## 4 Discussion  
Nowadays, Optisol-GS remains the most widely employed commercial solution for tissue preservation aimed at cornea transplantation worldwide. Eye banking documents recommend using an appropriate corneal storage solution following good manufacturing practices, which shall be used and stored according to the manufacturer's recommendations for temperature and time limit [35; 37]. The differences between this gold standard product and other available solutions have been the object of research in several studies. These studies show methodological differences in their evaluation protocols, ranging from very diverse numbers of corneas used to different preservation times, temperature range, and composition of preservative solutions. Nevertheless, most studies do not identify relevant differences between Optisol-GS and other products, as assessed by both qualitative and quantitative parameters, in the usual 7-day period of preservation common to cornea banking. However, the literature presents some evidence of differences, especially in longer times, that may be related to the chemical composition of the preservation media, as will be discussed below. One of the factors that may contribute to the preservation and reduction of the rate of vacuolization of endothelial cells in the Optisol-GS medium is the presence of chondro sulfate, non-essential amino acids, ascorbic acid, vitamins, purines, and lipid components with antioxidant action. These components were initially advertised, along with an increased concentration of chondro sulfate, as improvements that should provide superior preservative properties when compared with Dexsol, an older preservative medium available in the USA [25]. However, the study of [PERSON] et al. [25] reported very similar performances between Dexsol and Optisol-GS. However, the authors observed a loss of epithelium that was related to the conservation time and suggested that the preservation time of corneas should be lower than four days, especially when performing penetrating keratoplasty in patients with alterations of the ocular surface. The authors stated that efforts must continue to find an optimal corneal storage medium that is cost effective, safe, and ensures minimal loss of epithelial cells [25]. Cornisol is an intermediate-term corneal storage medium developed in India, aimed at improving the affordability and availability of preservation media for keratoplasty in developing countries. It differs from Optisol GS by adding insulin, vitamins, coenzymes, and trace elements. Cornisol is available only outside the US and approved in India for cold storage up to 14 days. A single study conducted by [PERSON] and [PERSON] [22] compared Cornisol and Optisol GS, reporting the absence of differences in 14 days regarding the endothelial cell loss, endothelial cell density (ECD), and percentage of hexagonality evaluation. The fact that that Cornisol costs usually half the price of Optisol-GS [22] is one important feature of this formulation if further studies confirm similar clinical outcomes after transplantations performed with corneas preserved with both media. The studies by [PERSON] et al. [20; 33] discuss the requirement of intermediate corneal storage in Japan since the government has regulated the quarantine time of donors to avoid infectious diseases. This requirement raised the need to develop a domestic composition with a simple formulation for longer preservation, which was investigated for safety reasons in rabbit corneas. The authors proposed in 2002 a new solution, whose main differences from Optisol GS are the absence of dextran, streptomycin, and ATP sources [33]. This new solution was formulated with a simple combination of MEM with 2.5% chondro sulfate. This formulation showed a similar corneal preservation ability as that presented by Optisol-GS in both rabbit [20] and human corneas [33].