Extraocular Muscle Repair and Regeneration

Abstract

Purpose of review: The goal of this review is to summarize the unique regenerative milieu within mature mammalian extraocular muscles (EOMs). This will aid in understanding disease propensity for and sparing of EOMs in skeletal muscle diseases as well as the recalcitrance of the EOM to injury.

Recent findings: The EOMs continually remodel throughout life and contain an extremely enriched number of myogenic precursor cells that differ in number and functional characteristics from those in limb skeletal muscle. The EOMs also contain a large population of Pitx2-positive myogenic precursor cells that provide the EOMs with many of their unusual biological characteristics, such as myofiber remodeling and skeletal muscle disease sparing. This environment provides for rapid and efficient remodeling and regeneration after various types of injury. In addition, the EOMs show a remarkable ability to respond to perturbations of single muscles with coordinated changes in the other EOMs that move in the same plane.

Summary: These data will inform Ophthalmologists as they work toward developing new treatments for eye movement disorders, new approaches for repair after nerve or direct EOMs injury, as well as suggest potential explanations for the unusual disease propensity and disease sparing characteristics of human EOM.

Keywords: Pax7; Pitx2; extraocular muscle; satellite cells; strabismus surgery.

Figure 1

Photomicrograph of (A) tibialis anterior and (B) extraocular muscle from the Pax7-lineage reporter mouse (red) immunostained for dystrophin (blue). Arrows indicate cell filopodia

Figure 2

Pax7 cells from a Pax7-tdTomato mouse isolated using flow cytometry analyzed (A) as a percent of all live mononuclear cells and (B) as number per milligram (mg) muscle weight. # indicates significant difference from soleus. * indicates significant difference from both diaphragm and EOM. Data analyzed with an ANOVA followed by a Tukey’s multiple comparisons test. Significance is p<0.05.

Figure 3

Photomicrograph of (A) triceps and (B) extraocular muscle from a Pax3 lineage reporter mouse (red) stained with dapi (blue). (A) Arrows indicate Pax3-positive nuclei (pink). Inset shows the Pax3-positive cells (pink). Bar is 75µm. Red myofibers indicate a previous contribution of Pax3-positive cells. (B) The dotted green line represents the edge of the sclera of the globe.

Figure 4

Photomicrograph of mouse EOM immunostained for pax7 (green) and wheat germ agglutinin (red). Pax7-positive cells can be seen both inside the sarcolemma (vertical arrow) and outside the sarcolemma (horizontal arrows). Bar is 20µm.

Figure 5

Photomicrograph of cross-sections stained for hematoxylin and eosin of mouse EOM at 18 months of age from an (A) mdx4cv mouse model of Duchenne muscular dystrophy and (B) an mdx4cv;mck-cre+/−;Pitx2fl/fl mouse. Note the relatively normal morphology in the mdx mouse EOM and the severe pathology in the mdx4cv;mck-cre+/−;Pitx2fl/fl EOM. Vertical arrow shows fatty infiltration, horizontal arrow denotes what is left of the levator palpebrae superioris muscle, which normally is affected in the mdx mouse. Arrow head indicates a centrally nucleated myofiber, which is a hallmark of the process of degeneration/regeneration that occurs in diseased or injured skeletal muscles. (C) Cross-section through the tibialis anterior of an mdx4cv mouse at 18 months of age. Bar is 100µm.

Figure 6

Cartoon of a single myofiber from the EOM in cross-section with the myogenic precursor cells that have been identified thus far indicated. Red: Pax7; Green: MyoD; Purple: EECD34; Orange: Pitx2; Blue: PW1; Black: myonucleus.