Ultrastructural characteristics of human mesenchymal stromal (stem) cells derived from bone marrow and term placenta

Human mesenchymal stromal ( stem) cells ( hMSCs) isolated from adult bone marrow ( BM- hMSCs) as well as amnion ( AM- hMSCs) and chorion ( CM- hMSCs) term placenta leaves were studied by transmission electron microscopy ( TEM) to investigate their ultrastructural basic phenotype. At flow cytometry, the isolated cells showed a homogeneous expression of markers commonly used to identify hMSCs, i. e., CD105, CD44, CD90, CD166, HLA- ABC positivities, and CD45, AC133, and HLADR negativities. However, TEM revealed subtle yet significant differences. BM- hMSCs had mesenchymal features with dilated cisternae of rough endoplasmic reticulum ( rER) and peripheral collections of multiloculated clear blisters; this latter finding mostly representing complex foldings of the plasma membrane could be revelatory of the in situ cell arrangement in the niche microenvironment. Unlike BM- hMSCs, CM- hMSCs were more primitive and metabolically quiescent, their major features being the presence of rER stacks and large peripheral collections of unbound glycogen. AM- hMSCs showed a hybrid epithelial - mesenchymal ultrastructural phenotype; epithelial characters included non- intestinal- type surface microvilli, intracytoplasmic lumina lined with microvilli, and intercellular junctions; mesenchymal features included rER profiles, lipid droplets, and well-developed foci of contractile filaments with dense bodies. These features are consistent with the view that AM- hMSCs have a pluripotent potential. In conclusion, this study documents that ultrastructural differences exist among phenotypically similar hMSCs derived from human bone marrow and term placenta leaves; such differences could be revelatory of the hMSCs in vitro differentiation potential and may provide useful clues to attempt their in situ identification.