Morphology and accommodative function of the vitreous zonule in human and monkey eyes

The age-related loss of accommodation (presbyopia) in human and nonhuman primates clearly involves loss of deformability of the crystalline lens. In addition, ciliary muscle mobility, as measured by both histologic1,2 and ultrasonographic (monkey)3 and magnetic resonance (human)4 imaging techniques, also diminishes with age. However, the isolated ciliary muscle in nonhuman primates maintains its full ability to contract in response to appropriate pharmacologic stimulation throughout life,1,2,5 and its contractile machinery and receptor biology also remain normal.6,7 Therefore, posterior restriction of muscle movement in the aging eye has been hypothesized to play a role in the pathophysiology of presbyopia as well, at least in monkeys.8 In addition, there are zonular attachments to the vitreous membrane in both species that have not yet been fully investigated. Earlier studies were focused mainly on the anterior hyaloid membrane. Wieger9 was among the first to describe attachments of the zonules to the anterior vitreous membrane. They were later termed Wieger's ligament.10–12 Streeten and Pulaski13 doubted the strength of Wieger's ligament, because, after lens extraction, it is not uncommon for a superficial flap of either the capsule or anterior hyaloid to be torn from Wieger's area.13 Bernal et al.,14 using a combination of environmental scanning electron microscopy (ESEM) combined with a custom-made manual lens stretcher to facilitate inside microscopy, were able to further analyze the complex connection between the anterior vitreous membrane and zonule.14 Attachments between the posterior pars plana zonule and the vitreous membrane have been observed in histologic sections and by SEM.15–17 Improved in vivo imaging by ultrasound biomicroscopy (UBM) allows visualization of zonular fibers in the living eye.18 Using UBM, we observed a straight line parallel to but separated from the pars plana by a cleft approximately 2 mm wide, extending from the region of the ciliary processes to the region of the ora serrata (see Fig. 1). There has been debate as to whether the structure observed by UBM was (1) the pars plana zonule, which in the living eye is normally separated from the pars plana ciliary epithelium, but after fixation for histology or air drying for scanning electron microscopy (SEM) becomes apposed to the pars plana ciliary epithelium (CE)19; (2) the vitreous membrane20; or (3) another component of the vitreozonular system that has not been well described or characterized. Figure 1. UBM overview image in a live rhesus monkey shows a prominent straight line (arrow) extending from the pars plicata region of the ciliary body to the ora serrata region and separated from the pars plana epithelium by a cleft. CP, ciliary processes, CB, ... We investigated the zonule, the posterior vitreous, and their attachments by novel scanning SEM and UBM techniques, and we now can hypothesize about how this system may affect ciliary muscle movement. In addition, we provide preliminary physiological evidence that ciliary muscle movement may be increased by perturbing the system.