Amniotic membrane

Amniotic membrane (also called amnion) is the innermost layer of the placenta, a form of donated human birth tissue that is recovered at scheduled cesarean delivery with maternal consent and infectious-disease screening, then processed into grafts used in transplantation and wound care.[1][2][3] As a clinical biomaterial it consists of a thin, transparent, avascular sheet (roughly 20 to 50 microns thick) made up of an epithelial layer, a thick basement membrane, and an avascular stroma.[1] It is valued because it is rich in growth factors, has anti-inflammatory, antifibrotic, and antimicrobial properties, promotes re-epithelialization, and has low immunogenicity, which means rejection is rare.[1][3][4]

In the United States, amniotic membrane is recovered and distributed by tissue banks operating under standards set by the American Association of Tissue Banks (AATB) and the U.S. Food and Drug Administration (FDA).[2][5] Its regulatory status depends on how it is processed and marketed: a minimally manipulated sheet used for ocular surface or wound covering is generally regulated as a "361" human cell, tissue, and cellular and tissue-based product (HCT/P), whereas micronized, particulated, or injectable amniotic products marketed for systemic or other non-homologous uses are generally regulated as "351" drugs or biologics that require FDA premarket approval.[6][7]

Structure and biology

The amniotic membrane is the layer of the placenta and fetal membranes closest to the fetus.[1] It comprises, from the fetal side outward, a single layer of cuboidal epithelial cells, a basement membrane, and an underlying stroma divided into compact, fibroblastic, and spongy sublayers.[1] The membrane is avascular, meaning it has no blood vessels, which contributes to its low immunogenicity.[1][4]

Biological properties

Amniotic membrane is biologically active rather than inert. Reported properties include:

• Anti-inflammatory and immunomodulatory activity. The tissue suppresses pro-inflammatory cytokines such as tumor necrosis factor alpha, interleukin-1 (IL-1alpha and IL-1beta), IL-6, and IL-8, in part through mediators such as the IL-1 receptor antagonist and IL-10.[4] It also contains protease inhibitors that help exclude inflammatory cells.[8]
• Antifibrotic and antiscarring activity. Amniotic membrane down-regulates transforming growth factor beta (TGF-beta) signaling and its receptor expression on fibroblasts, reducing fibroblast activation and scar formation.[4][8]
• Promotion of epithelialization. The membrane serves as a substrate (scaffold) for epithelial cell growth and secretes growth factors including epidermal growth factor (EGF), keratinocyte growth factor (KGF), hepatocyte growth factor (HGF), basic fibroblast growth factor (bFGF/FGF-2), vascular endothelial growth factor (VEGF), and platelet-derived growth factor (PDGF), all of which support healing.[1][4]
• Antimicrobial properties. The membrane has reported antimicrobial activity that can reduce infection at the application site.[3][4]
• Low immunogenicity. Because the tissue is avascular and expresses little of the antigens that trigger rejection, immune rejection is rare and systemic immunosuppression is generally not required.[1][3][4]

History

The first documented surgical use of fetal membranes was reported in 1910 by John Staige Davis, who described their use as a material in skin transplantation.[9] In 1940, Andrew de Rotth reported the first ophthalmic use, for conjunctival reconstruction.[9] After decades of limited use, interest in ophthalmology was revived in the mid-1990s by work from Kim and Tseng demonstrating successful ocular surface reconstruction, after which the technique became widespread.[9]

Clinical uses

Ophthalmology

Ophthalmology is the most established surgical application. Amniotic membrane is used both as a graft, in which it is incorporated into the healing tissue, and as a patch or bandage, in which it covers and protects the surface.[1][10] Reported ocular indications include:

• Ocular surface reconstruction after removal of scar tissue, tumors (for example ocular surface squamous neoplasia), or in fornix and eyelid-margin reconstruction.[1][10]
• Persistent (non-healing) epithelial defects and neurotrophic ulcers, where the membrane promotes epithelial healing.[1][10]
• Pterygium surgery, where the membrane is used to cover the bare area after excision; reported recurrence after amniotic membrane grafting in one study was about 3.8 percent, compared with conjunctival autograft and topical mitomycin C.[1]
• Chemical and thermal burns of the ocular surface; in one series of 72 eyes, the epithelial defect healed in about 94 percent of cases.[1]
• Stevens-Johnson syndrome, where early application within roughly 7 to 10 days has been reported to reduce long-term ocular sequelae.[1]
• Limbal stem cell deficiency, where the membrane serves as a carrier scaffold for techniques such as cultivated limbal epithelial transplantation (CLET) and simple limbal epithelial transplantation (SLET).[1]

The membrane is fixed using fine sutures (for example 10-0 nylon), absorbable sutures, or fibrin glue, and self-retained sutureless ring devices allow in-office application without surgery.[1][10]

