Amniotic fluid contains multiple cell types derived from the developing fetus, including some that can give rise to differentiated adipose, muscle, bone, and neuronal cell lines. The present investigators have identified lines of broadly multipotent amniotic fluid-derived stem (AFS) cells that can give rise to a wide range of lineages including those in all embryonic germ layers, thereby meeting the criterion for pluripotent stem cells. Immunoselection with magnetic microspheres was used to isolate, from cultures of human amniocentesis specimens taken for prenatal genetic diagnosis, cells bearing the surface antigen c-Kit, the receptor for stem cell factor. Flow cytometry served to assess markers expressed by human AFS cells.
AFS cells from 19 amniocentesis donors were able to differentiate along adipogenic, osteogenic, myogenic, endothelial, neurogenic, and hepatic pathways. Induced differentiation along multiple pathways was documented by the expression of mRNAs for lineage-specific genes. Multilineage differentiation was characteristic of AFS cells that were cloned by limiting dilution. Studies based on marking with a retroviral vector confirmed that cloned AFS cells and their differentiated derivatives did in fact descend from a single cell, and that the AFS cells are pluripotent stem cells. Feeders were not necessary for the undifferentiated AFS cells to expand extensively. The cells doubled in 36 hours and were not tumorigenic. Lines maintained for more than 250 population doublings continued to have long telomeres and normal karyotypes. The differentiated cells derived from AFS cells included neuronal lineage cells secreting the neurotransmitter L-glutamate or expressing G-protein-gated inwardly rectifying potassium channels, hepatic lineage cells producing urea, and osteogenic lineage cells that formed tissue-engineered bone.
These studies affirm that stem cells capable of extensive self-renewal can routinely be obtained from human amniotic fluid. AFS cells can serve as precursors to a broad range of differentiated cell types that potentially have therapeutic applications. Banking of cells that would otherwise be discarded could provide a convenient source not only for autologous treatment later in life, but for matching of histocompatible donor cells with prospective recipients.