A Review on Modifications of Amniotic Membrane for Biomedical Applications

Mesenchymal stem cells (MSCs) are multipotent cells defined by multilineage potential, ease to gene modification, and immunosuppressive ability, thus holding promise for tissue engineering, gene therapy, and immunotherapy. They exhibit a unique in vitro expansion capacity, which, however, does not compensate for the very low percentage in their niches given the vast numbers of cells required for the relative studies. Taking into consideration the lack of a uniform approach for MSC isolation and expansion, we attempted in this study, by comparing various culture conditions, to identify the optimal protocol for the large-scale production of MSCs while maintaining their multilineage and immunosuppressive capacities. Our data indicate that, apart from the quality of fetal calf serum, other culture parameters, including basal medium, glucose concentration, stable glutamine, bone marrow mononuclear cell plating density, MSC passaging density, and plastic surface quality, affect the final outcome. Furthermore, the use of basic fibroblast growth factor (bFGF), the most common growth supplement in MSC culture media, greatly increases the proliferation rate but also upregulates HLA-class I and induces low HLA-DR expression. However, not only does this upregulation not elicit significant in vitro allogeneic T cell responses, but also bFGF-cultured MSCs exhibit enhanced in vivo immunosuppressive potential. Besides, addition of bFGF affects MSC multilineage differentiation capacity, favoring differentiation toward the osteogenic lineage and limiting neurogenic potential. In conclusion, in this report we define the optimal culture conditions for the successful isolation and expansion of human MSCs in high numbers for subsequent cellular therapeutic approaches.

After n-heptanol removal of the total corneal epithelium and a limbal lamellar keratectomy, 23 rabbit eyes developed features of limbal stem cell deficiency including conjunctival epithelial ingrowth, vascularization and chronic inflammation. One month later, 10 control eyes received a total keratectomy, and 13 experimental eyes received additional transplantation of glycerin-preserved human amniotic membrane. In 3 months of follow-up, all control corneas were revascularized to the center with granuloma and retained a conjunctival epithelial phenotype. In contrast, five corneas in the experimental group became clear with either minimal or no vascularization; the rest had either mid peripheral (n = 5) or total (n = 3) vascularization and cloudier stroma. The success of corneal surface reconstruction correlated with the return of a cornea-like epithelial phenotype and the preservation of amniotic membrane, whereas the failure maintained a conjunctival epithelial phenotype and the amniotic membrane was either partially degraded or covered by host fibrovascular stroma. These results suggest that measures taken to facilitate epithelialization without allowing host fibrovascular ingrowth onto the amniotic membrane might prove this procedure clinically useful for ocular surface reconstruction.

An important component of tissue engineering (TE) is the supporting matrix upon which cells and tissues grow, also known as the scaffold. Scaffolds must easily integrate with host tissue and provide an excellent environment for cell growth and differentiation. Most scaffold materials are naturally derived from mammalian tissues. The amniotic membrane (AM) is considered an important potential source for scaffolding material. The AM represents the innermost layer of the placenta and is composed of a single epithelial layer, a thick basement membrane and an avascular stroma. The special structure and biological viability of the AM allows it to be an ideal candidate for creating scaffolds used in TE. Epithelial cells derived from the AM have the advantages of stem cells, yet are a more suitable source of cells for TE than stem cells. The extracellular matrix components of the basement membrane of the AM create an almost native scaffold for cell seeding in TE. In addition, the AM has other biological properties important for TE, including anti-inflammatory, anti-microbial, anti-fibrosis, anti-scarring, as well as reasonable mechanical property and low immunogenicity. In this review, the various properties of the AM are discussed in light of their potential use for TE.