Preservation, sterilization and de-epithelialization of human amniotic membrane for use in ocular surface reconstruction

Abstract

In the past 20 years, human amniotic membrane (AM) has become widely used as an ophthalmic surgical patch as well as a substrate for stem cell tissue equivalents for ocular surface reconstruction. AM reduces ocular surface scarring and inflammation, and enhances epithelialization. In addition, it shows limited immunogenicity and some anti-microbial properties. Before being applied clinically, the donor of AM is required to undergo a thorough health screening and the membrane has to undergo an accepted processing routine, which includes preservation, sterilization and de-epithelialization. There have been various articles describing methods in preserving, sterilizing and de-epithelializing AM. Each preparation technique has been reported to have differential effects on the physical and biological properties of the AM. Therefore, it is difficult to establish a standardized procedure. In this review, we discuss the present techniques and several novel, new approaches in the preparation of AM for use in ocular surface reconstruction, and their impact on AM structure and biological activity.

Introduction

Human amniotic membrane (AM) has been used as a biomaterial for surgical reconstruction for nearly 100 years. In 1910, Davis [1] was the first to use AM in skin transplantation. Subsequently, it has been widely used in management of burns [2], [3]; as a surgical dressing [4], [5]; surgical reconstruction of the oral cavity [6], bladder [7], and vagina [8], [9]; occlusion of pericardium [10]; and in the prevention of surgical adhesions [11], [12]. Although, as discussed below the inherent biological qualities in this membrane substrate have prompted its widespread and successful use, there are some limitations. Therefore, in more advanced applications it would be desirable to have a range of properties which could be modifiable. This has prompted a search for development of new artificial membranes [13], [14], [15]. Nonetheless, the properties of AM become important to use as model to guide the development of new synthetic substitutes.

The first documented use of AM in ophthalmic surgery appeared in 1940. It was reported by de Rotth [16] and Sorsby [17] who examined its role in the management of ocular burns. Five decades later, in 1990, Kim and Tseng [18] propelled it into ophthalmic practice by their success in repairing corneal defects with AM. Now, in the 21st century, AM has gained significance because of its reported ability to reduce scarring and inflammation [19], [20], enhance wound healing and epithelialization [21], as well as its anti-microbial and anti-viral properties [22], [23]. Moreover, it has been shown to have low immunogenicity since it has not been associated with graft rejection after transplantation [24], [25], [26]. Indications for its use have expanded over the last two decades, with a vast and growing list of procedures for which it has been used [27], [28]. These include persistent corneal epithelial defects [29], [30], [31], neurotrophic corneal ulcers [32], corneal perforations [33], [34], shield ulcers [35], infectious keratitis [36], bullous keratopathy [37], [38], band keratopathy [39], and following chemical injury [40], [41]. More recently, it has been used as a substrate for ocular surface epithelial stem cell transplants, corneal endothelial cell and retinal pigment epithelial substrate [42], [43], [44], [45].

The pathway from donor placenta retrieval to AM transplantation is complex. Before the retrieval, screening of the potential donor is required to avoid possible disease transmission to the AM recipient. There is the potential for disease transmission due to inadequate aseptic preparation, and the subsequent need for expensive storage. Another important step before AM transplantation is its de-epithelialization. It has been demonstrated that denuded AM promotes better cell proliferation and differentiation, better structural integrity, as well as more uniform cell outgrowth compared to intact AM (epithelialized) [46], [47].

There have been many publications in the literature describing the effects of different agents or techniques in the preparation of AM, which includes preservation, sterilization and de-epithelialization. In this review, we describe the basic structure and the biological properties of AM with respect to extracellular matrix (ECM) components and growth factors, the consequences of different techniques employed in its preservation and sterilization, and methods for removing the epithelium. Changes in mechanical properties are also compared among several techniques.