Wound care and other surgical uses

Outside the eye, amniotic membrane is used as a biological dressing and graft for wound healing, including chronic and diabetic foot ulcers, pressure ulcers, and burns, where its growth factors and anti-inflammatory properties support epithelialization.[3][11] It has also been studied as a surgical adjunct and barrier in settings such as oral and periodontal surgery.[12] As with any allograft, reported limitations include limited tensile strength for structural (tectonic) support and a small risk of infectious disease transmission.[1]

Forms

Amniotic membrane grafts are supplied in two main forms, distinguished by how they are preserved:

• Cryopreserved amniotic membrane is frozen and stored, typically at around minus 75 to minus 80 degrees Celsius, in a medium containing a cryoprotectant such as glycerol.[1][10] Cryopreservation is reported to retain more of the tissue's cellular and biochemical components, including certain immune-modulating proteins, but requires frozen storage and shipping.[10]
• Dehydrated (dried or freeze-dried) amniotic membrane has had its moisture removed, often with a sugar protectant such as trehalose, allowing room-temperature storage for an extended shelf life and ready-to-use handling, though processing may reduce some biological activity.[10]

Both forms are marketed under various commercial product names and may be configured as sheets, discs, or self-retained ring devices for ocular use.[10]

Recovery and processing

Amniotic membrane is recovered from the placenta after scheduled (planned, uncomplicated) cesarean delivery.[1][2] The donating mother provides informed consent, and the donation does not alter the care she or her baby receives and poses no added risk to either; placentas not donated are otherwise discarded as medical waste.[2][13]

Before tissue is released, the donor undergoes medical and social history screening and serologic (blood) testing.[1][2] Testing must be negative or non-reactive for transmissible agents such as HIV, hepatitis B, hepatitis C, human T-lymphotropic virus (HTLV), and syphilis, with additional testing as required by current standards.[1][2] Processing typically involves separating the amnion from the chorion, rinsing to remove blood and debris (often with antimicrobial solutions), cutting the tissue to size, and preserving it by cryopreservation or dehydration.[1][2] Recovery, screening, testing, and processing are carried out under AATB standards and FDA Current Good Tissue Practice.[2][5]

FDA regulation

Human amniotic membrane is regulated by the FDA as a human cell, tissue, and cellular or tissue-based product (HCT/P) under Title 21 of the Code of Federal Regulations, Part 1271.[6][7] The regulatory pathway turns on whether the product meets all four criteria in 21 CFR 1271.10(a): it must be minimally manipulated; intended for homologous use only; not combined with another article (with limited exceptions); and either have no systemic effect and not depend on the metabolic activity of living cells, or, if it does, be for autologous use or certain related uses.[6] An HCT/P meeting these criteria is regulated solely under Section 361 of the Public Health Service Act and 21 CFR Part 1271 (a "361 HCT/P"), and does not require premarket approval. An HCT/P that does not meet them is regulated as a drug, device, or biological product (a "351" product) and requires premarket review and approval, for example a Biologics License Application (BLA).[6][7]

"Homologous use" means the repair, reconstruction, replacement, or supplementation of a recipient's cells or tissues with an HCT/P that performs the same basic function in the recipient as in the donor.[6] In FDA guidance, an amniotic membrane product applied to the surface of the eye to cover and protect during ocular repair and reconstruction is considered homologous use, because serving as a covering and barrier is a basic function of the membrane; a minimally manipulated sheet used this way generally qualifies as a 361 HCT/P.[6][7]

By contrast, FDA guidance gives the example that grinding and lyophilizing (freeze-drying) amniotic membrane and packaging it as particles is considered more than minimally manipulated, because the processing alters the membrane's original characteristics relevant to its function as a barrier.[7] Such micronized or particulated products, and amniotic products marketed for injection or for systemic or other non-homologous uses, generally fall outside the 361 pathway and are regulated as 351 drugs or biologics requiring FDA approval.[6][7] The current FDA guidance on these criteria, Regulatory Considerations for Human Cells, Tissues, and Cellular and Tissue-Based Products: Minimal Manipulation and Homologous Use, was issued in July 2020.[7]

Enforcement and warning letters

The FDA established a period of enforcement discretion to give manufacturers time to determine whether their products required premarket approval and, if so, to submit applications. That period ended on May 31, 2021, after which the FDA stated it expected to apply its full enforcement authorities.[14] (Status is point-in-time; re-confirm against FDA primary sources.)

