Mesenchymal stem cells (MSCs) also known as mesenchymal stromal cells or medicinal signaling cells are multipotent stromal cells that can differentiate into a variety of cell types, including osteoblasts (bone cells), chondrocytes (cartilage cells), myocytes (muscle cells) and adipocytes (fat cells which give rise to marrow adipose tissue).
Structure
Definition
While the terms mesenchymal stem cell (MSC) and marrow stromal cell have been used interchangeably for many years, neither term is sufficiently descriptive:
Mesenchyme is embryonic connective tissue that is derived from the mesoderm and that differentiates into hematopoietic and connective tissue, whereas MSCs do not differentiate into hematopoietic cells. Stromal cells are connective tissue cells that form the supportive structure in which the functional cells of the tissue reside. While this is an accurate description for one function of MSCs, the term fails to convey the relatively recently discovered roles of MSCs in the repair of tissue. The term encompasses multipotent cells derived from other non-marrow tissues, such as placenta, umbilical cord blood, adipose tissue, adult muscle, corneal stroma, or the dental pulp of deciduous (baby) teeth. The cells do not have the capacity to reconstitute an entire organ.
Morphology
Mesenchymal stem cells (MSCs), a term first coined by Arnold I. Caplan in 1991, are characterized morphologically by a small cell body with a few cell processes that are long and thin. The cell body contains a large, round nucleus with a prominent nucleolus, which is surrounded by finely dispersed chromatin particles, giving the nucleus a clear appearance. The remainder of the cell body contains a small amount of Golgi apparatus, rough endoplasmic reticulum, mitochondria, and polyribosomes. The cells, which are long and thin, are widely dispersed, and the adjacent extracellular matrix is populated by a few reticular fibrils, but is devoid of the other types of collagen fibrils. These distinctive morphological features of mesenchymal stem cells can be visualized label-free using live cell imaging.
Classification
The International Society for Cellular Therapy (ISCT) has proposed a set of standards to define MSCs. A cell can be classified as an MSC if it shows plastic adherent properties under normal culture conditions and has a fibroblast-like morphology. In fact, some argue that MSCs and fibroblasts are functionally identical. Furthermore, MSCs can undergo osteogenic, adipogenic and chondrogenic differentiation ex vivo. The cultured MSCs also express on their surface CD73, CD90 and CD105, while lacking the expression of CD11b, CD14, CD19, CD34, CD45, CD79a and HLA-DR surface markers.
Location in the body
MSCs are found throughout the human body.
Bone marrow
Bone marrow was the original source of MSCs, and is still the most frequently utilized source. These bone marrow stem cells do not contribute to the formation of blood cells, and so do not express the hematopoietic stem cell marker CD34. They are sometimes referred to as bone marrow stromal stem cells.
Cord cells
The youngest and most primitive MSCs may be obtained from umbilical cord tissue, namely Wharton's jelly and the umbilical cord blood. However, MSCs are found in much higher concentration in the Wharton's jelly compared to cord blood, which is a rich source of hematopoietic stem cells. The umbilical cord is available after a birth. It is normally discarded, and poses no risk for collection. These MSCs may prove to be a useful source of MSCs for clinical applications, due to their primitive properties and fast growth rate.
These cells have several advantages over bone-marrow-derived MSCs. Adipose-tissue-derived MSCs (AdMSCs), in addition to being easier and safer to isolate than bone-marrow-derived MSCs, can be obtained in larger quantities.
Adipose tissue
Apisode-tissue-derived MSCs (AdMSCs), in addition to being easier and safer to isolate than bone-marrow-derived MSCs, can be obtained in larger quantities.
Molar cells
The developing tooth bud of the mandibular third molar is a rich source of MSCs. While they are described as multipotent, it is possible that they are pluripotent. They eventually form enamel, dentin, blood vessels, dental pulp, and nervous tissues. These stem cells are capable of differentiating into chondrocytes, cardiomyocytes, melanocytes, and hepatocyte‐like cells in vitro.
Amniotic fluid
Stem cells are present in amniotic fluid. As many as 1 in 100 cells collected during amniocentesis are pluripotent mesenchymal stem cells.