The amniotic membrane is the thin, innermost layer of the placenta that surrounds and protects a developing baby during pregnancy. After delivery, this tissue can be collected, processed, and used as a biological graft to help heal wounds on the eye surface, skin, and other parts of the body. Its unique combination of growth-promoting proteins, anti-inflammatory properties, and low risk of immune rejection has made it one of the most widely used biological tissues in medicine.
Structure of the Amniotic Membrane
The human amniotic membrane consists of five distinct layers: the epithelium, basement membrane, compact layer, fibroblast layer, and spongy layer. The epithelium faces inward toward the amniotic fluid that cushions the baby, while the outermost spongy layer connects to the chorion, the thicker outer membrane of the placenta. Despite being less than half a millimeter thick, these layers contain living cells, structural proteins, and a rich mix of signaling molecules that play active roles in healing when applied to damaged tissue.
The basement membrane layer is especially important in medical applications. When placed on a wound, it acts as a scaffold that cells can migrate across and attach to, essentially giving the body a template to rebuild on. This is why the membrane works so well for reconstructing surfaces like the cornea, where cells need a smooth, organized foundation to regenerate properly.
Why the Body Doesn’t Reject It
One of the most useful features of amniotic membrane is that it rarely triggers an immune response when transplanted into another person. The tissue expresses a molecule called HLA-G, which has very low variation between individuals compared to the standard immune-recognition proteins found on most human cells. This makes it difficult for a recipient’s immune system to identify the graft as foreign. Certain cells within the membrane’s connective tissue also carry a protein called Fas ligand, which can prevent immune cells from infiltrating the graft and mounting an attack.
This immune tolerance means patients receiving amniotic membrane grafts typically do not need immunosuppressive medications, a significant advantage over many other types of tissue transplants.
Growth Factors and Healing Properties
The amniotic membrane is loaded with proteins that actively promote tissue repair. These include factors that stimulate skin and surface cell growth (epidermal growth factor, keratinocyte growth factor), factors that encourage new blood vessel formation (basic fibroblast growth factor), and factors that drive the proliferation of connective tissue cells (platelet-derived growth factor). It also contains hepatocyte growth factor, which supports cell survival, and transforming growth factor beta, which plays a role in cell differentiation and tissue remodeling.
On the anti-inflammatory side, the membrane produces interleukin-10, a powerful immune-calming protein, along with a molecule that blocks the inflammatory signaling of interleukin-1. It also contains tissue inhibitors of metalloproteinases, proteins that prevent enzymes from breaking down healing tissue. This combination of growth promotion and inflammation control is what makes amniotic membrane effective for chronic wounds that have stalled in the healing process.
Uses in Eye Surgery
Ophthalmology is where amniotic membrane grafts have the longest track record. The membrane supports damaged tissue, shields defects from further breakdown, and promotes the regrowth of surface cells on the eye. It is used for a wide range of conditions: corneal ulcers, chemical and thermal burns, severe dry eye, pterygium (a fleshy growth on the eye surface), and deficiencies in the stem cells that maintain the cornea. Surgeons also use it after removing tumors from the eye surface, during glaucoma procedures to repair leaking surgical sites, and even as plugs placed inside the eye to close retinal holes.
When grafted onto the cornea, the membrane can be applied in two ways. In the inlay technique, the tissue is tucked into a wound or ulcer and secured with tiny sutures. The patient’s own corneal cells then grow over and incorporate the graft, which remodels into the surrounding tissue over time. In the overlay technique, the membrane is draped across the entire surface like a bandage. Clinical studies comparing the two approaches have found no significant difference in healing time or recurrence rates, so the choice depends on the specific injury.
In one study of chemical and thermal eye injuries, epithelial defects healed in over 94% of cases treated with amniotic membrane transplantation, with symptom relief reported in 88% of patients.
Uses in Wound Care
Chronic wounds that resist conventional treatment, particularly diabetic foot ulcers, are another major application. In a clinical study of patients with chronic diabetic foot ulcers, dehydrated amniotic membrane allografts produced a mean healing time of about three weeks, with a median of just two weeks. At follow-up nine to twelve months later, 94.4% of those healed wounds remained closed, suggesting durable results rather than temporary improvement.
The membrane is also used in orthopedic and reconstructive surgery to reduce scarring and adhesion formation, and in dental and oral surgery to support tissue regeneration after procedures.
How the Tissue Is Collected and Processed
Amniotic membrane is donated by mothers who have safely delivered their baby by elective cesarean section. The entire placenta and umbilical cord are collected after delivery, and the amniotic membrane is carefully separated for processing. Donors undergo blood screening prior to the procedure to test for infectious diseases, and certain illnesses or lifestyle factors can disqualify potential donors.
Once collected, the tissue is preserved using one of two main methods. Cryopreservation involves freezing the membrane in a protective solution, which requires a cold chain for storage and transport. Dehydrated (dried) preparations can be stored at room temperature, making them far more practical for clinics and operating rooms. Research comparing the two methods has found that dried membranes, particularly those treated with stabilizing sugars like trehalose, actually retain growth factors more effectively and release them more gradually than cryopreserved tissue. Cryopreserved membranes also show a significant decrease in certain growth factors during extended storage, while dried preparations remain stable at ambient temperature.
Risks and Limitations
Amniotic membrane grafts have a strong safety profile overall, with no major complications commonly reported. The risks that do exist are relatively minor. The membrane is not structurally strong, so it cannot provide support for large corneal perforations. If the graft is not anchored securely during surgery, it can detach early. There is a small risk of infection from contamination of the membrane itself, and rare cases of sterile inflammation inside the eye have been reported after grafting.
Transmission of bloodborne infections like HIV or hepatitis is theoretically possible but rare, given the screening protocols donors undergo. One specific concern: if a patient needs a repeat graft, using tissue from the same donor can trigger a hypersensitivity reaction, so a different donor source is preferred for subsequent procedures.
Regulatory Classification
In the United States, amniotic membrane products fall under the FDA’s framework for human cells, tissues, and cellular or tissue-based products. Products that are minimally processed and used for the same basic function the tissue serves in the body (called “homologous use”) can be regulated under a lighter pathway that does not require premarket approval. Products that are more extensively manipulated or used for purposes beyond the tissue’s natural function are regulated as drugs, devices, or biological products and must go through a formal review process. This distinction matters because it means the level of clinical evidence behind different amniotic membrane products on the market can vary considerably.