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Original article
Submitted: 2021-03-13
Accepted: 2021-04-09
Published online: 2021-04-28
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Inferior oblique muscle of the eye: its fetal development with special reference to understanding of the frequent variants in adults

Z. W. Jin1, S. Umeki2, Y. Takeuchi2, M. Yamamoto2, G. Murakami23, S. Abe2, J. F. Rodríguez-Vázquez4
DOI: 10.5603/FM.a2021.0043
·
Pubmed: 33954958
Affiliations
  1. Department of Anatomy, Wuxi School of Medicine, Jiangnan University, Wuxi, China
  2. Department of Anatomy, Tokyo Dental College, Tokyo, Japan
  3. Division of Internal Medicine, Cupid Clinic, Iwamizawa, Japan
  4. Department of Anatomy and Embryology, School of Medicine, Complutense University, Madrid, Spain

open access

Ahead of Print
ORIGINAL ARTICLES
Submitted: 2021-03-13
Accepted: 2021-04-09
Published online: 2021-04-28

Abstract

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.

Abstract

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.

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Keywords

inferior oblique muscle of the eye, inferior rectus, lateral rectus, human fetus, muscular bridge, double muscle bellies

About this article
Title

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

Journal

Folia Morphologica

Issue

Ahead of Print

Article type

Original article

Published online

2021-04-28

Page views

675

Article views/downloads

333

DOI

10.5603/FM.a2021.0043

Pubmed

33954958

Keywords

inferior oblique muscle of the eye
inferior rectus
lateral rectus
human fetus
muscular bridge
double muscle bellies

Authors

Z. W. Jin
S. Umeki
Y. Takeuchi
M. Yamamoto
G. Murakami
S. Abe
J. F. Rodríguez-Vázquez

References (15)
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