Human placental stem cells: biomedical potential and clinical relevance.

Emerging evidence shows that different adult tissues of the human body harbour small amounts of adult mesenchymal stem cells (MSCs) which have the potential to give a rise to all cell types of the specific tissue in which they reside. These cells were found in human adult bone marrow, muscle, fat, brain, cartilage, dental pulp, etc. Depending on the type of host tissue as well as age and state of the donor's health, the plasticity of MSCs can vary considerably. Previously accomplished research efforts have shown that adult MSCs have a broad therapeutic potential due to their ability to differentiate towards multiple adult cell types under appropriate conditions. This unique property characterizes adult MSCs as potential candidates for biomedical and clinical applications. In the last decade human extra-embryonic tissues (placenta, amniotic membranes, and umbilical cord) as well as human placental fluids (amniotic fluid and umbilical cord blood), were also defined as sources of MSCs, which represents a great interest for the biomedical applications. Human placenta is a feto-maternal organ which works as a natural barrier between mother and foetus and playing a crucial role for the nutrition and immune-tolerance during the whole period of the fetal development. Anatomically, the placenta can be divided in four regions: amniotic epithelial, amniotic mesenchymal, chorionic mesenchymal and chorionic trophoblastic. Immunomodulatory properties of human placenta as well as the clinical applicability of such placental parts as amniotic membranes for wound healing, treatment of burns, and reconstruction of the oral cavity, will be described. Due to the specific structure and functions of placenta, human extra-embryonic MSCs represent stem cell types which combine some properties of pluripotent embryonic stem cells with other properties of multipotent mesenchymal stem cells. Due to the close ontogenic relationship to embryonic stem cells, MSCs derived from different placental regions as well as from amniotic fluid and umbilical cord blood have immunoprivileged characteristics, posses a broader plasticity, and proliferate faster than adult MSCs. Moreover, the human placenta is normally discarded after birth, and cells can be isolated avoiding any ethical concerns. In this review we summarize and discuss different aspects of development, ontogenetic properties, plasticity, and immunomodulatory characteristics of MSCs from different placental regions as well as from amniotic fluid and umbilical cord blood. In addition the potential of placental MSCs for biomedical and clinical applications are discussed.