Inferior oblique muscle of the eye: its fetal development with special reference to understanding of the frequent variants in adults

To provide better understanding of frequent variations of the inferior oblique (IO) of adult extraocular muscles, we observed sagittal and horizontal histological sections of the eye and orbits from 32 fetuses (approximately 7-34 weeks of gestational age; 24-295 mm of crown-rump length). In early fetuses (7-8 weeks), the IO was restricted at an antero-infero-medial angle of the future orbit. In contrast to extraocular recti, the IO appeared to extend along the mediolateral axis and had no definite tendon. At midterm, the IO tendon became evident. Sometimes, the IO muscle belly attached to the inferior rectus or, the IO tendon divided into two laminae to enclose the lateral rectus. At late-term, a multilayered sheath was evident around the sclera and, via one or some of the fascial layers, the IO was communicated with a fascia enclosing the inferior rectus. At midterm and late-term, the IO originated not only from the maxilla near the orbicularis oculi origin but also from a vein-rich fibrous tissue around the lacrimal sac. Both origins were muscular without intermittent tendon or ligament. Therefore, the fascial connection as well as a direct contact between the IO and the inferior or lateral rectus seemed to provide variant muscular bridges as reported in adults. Moreover, the two attachment sites at the origin seemed to provide double muscle bellies of the adult IO. Consequently, the present specimens contained seeds of any types of adult variations. The muscle fibers from the lacrimal sac might play a role for the lacrimal drainage.