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C.S. Nickerson, H.L. Karageozian, J. Park, J.A. Kornfield; Chemical Resilience of the Vitreous and Vitreoretinal Interface . Invest. Ophthalmol. Vis. Sci. 2005;46(13):5165.
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Purpose: Understanding the molecular–level interactions responsible for the structure of the vitreous and its adhesion to the retina is of fundamental and therapeutic interest, providing insight into natural liquefaction and directions for pharmacologic vitrectomy. To elucidate the key chemical interactions required for vitreoretinal adhesion and gel network stability, the present study examines the relative importance of covalent bonds, hydrogen bonds, and electrostatic interactions in the vitreous and vitreoretinal interface. Methods: Whole vitreous specimens were gently removed from fresh porcine eyes (<36 hrs. post mortem) and placed in 10 mL solutions containing one of the following: collagenase or hyaluronidase (cleave covalent bonds), carbamide or 0.9% saline with pH varied (disrupt H–bonds), or NaCl (screens electrostatic interactions). The retinas of some vitreous specimens were left attached, to evaluate the effect of treatment on vitreoretinal adhesion. Samples denuded of retina were weighed, incubated at 37°C for 24 hrs. and reweighed to determine mass loss. Specimens with attached retinas were incubated at 5°C for up to 1 hr.; retinas were then carefully peeled and examined microscopically. Results: Collagenase digestion destroyed the vitreous, leaving a viscous liquid. In contrast, hyaluronidase digestion merely softened and reduced the weight (∼40%) of the vitreous body. As a function of incubation conditions there is a sharp transition from moderate weight loss (∼50% or less) to dramatic weight loss (98% or more) with pH (3≤pH≤10 gives moderate loss; pH=1.5 or 12.5 gives "catastrophic collapse") and carbamide concentration (<5M, moderate; ≥ 5M, catastrophic). For all salt concentrations examined ([NaCl] ≤ 1M) the weight loss was ∼50% or less. Vitreoretinal adhesion appears to be weakened after just one hour of incubation in 4M carbamide at 5°C relative to saline controls without obvious damage to the retinal surface. Conclusions: Retention of covalent collagen bonds is necessary, but not sufficient to maintain vitreous structure and retinal adhesion. In addition to covalent bonding, H–bonding (disrupted by carbamide) appears to play an important structural role in the vitreous while electrostatic forces (greatly modified by 1M salt or pH changes from 3–10) appear to have a less significant role. Thus, the most promising non–covalent interactions to target for pharmacologic vitrectomy agents are H–bonds.
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