Amnios et problèmes de surface oculaire Amnion and ocular surface problems

Résumé

La membrane amniotique, enclave interne de la vie naissante, présente diverses propriétés exploitées en ophtalmologie. Elle est utile pour : (1) limiter la formation d’adhésions fibreuses entre la paupière et le globe oculaire (symblépharon) ou la progression d’excroissances fibrovasculaires vers la cornée (ptérygion) ; (2) contribuer à la guérison d’ulcères cornéens, de kératites bulleuses et des déficiences en cellules souches de la cornée dues à des brûlures thermiques, chimiques ou d’autre origine. L’amnios, alors greffé avec des cellules limbiques saines, favoriserait la prolifération de cellules moins différenciées, aptes à reconstruire l’épithélium cornéen. La membrane amniotique contient des cytokines, réduit l’acuité des réactions immunologiques et possède des propriétés antalgiques, anti-bactériennes et anti-inflammatoires ; de plus, elle favorise, comme le fait la peau foetale, une guérison avec un minimum de cicatrices. La connaissance des mécanismes d’action de la membrane amniotique obtenue grâce à la recherche pourrait fournir de nouvelles avenues pharmacologiques afin de traiter des maladies de la surface oculaire.

Summary

The amniotic membrane, the most internal placental membrane, has various properties useful in ophthalmology. Collected on delivery by elective Caesarean section, the amnion is prepared under sterile conditions, and, usually, cryopreserved until its use as a biological bandage or as a substrate for epithelial growth in the management of various ocular surface conditions. Specifically, the amnion is used to : (1) limit formation of adhesive bands between eyelids and eyeball (symblepharon) or the progression of a fibrovascular outgrowth towards the cornea (pterygium) or to (2) facilitate the healing of corneal ulcers, bullous keratopathy, and corneal stem cell deficiency. In this last condition, either hereditary or acquired after a thermal or a chemical burn, corneal stem cells, located at a transitional zone between the cornea and conjunctiva, are lost. These cells are essential for renewal of corneal epithelium in normal and in diseased states. The loss of these cells leaves the corneal surface free for invasion by conjunctival epithelium. Not only, does conjunctival epithelium support the development of vascularisation on the normally avascular cornea, but some conjunctival cells differentiate into mucus secreting goblet cells. Such a change in phenotype leads to loss of corneal transparency and visual disability. The removal of this fibro-vascular outgrowth in combination with transplantation of both amniotic membrane and corneal stem cells are used to treat this condition. The amnion stimulates the proliferation of less differentiated cells which have the potential to reconstruct the cornea. This potential is at the origin of the hypothesis that the amnion may provide an alternative niche for limbal stem cells of the corneal epithelium. It abounds in cytokines and has antalgic, anti-bacterial, anti-inflammatory and anti-immunogenic properties, in addition to allowing, like fetal skin does, wound healing with minimal scar formation. These desirable properties are responsible for the increasing use of amniotic membrane in ophthalmology. The complete understanding of the mechanisms of action of amniotic membrane for ocular surface diseases has yet to be understood. Once revealed by research, they may provide new pharmacological avenues to treat ocular surface diseases.