Amniotic membrane, or amnion, can provide great utility as a surgical adjunct to treat ocular surface indications such as persistent epithelial defects, corneal stromal ulcers, keratitis and ocular surface trauma, and save sight.
Since working with both human and veterinary ophthalmologists, I am frequently asked “which species is the best source of amnion”.
My opinion is that as human and large animal species (bovine, equine and porcine) have good reported xeno-compatibility it is unlikely that there is a significant clinical difference between fresh amnion from these species.
However, the absolutely essential differences originates from the following critical factors:
- When and how amnion is collected.
- The way amnion is processed for clinical use.
- The way amnion is preserved for clinical use.
When and how amnion is collected.
Clinically available human and veterinary amnion products are often manufactured from tissues collected via both normal vaginal delivery (NVD) and ‘elective’ caesarean sections (ESC). Despite being widely used and referred to as ‘comparable’ products, there are two important caveats with using tissue collected via NVD.
First, when we consider the events of normal birth, this process requires the natural onset of labour, which begins with the rupture of the amniotic sac. It is widely understood in literature that this event is initiated by an increased production of a variety of tissue degrading enzymes, which triggers a cascade of changes that degenerates and weakens the amnion.
As amnion often begins to disintegrate, the structural and ‘tectonic’ properties of the amnion as a transplant material could therefore be severely compromised.
Amnion at ‘term’ also undergoes extensive gene and protein expression changes, and the negative effect of this on the regenerative function and clinical efficacy are still not fully known.
Second, the structurally compromised tissue delivered via NVD becomes heavily contaminated with excessive blood and potentially pathogenic bioburden. NVD tissue therefore requires more aggressive decontamination procedures, which potentially further compromises the tissue quality.
ESC amnions are collected prior full labour onset, and before degradation and stress related changes are initiated. ESC amnions are collected in a clean theatre environment and contain significantly less contamination.
Products manufactured solely from ECS could therefore be considered superior quality over amnion harvested through NVD, which would not be ideal for surgical reconstruction and regeneration of the ocular surface.
The way amnion is processed for clinical use.
Procedures for processing collected amnion for clinical application vary extensively. The time taken to process tissues can vary between 2 and 24 hours; processing/storage temperature; buffers and reagents used to wash amnion; and the aggressiveness and mode of tissue ‘cleaning’ all have the potential to cause extensive changes to the biochemical content and structural quality of the issues. Researchers at the University of Nottingham, Ophthalmology Department have extensively characterised these effects over the last 15 years[2-5].
The way amnion is preserved for clinical use.
The most critical, and potentially negative, influence on amnion quality, properties and regenerative function is the way the tissue is preserved. Amnion is mainly preserved ‘wet’ through cryopreservation procedures, typically at -80˚C, or dehydrated using freeze-drying, heat desiccation or vacuum drying.
Though all these methods render the tissue no-viable, freezing causes widespread damage to the tissue. Ice-crystal formation causes extensive damage to the structural organisation of amnion and epithelial capsules, which act as a vest reservoir of important trophic factors.
Freeze-drying not only employs a destructive pre-freeze step with the same result as cryo-preservation, without careful optimisation and development, freeze dried amnion is also known not to effectively rehydrate.
Any freeze-preserved amnion product will therefore have compromised biochemical properties and is likely to function more as a biological contact lens.
Heat desiccation may improve tissue quality but is difficult to standardise.
Researchers at the Nottingham Centre for Eye Research, and NuVision Biotherapies have developed a highly standardised Tereo™ manufacturing process, to delicately dehydrate human amnion, through vacuum drying to produce a product called Omnigen™. Tereo addresses all of the aforementioned issues; Omnigen is manufactured from human amnion collected solely from ECS; is carefully processed using simple and protective standardised steps; and is dry preserved within 6 hours of birth to minimise any negative changes. The vacuum drying process delicately protects the structure and integrity of the tissue to preserve the biochemical and functional benefits of amnion, whilst allowing rapid and effective rehydration of the tissue into a superior wound healing transplant.
Compared to frozen amnion, and acellular biological collagen scaffolds, Omnigen has superior wound healing and regenerative properties, both as a removable patch (also known as an overlay), and as a permanent stromal replacement graft ‘in lay’ [ 6].
Though there is little evidence to suggest advantages of a species-specific amnion, human amnion based clinical products, such as Omnigen, are collected, screened and manufactured using most stringent, high quality, highly regulated and validated procedures. This would therefore suggest that human licenced amnion is likely to have superior outcomes over animal based products.
- Adds PJ, Hunt C, Hartley S (2001) Bacterial contamination of amniotic membrane. The British journal of ophthalmology 85: 228-230.
- Hopkinson A, McIntosh RS, Tighe PJ, James DK, Dua HS (2006) Amniotic membrane for ocular surface reconstruction: donor variations and the effect of handling on TGF-beta content. Invest Ophthalmol Vis Sci 47: 4316-4322.
- Gicquel JJ, Dua HS, Brodie A, Mohammed I, Suleman H, et al. (2009) Epidermal growth factor variations in amniotic membrane used for ex vivo tissue constructs. Tissue Eng Part A 15: 1919-1927.
- Hopkinson A, McIntosh RS, Shanmuganathan V, Tighe PJ, Dua HS (2006) Proteomic analysis of amniotic membrane prepared for human transplantation: characterization of proteins and clinical implications. J Proteome Res 5: 2226-2235.
- Hopkinson A, McIntosh RS, Layfield R, Keyte J, Dua HS, et al. (2005) Optimised two-dimensional electrophoresis procedures for the protein characterisation of structural tissues. Proteomics 5: 1967-1979.
- Allen CL, Clare G, Stewart EA, Branch MJ, McIntosh OD, et al. (2013) Augmented dried versus cryopreserved amniotic membrane as an ocular surface dressing. PLoS One 8: e78441.