Mesenchymal stem cells are a virtually ubiquitous population of adult stem cells, able to differentiate into various tissue lineages. As they are multipotent and easy to grow in culture, they are at present considered very attractive candidates for tissue repair and gene therapy. With the exception of a few reports, mesenchymal stem cell morphology has been widely disregarded in the past years. In this paper we discuss the establishment of mesenchymal stem cell cultures from equine adipose tissue and describe their fine structure by transmission electron microscopy. The cultured cells revealed a fibroblastoid appearance and were characterized by an eccentric nucleus with multiple nucleoli, dense cytoplasm rich in ribosomes, a rough endoplasmic reticulum with dilated cisternae, elongated mitochondria and heterogeneous vacuolar inclusions. In addition, they were often interconnected by adhesion structures located on the cell body and on cytoplasmic processes contacting other cells. The features observed are evocative of an undifferentiated cellular phenotype and of an intense synthetic and metabolic activity.