The biology of equine mesenchymal stem cells: phenotypic characterization, cell surface markers and multilineage differentiation.

Mesenchymal stem cells (MSCs) are multipotent stem cells that can give rise to a range of connective tissue cells including osteoblasts, chondrocytes and adipocytes. MSCs have been isolated from humans and a variety of animal species including rodents, dogs, horses and rabbits. There is currently no consensus on how these cells are identified and characterized. This is partly due to the lack of standardized specific cell surface markers for MSCs. The aim of this review is to examine the literature on equine MSCs and establish whether there is a well-defined phenotype for these cells. Equine MSCs have been obtained from four main sources, bone marrow, adipose tissue, umbilical cord (blood and matrix) and peripheral blood. MSCs from these tissue sources have been shown to undergo chondrogenic, adipogenic and osteogenic differentiation. However the markers used to identify these cells vary significantly in the literature. Despite this, CD90 and CD34 seem to be reliable positive and negative markers respectively. Our understanding of the biology of equine MSCs will benefit from better reagents for their phenotypic characterization. The antibodies and molecular probes needed for the reliable identification of equine MSCs are not standardized and this is a high priority for future research.