Fascicular specialization in human and monkey rectus muscles: evidence for anatomic independence of global and orbital layers

Abstract

Purpose: Connective tissue pulleys inflect the extraocular muscles (EOMs) and receive insertions from some fibers. The authors investigated insertions and anatomic relationships of fiber fascicles within rectus EOMs to clarify the relationship to their pulleys.

Methods: Two human and two monkey orbits were removed intact, serially sectioned in the coronal plane, histologically stained, and digitally photographed. The authors traced representative fascicles in the human medial rectus (MR) and inferior rectus and monkey lateral rectus and superior rectus muscles. In the human MR, the authors computed average collagen fractions in the orbital layer (OL) and the global layer (GL).

Results: In human and monkey, OL fascicles remained distinct from each other and from the GL throughout. Most OL fascicles were inserted into the pulley through short tendons. Most GL fascicles bypassed the pulley without insertion. Collagen content in the human MR OL increased from 29% +/- 5% (SD) in midorbit to 65% +/- 9% in the anterior orbit but slightly decreased from 26% +/- 6% to 23% +/- 1% in the GL. Tracing of every fiber in a human MR OL fascicle demonstrated terminations on pulley tendons without myomyous junctions.

Conclusions: Fibers in the primate rectus OL lack myomyous or GL junctions, but nearly all insert on the pulley through a broad distribution of short tendons and dense intercalated collagen. Fibers in the GL generally do not insert on pulley tissues and are associated with less collagen. These features support the distinct role of the OL in anteroposterior positioning of connective tissues proposed in the active pulley hypothesis and substantial mechanical independence of the OL and GL.

Figure 1

Fascicles sampled in OL and GL of a 17-month-old human MR muscle (17.97 mm posterior to the corneal surface). All numbered fascicles were initially selected in this section, and each was traced anteriorly to its insertion. Serial tracing showed that seven of eight fascicles initially classified as OL (1–8) inserted on the pulley by way of short tendons distributed over a wide anteroposterior extent. The exception, fascicle 8, was a global surface fascicle that inserted on the insertional tendon and that might have been misclassified because 8 is actually on the global surface. Of six fascicles initially classified as GL (11–16), five bypassed the pulley without insertion, and GL 11, located at the OL-GL border, inserted on the pulley (blue line; OL-GL border, based on fiber size and color).

Figure 2

Collagen fraction in the GL and OL of a 17-month-old human MR muscle. Collagen content in the OL increased from 29% ± 5% in midorbit to 65% ± 9% in anterior orbit but slightly decreased from 26% ± 6% to 23% ± 1% in the GL.

Figure 3

Collagen fraction plotted in comparison with muscle fiber number in a representative fascicle from a 17-month-old human MR OL. When traced anteriorly, the collagen fraction increased as the number of muscle fibers decreased to zero, and the fiber ultimately terminated in a tendon continuous with the pulley.

Figure 4

Fascicle 4 in a 17-month-old human MR OL inserts on the pulley by way of a short tendon. Scale bars apply to corresponding rows (Masson trichrome stain, Movie 1). (A) 17.97 mm posterior to the corneal surface, the fascicle consists of a well-demarcated bundle (arrows) containing 120 individual fibers. (B) 13.79 mm posterior to the corneal surface, all muscle fibers terminated in a pulley tendon (arrows). (C) 13.70 mm posterior to the corneal surface, van Gieson elastin stain shows heavy deposits of black elastic fibers (arrows) at the site of the fascicle's insertion into the pulley tendon. (D) Higher magnification of fascicle in (A). (E) Higher magnification of fascicle termination in pulley tendon in (B). (F) Higher magnification of elastin fibers (black) at fascicle insertion into the pulley tendon.

Figure 5

Masson trichrome stain showing tracing of individual fibers in a 17-month-old human MR OL to their terminations onto a pulley tendon. The transition to tendon is gradual, with individual fibers replaced by collagen at varying distances. Arrow: muscle fiber 14. (A) 16.99 mm posterior to corneal surface, fiber 14 has large diameter and is not heavily invested with collagen. (B) 16.77 mm posterior to corneal surface, fiber 14 becomes smaller and heavily invested by collagen. (C) 16.76 mm posterior to corneal surface, fiber 14 is no longer visible but is replaced by collagenous pulley tendon.

Figure 6

Serial tracing of fascicle in the inferior rectus muscle of a 4-year-old human, in coronal 10-μm–thick sections stained with Masson trichrome. (A) 20.61 mm posterior to corneal surface, low-power view shows the selected wedge-shaped fascicle (arrow) in the middle of the orbital surface of the muscle. Bone of the orbital floor stains red. (B) 20.61 mm posterior to corneal surface, higher-power view shows fascicle delineated by arrow. Scale bar, 300 μm (B–F). (C) 19.01 mm posterior to corneal surface, the fascicle (arrow) was well delineated by a collagen sheath. (D) 18.11 mm posterior to corneal surface, some fibers within the fascicle (arrow) were replaced by collagenous tendon, while many remaining fibers were individually surrounded by collagen. (E) 17.62 mm posterior to the corneal surface, more fibers were replaced by collagenous tendon (arrow). (F) 15.01 mm posterior to the corneal surface, the tendinous termination of the fascicle merged with the collagen of the IR pulley (arrow).

Figure 7

Serial tracing of fascicles in the lateral rectus muscle of a 2-year-old macaque monkey. (A) 15.57 mm posterior to corneal surface, we selected one OL fascicle (green arrowheads) and one GL fascicle (yellow arrowheads) for tracing. Masson trichrome stain. (B) Higher-power view of GL fascicle shown in (A). (C) Higher-power view of OL fascicle shown in (A). (D) 10.46 mm posterior to corneal surface, the OL fascicle inserted on the pulley through a short tendon (green arrowheads). van Gieson elastin stain shows dense, black-staining elastin anchoring the OL fibers in the pulley tendon. (E) High magnification of (D) shows abundant elastin fibers in the pulley. van Gieson elastin stain. (F) 11.96 mm posterior to corneal surface, the GL fascicle remained deep in the muscle and bypassed the pulley. Note the black elastin fibers in the connective tissue sleeve at this level (van Gieson elastin stain).

Figure 8

Serial tracing of fascicle in the superior rectus (SR) muscle of an 11-year-old nemestrina monkey, in coronal 10-μm–thick sections stained with Masson trichrome. (A) 21.81 mm posterior to the corneal surface, low-power view shows that the selected fascicle lies on the orbital surface of the muscle inferior to the levator palpebrae superioris muscle. Dilated superior ophthalmic vein lies inferior to the SR. (B) 21.81 mm posterior to the corneal surface, higher-power view shows selected fascicle (arrow) demarcated by thin fissure. Scale bar, 300 μm (B–E). (C) 17.42 mm posterior to the corneal surface, the selected fascicle (arrow) remains among the smaller and darker-staining fibers of the orbital layer (OL), which is demarcated from the larger and brighter-staining fibers of the global layer (GL) by a thin fissure. Note the similarity of the GL fibers to levator fibers, separated from the SR by the thick collagen sheath of the pulley. (D) 15.82 mm posterior to the corneal surface, the OL fascicle remains distinct from the GL by a wider fissure between them. (E) 14.22 mm posterior to the corneal surface, fibers of the traced fascicle transition to a short, collagenous tendon that begins to merge with the dense collagen of the pulley. The OL is separated from the GL by a wider fissure. (F) 14.22 mm posterior to the corneal surface at higher power, OL fibers of the fascicle are surrounded by dense collagen contiguous with the pulley.