BLOG: How do amniotic membranes work?

ByDoug Rett, OD, FAAO

The fact that we, as eye doctors, can use amniotic membranes in our daily practice is a pretty amazing testament to how far medicine has come in the 21st century. But although the process has certainly been streamlined lately, amniotic membranes have been used in medicine for over 100 years.

There’s literature dating back to 1910 in which amniotic membranes were used for dressings in skin injuries like burns and wounds. Over the years it fell out of favor, but occasionally popped up in various surgeries throughout the body. But near the end of the 20th century, it started being used in ocular disease, for which its use seemed perfectly designed.

This month we’ll talk about amniotic membranes (AMs), specifically, their history and anatomical properties that allow them to work so well on the ocular surface.

Before the ocular use of AMs, we had conjunctival flaps. This is another procedure that’s been done for over 100 years, but popularized by Dr. Gunderson, who described the technique in the 1950s. Part of the conjunctiva (usually the superior location) is dissected away, slid over the cornea and sutured into place. There are fewer indications for this procedure in today’s practice, as alternatives like bandage contact lenses, amniotic membrane transplants (AMTs) and other corneal surgeries have improved. But conjunctival flaps are still used today, often in patients with little visual potential, to aid chronically irritated ocular surfaces. In addition, it’s a procedure that can be reversed, perhaps to attempt a surgery at a later date once inflammation subsides.

Now that AMs are more widely available, they are being used much more frequently in ocular surgery than are conjunctival flaps. But to understand their use, it’s important to understand their anatomy.

For mammals, our embryos are held in a sac made up of fetal membranes. The outermost layer of this sac is called the chorion, the innermost is called the amnion and, inside, the sac is filled with amniotic fluid. The amnion itself is avascular and has three layers: the epithelium, basement membrane (BM) and stroma. These layers are important to ocular surface disease for different reasons.

The epithelium has nondifferentiated epithelial cells, which allow it to be used for corneal or conjunctival (or other mucous membrane) reconstruction. The BM has laminin and fibronectin, which improve epithelial cell migration and strengthen adhesions on the basal cells. The BM also has growth factors and cytokines, which help prevent epithelial apoptosis and is where epithelial cell differentiation occurs. The stroma has anti-inflammatory and anti-angiogenic proteins, which inhibit inflammation, neovascularization and scarring. And this all makes sense, because the eye and the embryo are two of the rare immune-privileged sites in our bodies. The enemy of both is inflammation, and the body has developed an amazing way to protect the embryo from an immune response. AMs are ideally suited for use in ocular surgeries.

Amniotic membranes can be used as a graft or as a patch. If the patient has missing stroma, then AMT is typically performed as a graft, with the epithelial side placed facing up (anteriorly), and the stroma side resting against the host. This typically requires suturing or gluing the graft into place. The AM replaces the missing stroma and provides a BM upon which normal corneal epithelium can grow. If the patient has normal stroma and is only missing epithelium, then AM is used as a patch, with the epithelial side placed resting against the host tissue (and the stromal side facing up). This both promotes epithelialization and reduces inflammation, all while protecting the ocular surface.

In the not-too-distant future we’ll talk more about the practical aspects of using AMs in your daily practice. But for now, I think it’s important to consider if AMs are right for use in your practice, because in a lot of anterior segment conditions, AMs have gone from the future of eye care to the current-day standard-of-care.

References:

Dua HS, et al. Br J Ophthalmol. 1999;doi 10.1136/bjo.83.6.748.

Krachmer J, et al. Cornea. 3rd ed. Mosby-Elsevier; 2011.

Li DQ, et al. J Cell Physiol. 1995;doi:10.1002/jcp.1041630108.

ElHeneidy H, et al. Int J Womens Health. 2016;doi:10.2147/IJWH.S96636.