Mesenchymal stem cell (MSC), discovered in the 1950s, is a type of pluripotent stem cell. It was found that it originated from the early development of mesoderm, has a high self-renewal capacity and has the potential to differentiate into a variety of cells. MSCs are induced to differentiate into adipose tissue cells, cartilage tissue cells, connective tissue cells, bone tissue cells and neural stem cells in vitro and in vivo in different ways. In addition, MSC may also be induced to differentiate into endoderm cells (lung cells, muscle cells and intestinal epithelial cells) and ectoderm cells (epithelial cells and neurons).
1. The biological characteristics of MSC
Current research shows that MSC can be obtained from in vitro culture expansion of bone marrow, umbilical cord blood, umbilical cord, placenta, mobilized peripheral blood, adipose tissue, dental pulp, and even fetal liver and lung tissue. Although MSCs come from a variety of sources, they still have some common characteristics: they show the shape of fibroblasts under a microscope, and are often fusiform or spindle-shaped. Markers CD90, CD105, CD44, CD73, CD9, very low levels of CD80, surface markers of hematopoietic cells include CD34, CD45, CD11b, CD11c, CD14, CD19, CD79a, CD86 and major histocompatibility complex (MHC) class II expressions are all negative. And it has the ability to secrete insulin-like growth factor, vascular endothelial growth factor and hepatocyte growth factor, etc. It can be differentiated into bone cells, chondrocytes and adipocytes under specific conditions, and its immunogenicity is low. MSC is not easy to cause immune rejection after transplantation,
With the need for the clinical treatment of MSC, the International Committee on Mesenchymal and Tissue Stem Cells has proposed a minimum identification standards for human-derived MSCs: ①It must have adhesion to plastic substrates under standard stem cell culture conditions. ②The positive rate of CD105, CD73 and CD90 expression on the surface MSC markers detected by flow cytometry should be ≥95%, and the negative expression rate of CD45, CD34, CD14, CD11b, CD79a, CD19, human leukocyte antigen-DR (human leukocyte antigen-DR, HLA-DR) ≥98%. ③After induction in vitro by standard methods, MSCs are able to induce differentiation into osteoblasts, chondrocytes and adipocytes.
2. The comparison of MSCs from different sources
At present, the clinical applications of MSCs are mostly bone marrow-MSC (BM-MSC), umbilical cord-MSC (UC-MSC) and umbilical cord blood -MSC (UCB-MSC). Although MSCs from different sources have some commonalities, they also have some different characteristics. MSC was first found in bone marrow, but the collection of bone marrow is an invasive operation, which limits the source of BM-MSC to a great extent. At the same time, BM-MSC has the risk of viral infection, and as the age of the collector grows, the number of cells, and the differentiation and expansion ability of the cells will show a clear downward trend. Through continuous research, MSCs have also been found in human umbilical cord and cord blood, and they all have the potential to differentiate into a variety of cells and the ability to support hematopoiesis.
2.1. The comparison of cell morphology
There are no significant differences in the cell morphology, colony number, and colony size of MSCs from two different sources (BM-MSC and UCB-MSC), after being cultured in different media. In the early stages, the colony formation is faster. BM-MSC and UC-MSC, MSCs of two different sources, were spindle-shaped under a microscope, and then their growth rate began to increase, forming spindle-shaped cells with a more uniform shape after 1 week, which grow in a spiral or parallel shape with similar growth pattern. There may be difference in colony formation or fusion time between the MSCs from different sources, but there is no significant difference in their cell morphology after culture.
2.2. The comparison of proliferation characteristics
Studies have shown that the gene expression profile of UC-MSC is similar to embryonic stem cells, and has a faster self-renewal ability than BM-MSC, which makes the proliferation time of UC-MSC significantly shorter than that of BM-MSC. Moreover, the doubling time of umbilical cord-derived P1 generation MSCs does not extend with the increase of the number of passages, while the doubling time of bone marrow-derived P1 generation MSCs is significantly longer when it is passaged to P6 generations. This suggests that MSCs with different sources and the same number can be expanded at the same time. UC-MSC can obtain more mesenchymal stem cells than BM-MSC. Compared with MSC from other sources, UC-MSC has shorter proliferation time and stronger proliferation ability. UCB-MSC has a strong proliferative ability in the early stage, but the success rate of differentiation in vitro is lower.
2.3. The comparison of surface markers
The analysis of cellular immune phenotypes shows that most of the immune markers of UC-MSC are similar to BM-MSC, but the expressions of HLA-ABC and CD106 are lower than those of BM-MSC. The HLA molecule can cause immune rejection during MSC transplantation, which suggests that UC-MSC has lower immunogenicity than BM-MSC. CD106 is a type of adhesion molecule related to the location, migration, proliferation and differentiation of hematopoietic stem cells and progenitor cells. The low expression of CD106 by UC-MSC may be one of the differences between UC-MSC and BM-MSC. Analysis of the percentage of cells on the surface markers of UCB-MSC and BM-MSC by flow cytometry showed that although the two types of MSC were from different sources, they all expressed markers of cell adhesion molecules such as CD29, CD44 and CD105, while hematopoietic marker CD13, CD14, CD34 and CD45 were all negative, and their immunophenotype did not change with the increase of cell passage. This suggests that UCB-MSC and BM-MSC have the same cell surface markers. Most of the three MSC immune markers from different sources expressed similar expression. UC-MSC may have lower immunogenicity. CD106 may become one of the differentiation points between BM-MSC and peripheral MSC.
To be continued in Part II…