amniotic membrane: Basic concepts, different kinds
Authors: Cocho Lidia 
Published: December 2022

Amniotic membrane (AM) use in ophthalmic surgery has provided an alternative for corneal and conjunctival reconstruction in many clinically challenging situations and it is currently part of our armamentarium to deal with ocular surface disease and corneal surgery. In 1910, AM was first utilized surgically as a skin graft substitute material. The first ophthalmic usage was in the 1940 s as a conjunctival substitute after removal of fibrotic tissue.

The AM is a thin, avascular membrane. It is the innermost layer of the foetal membranes and is composed of three layers: the epithelium, a monolayer of cuboidal cells attached to the basal lamina by numerous hemidesmosomes; the basement membrane (BM), composed of collagen, fibronectin and laminin. and the stroma. Finally, the loose connective tissue of the stroma confers tensile strength.

AMT is useful clinically due to its unique structure, biocompatible composition and subsequent biological functions. Studies on human AM have shown important biological properties: enhanced epithelialization by facilitating the migration of epithelial cells, reinforcing adhesion of basal epithelial cells, promoting epithelial differentiation maintaining the epithelial phenotype, and preventing apoptosis; inhibition of inflammation, neovascularization and scarring; and antimicrobial properties.

Diverse Growth factors have been found, that may have an epithelial origin. The epithelium also provides cytokines that maintain the microenvironment of the stem cells (SCs) of the corneal epithelium. BM facilitates migration of epithelial cells, reinforces adhesion of basal epithelial cells, and prevents apoptosis. Finally, a component of the stroma suppresses signalling via TGF‐beta, which modulates the proliferation of normal corneal, conjunctival, and limbal fibroblasts, and reduces fibrosis. The stroma also contains antiinflammatory and antiangiogenic proteins and protease inhibitors that reduce stromal inflammation and ulceration.

Different methods of AM preservation have been described, including Cryopreservation (the most common technique of preservation), lyophilisation and dehydration and low‐temperature vacuum evaporation.

Preserved AM may be used as a graft, as a patch, or a combination of both. As a graft (epithelial‐side up) AM is used in cases of both epithelial and stromal defects. The AM replaces the missing stromal matrix and provides a BM for cells to grow on. As a patch (epithelial‐side down), the AM covers an epithelial defect without stromal loss, and is sutured to the episclera near the limbus and protects the ocular surface from external insults and provides substances that reduce inflammation and promote epithelialization. Combined transplantation utilizes both the inlay and onlay methodologies of transplantation, where the graft provides structural integrity, whilst the patch allows for the protection of the graft.

Indications for AMT in Ocular Surface Diseases include (1) conjunctival reconstruction in areas with conjunctival tissue defects, (2) corneal surface reconstruction: AM as a graft provides a stroma and a BM on which normal corneal epithelium grows. AM heals corneal ulcers with stromal thinning of different aetiologies, the most common being neurotrophic corneal ulcers. Additionally, AMT can be used in cicatrizing disease associated with acute or chronic SC loss and acts as a substrate for culturing epithelial stem cells of the sclerocorneal limbus.