Absence of Intramuscular Elastin Correlates with Independent Mechanical Behavior of Extraocular Muscle (EOM) Compartments During Active and Passive Mechanical Loading.

Abstract

 

Purpose

 

Intramuscular innervation of horizontal rectus EOMs is segregated into superior & inferior (transverse), compartments while all EOMs are also divided into global (GL) & orbital (OL) layers with scleral & pulley insertions. Mechanical coupling among EOM compartments was investigated during passive tensile and active loadings, and correlated with histological distribution of elastin, a connective tissue constituent localized to high mechanical stress.

 

Methods

 

Bovine rectus EOMs were rapidly removed after slaughter. For passive loading, one compartment was extended, while the other was stationary, as forces in both channels were monitored. For active contraction, the EOM was immersed in 50 mM CaCl₂ labeled with fluorescein while one compartment was coated with petrolatum to prevent permeation; contraction was in the other compartment while tensions in both channels were monitored. Fresh EOMs were then sectioned transversely for yellow-green fluorescence photography under blue light, and a formalin-fixed whole orbit was transversely sectioned and stained with elastin van Geison stain for histology.

 

Results

 

There was abundant elastin in pulley tissues, but intramuscular elastin only in blood vessels, not in intercompartmental boundaries. (Fig. A). Fluorescein penetration confirmed selective CaCl₂ permeation inducing selective active contraction (Fig. B). For both passive elongation and active contraction, intercompartmental force coupling was always ≤10%. Active tensions in GL and OL could change in opposite directions (Fig. C).

 

Conclusions

 

Absence of elastin in intercompartmental regions of EOMs correlates with minimal active and passive force coupling. Marked mechanical independence of EOM compartments under both passive and active loading endows each EOM with potential for separately controllable actions required by the active pulley & transverse compartmental hypotheses.  

 

A. Low magnification cross section of lateral rectus muscle. A.(a) Boundary between OL and GL. A(b) Pulley contains abundant elastin (black). A(c) Elastin present only in blood vessel walls, not among muscle fiber bundles.<br /> B. Cross section showing fluorescein permeated only GL of lateral rectus muscle. OL was coated by petrolatum.<br /> C. in B. After initial buoyancy effect, force increased only in GL, while decreasing in OL.