Presence and morphological variability of Golgi tendon organs in the distal portion of sheep extraocular muscle

Abstract

This study was undertaken to demonstrate the presence of Golgi tendon organs (GTOs) in the distal portion of sheep extraocular muscle (EOM) and to describe the morphological variability of these receptors. Extraocular muscles of a young and an old sheep were perfusion fixed and/or immersion fixed. Tissue was prepared for light microscopy and transmission electron microscopy. Immunohistochemistry was done to demonstrate the myosin pattern of the intracapsular muscle fibers of the GTOs. All GTOs in the distal portions of the sheep EOMs were located in a distinct muscle layer which was designated in a former investigation as the so-called peripheral patch layer. Each EOM of the young sheep contained GTOs; between four and 15 GTOs were counted in the rectus EOMs. Eight GTOs were found in the superior rectus of the old sheep. Golgi tendon organs in EOMs of the young and the old sheep did not differ in their morphology. In the young sheep the mean length of the GTOs was 447 +/- 132 microm (n = 60) and their mean width 101 +/- 26 microm (n = 60). In the old sheep values were 576 +/- 188 microm (mean length, n = 8) and 103 +/- 18 microm (mean width, n = 8). The GTOs were encapsulated by perineurial cells. In 12 GTOs, only collagen bundles were inside. In the remaining GTOs (56), intracapsular muscle fibers were present. Muscle fibers entered the proximal poles of the GTOs and either terminated inside the receptors or muscle fibers left the GTOs at their distal poles. These intracapsular muscle fibers were of the multiply-innervated type. In the GTOs variably shaped nerve terminals were found which contained a high number of mitochondria. In two GTOs, additionally, nerve terminals with aggregates of densely packed vesicles were present.