The levator aponeurosis consists of two layers that include smooth muscle.
OBJECTIVE
To investigate the two-fold structure of the levator aponeurosis, which is partly composed of independent smooth muscles.


METHODS
Fifteen upper eyelids of 12 Asian postmortems, with age at death ranging from 72 to 91 years, were examined. In 9 eyelids, posterior lamella tissue of the upper eyelid was removed to observe the stratified structures of the levator aponeurosis. Six full-thickness eyelids were used to observe the attachment site or the continuity between the levator aponeurosis and its surrounding tissues. The eyelids were incised perpendicularly in the center of the eyelid; samples were stained with Masson trichrome and antismooth muscle actin antibody and examined microscopically.


RESULTS
Masson trichrome staining demonstrated the two-layered nature of the levator aponeurosis. The anterior layer was characterized by thick, robust fibrous tissue, and the posterior by thinner fibrous tissue. Although both layers contained muscle structures, the posterior layer contained more than the anterior. Immunostaining with antismooth muscle actin antibody revealed that the muscle in both layers was smooth muscle. The anterior layer continued to the orbital septum and the submuscular fibroadipose tissue; the posterior layers, located in front of Müller muscle and its tendon, attached to the anterior inferior one-third of the tarsus. Part of the anterior layer went through the orbicularis oculi muscle and attached to the subcuticular tissue.


CONCLUSIONS
The levator aponeurosis is stratified, consisting of two layers than contain smooth muscle components in their proximal portions. It pulls mainly the preaponeurotic fat and anterior eyelid lamella. This partially regulates the tension of the eyelid and contributes to the ordered movement of the upper eyelid.
Kakizaki
Hirohiko
H
Department of Ophthalmology, Aichi Medical University, Nagakute-cho, Aichi-gun, Aichi-ken 480-1195, Japan. cosme@d1.dion.ne.jp
Zako
Masahiro
M
Nakano
Takashi
T
Asamoto
Ken
K
Miyaishi
Osamu
O
Iwaki
Masayoshi
M
D000199:Actins / Q000378:metabolism; D000368:Aged; D000369:Aged, 80 and over; D001391:Azo Compounds; D004801:Eosine Yellowish-(YS); D005143:Eyelids / Q000033:anatomy & histology*; D006801:Humans; D008739:Methyl Green; D009130:Muscle, Smooth / Q000033:anatomy & histology* / Q000378:metabolism; D009801:Oculomotor Muscles / Q000033:anatomy & histology* / Q000378:metabolism; D013194:Staining and Labeling / Q000379:methods
D000199:Actins; D001391:Azo Compounds; C017768:trichrome stain; D008739:Methyl Green; D004801:Eosine Yellowish-(YS)