbit. A reappraisal of 54 specimens ranging in size from 8 mms to term indicates that human extraocular muscles develop from the mesoderm within the orbit and that these muscles do not develop from the apex of the orbit and grow anteriorly. Furthermore, the origin, belly, and insertion of the extraocular muscles develop contemporaneously, and each individual muscle develops at the same time. The morphogenesis of extraocular muscles includes the stages of mesenchymal cell, early myoblasts cells, myoblast cell, fusion of myoblast cell, myotube cell, and mature muscle cell.