The FDA has taken enforcement action against companies marketing unapproved amniotic and related products, including those promoted as "stem cell" treatments or for unproven uses.[14][15] For example, the agency issued a warning letter to BioLab Sciences, Inc. (dated August 23, 2022) regarding amniotic fluid-derived products that the FDA determined were drugs and biological products under Section 351 of the Public Health Service Act, marketed without an approved BLA or an investigational new drug application in effect.[16] In a separate action, the FDA issued a warning letter to Amnio Technology, LLC (dated October 1, 2024) finding that one of its amniotic products met the definition of a drug and biological product requiring premarket review and approval.[17] The FDA has also issued public safety notifications and untitled letters concerning amniotic fluid eyedrops.[18] The FDA has warned patients and clinicians that unapproved products derived from human cells or tissues can carry serious risks, including infection and contamination, and that legitimate stem cell or regenerative products generally require FDA approval.[15][19]

Ethics and consent

Donation of birth tissue raises the same ethical principles that govern other human tissue donation: informed and voluntary consent, donor and recipient safety, and prohibition of payment for the tissue itself.[2][13] Consent is obtained from the mother before a scheduled cesarean delivery, and the decision to donate does not affect the medical care provided to mother or infant.[2][13] Under U.S. federal law, donors are not paid for donated tissue, although tissue banks and processors may recover costs associated with recovery and processing.[13] Screening and testing are intended to protect recipients from transmissible disease, consistent with FDA and AATB requirements.[2][5]

See also

• Tissue bank
• FDA regulation of human tissue (HCT/Ps)
• Corneal transplantation (keratoplasty)
• Organ donation

References

• Walkden A. Amniotic Membrane Graft. StatPearls. NCBI Bookshelf. https://www.ncbi.nlm.nih.gov/books/NBK567771/
• Procurement, Processing, and Storage of Human Amniotic Membranes for Implantation Purposes in Non-Healing Pressure Ulcers. PMC. https://pmc.ncbi.nlm.nih.gov/articles/PMC11858804/
• Biological properties and surgical applications of the human amniotic membrane. Frontiers in Bioengineering and Biotechnology. 2022. https://www.frontiersin.org/journals/bioengineering-and-biotechnology/articles/10.3389/fbioe.2022.1067480/full
• Human Amnion Membrane: Potential Applications in Oral and Periodontal Field. PMC. https://pmc.ncbi.nlm.nih.gov/articles/PMC5343678/
• American Association of Tissue Banks (AATB). https://www.aatb.org/
• U.S. Food and Drug Administration. Regulatory Considerations for HCT/Ps: Minimal Manipulation and Homologous Use (guidance PDF). https://www.fda.gov/media/109176/download
• Federal Register. Regulatory Considerations for Human Cells, Tissues, and Cellular and Tissue-Based Products: Minimal Manipulation and Homologous Use; Guidance Availability. July 21, 2020. https://www.federalregister.gov/documents/2020/07/21/2020-15718/regulatory-considerations-for-human-cells-tissues-and-cellular-and-tissue-based-products-minimal
• Amniotic membrane transplantation for ocular surface reconstruction: updated clinical review. Annals of Eye Science. https://aes.amegroups.org/article/view/7312/html
• Treatment Primer for Utilizing Amniotic Membrane Tissue. Eyes On Eyecare. https://eyesoneyecare.com/resources/treatment-primer-amniotic-membrane-tissue/
• Amniotic Membrane Transplant. EyeWiki (American Academy of Ophthalmology). https://eyewiki.org/Amniotic_Membrane_Transplant
• Amniotic membrane application for the healing of chronic wounds and ulcers. ScienceDirect. https://www.sciencedirect.com/science/article/abs/pii/S0143400417302278
• The Human Amniotic Membrane: A Rediscovered Tool to Improve Wound Healing in Oral Surgery. PMC. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC12429774/
• Birth Tissue Donation. Solvita. https://solvita.org/tissue-donation/birth-tissue-donation/
• Harms Linked to Unapproved Stem Cell Interventions Highlight Need for Greater FDA Enforcement. The Pew Charitable Trusts. June 2021. https://www.pew.org/en/research-and-analysis/issue-briefs/2021/06/harms-linked-to-unapproved-stem-cell-interventions-highlight-need-for-greater-fda-enforcement
• U.S. Food and Drug Administration. Important Patient and Consumer Information About Regenerative Medicine Therapies. https://www.fda.gov/vaccines-blood-biologics/consumers-biologics/important-patient-and-consumer-information-about-regenerative-medicine-therapies
• U.S. Food and Drug Administration. Warning Letter: BioLab Sciences, Inc. (621465), August 23, 2022. https://www.fda.gov/inspections-compliance-enforcement-and-criminal-investigations/warning-letters/biolab-sciences-inc-621465-08232022
• U.S. Food and Drug Administration. Warning Letter: Amnio Technology, LLC (646460), October 1, 2024. https://www.fda.gov/inspections-compliance-enforcement-and-criminal-investigations/warning-letters/amnio-technology-llc-646460-10012024
• U.S. Food and Drug Administration. Public Safety Notification on Amniotic Fluid Eyedrops. https://www.fda.gov/vaccines-blood-biologics/safety-availability-biologics/public-safety-notification-amniotic-fluid-eyedrops
• U.S. Food and Drug Administration. Patient and Consumer Warning about Potential Serious Risks of Harm following Use of Unapproved Products from Human Cells or Tissues. https://www.fda.gov/vaccines-blood-biologics/safety-availability-biologics/patient-and-consumer-warning-about-potential-serious-risks-harm-following-use-unapproved-products