Introduction of Mesenchymal Stem Cells
Mesenchymal stem cells (MSCs) are important members of the stem cell family. They are also adult stem cells with self-replication ability and multi-directional differentiation potential. Such cells are capable of differentiating into hard bone, cartilage, fat and other types of cells. Mesenchymal stem cells are mainly derived from the mesoderm. They are first found in bone marrow and can also be isolated from tissues such as fat, cord blood, amniotic fluid and tooth body. It is mainly found in connective tissue and organ stroma, especially in bone marrow tissue. Since bone marrow is the main source, it is collectively called bone marrow mesenchymal stem cells. In the mid-1970s, Fridenstein et al reported for the first time, a small part of adherent cells in bone marrow specimens can differentiate into 2 colonies similar to bone and cartilage during culture, and become bone marrow pluripotent mesenchymal stem cells. In 1997, Prockop successfully isolated bone marrow mesenchymal stem cells and found that it has multi-directional differentiation potential and can differentiate into osteoblasts, chondrocytes, adipocytes, neuron-like nuclei and myoblasts, etc. With the progress of the research, the international mesenchyme and stem cell committee in 2006 proposed three minimum criteria for human-derived mesenchymal stem cell experiments: (1)Under standard culture conditions, mesenchymal stem cells must have attachment properties to plastic substrates; (2)The positive rate of CD105, CD73 and CD90 expression in mesenchymal stem cell population was greater than or equal to 95% by flow cytometry, and the negative expression rate of CD45, CD34, CD14 or CD11b,CD79a or CD19, HLA-DR greater than 98%; (3)In vitro induction by standard methods, mesenchymal stem cells must be able to differentiate into osteoblasts, chondrocytes, and adipocytes. Current clinical research on mesenchymal stem cells is mainly used for anti-inflammatory treatment, tissue regeneration, great versus host response, autoimmune diseases and gene transfection. Nowadays, commonly used mesenchymal stem cell sorting methods include immunomagnetic bead sorting, whole bone marrow adherence, and tissue digestion.
Main characteristics of mesenchymal stem cells
Mesenchymal stem cells have the following characteristics: (1) it has strong proliferative ability and multi-directional differentiation potential, and has the ability to differentiate into various cells such as myocytes, hepatocytes, osteoblasts, fat cells, chondrocytes, stromal cells, etc. in a suitable in vivo or in vitro environment. (2) in has an immunoregulatory function, inhibits the proliferation of T cells and its immune response through cell-cell interactions and the production of cytokines, thereby exerting the function of immune reconstitution. (3) it has convenient source, easy to separate, culture, expand and purify. It has stem cell characteristics after multiple passages and has no immunological rejection. (4) the appearance is blurred, the surface antigen is not obvious, the allograft rejection is lighter, and the matching requirements are not strict. It is precisely because of these immunological characteristics of mesenchymal stem cells that it has broad clinical application prospects in the treatment of blood diseases. Autologous transplantation can reconstruct the structure and function of tissues and organs, and can avoid immune rejection.
Figure 1. The differentiation process of mesenchymal stem cells
Physiological function of mesenchymal stem cells
MSCs can support hematopoiesis. MSCs can form a connective tissue skeleton with other mesenchymal progenitor cells, secreting cytokines and extracellular matrix proteins, regulate the proliferation and homing of hematopoietic cells. The surface of MSCs has many receptors that adhere to hematopoietic cells, these receptors includes: ICAM, VCAM, L-selectin, and P-selectin. These molecules are responsible for binding to fibronectin, laminin or collagen. The differentiation and maturation of red blood cells, granulocytes, macrophages, platelets and other cells in the bone narrow depend on the contact of hematopoietic stem and progenitor cells with the microenvironment composed of MSCs and extracellular matrix and blood vessels, including direct contact between hematopoietic cells via surface adhesion-related molecules and MSCs, and MSCs regulate the differentiation and maturation of hematopoietic cells by secreting various cytokines. In 1998, Zannetino et al reported that homologous binding experiments between hematopoietic progenitor cells and MSCs by CD164 confirmed that CD164 receptor plays an important role in cell homing. MSCs also have the application of regulating immunity. In 1994, Ishida et al showed that MSC can not only prevent graft failure, but also play a tole in complex immune regulation of T and B lymphocytes. In 1998, Klyushnenkova et al pointed out that in vitro expansion of MSC can inhibit T lymphocyte reaction in vitro. In 1966, Barda-Saad et al. pointed out that MSCs play a role in the early development of T lymphocytes by providing the necessary binding sites, and inoculation of providing the necessary binding sites, and inoculation of murine thymocytes in bone marrow stromal fluids reveals that T cells have different sensitivities to adhesion. This experiment suggests that the bone marrow stroma may be a place for extrathymic T cells to mature. It can be seen from the above that MSC plays an important role in immune regulation in vivo.
Problems in research
Although the research on MSCs has made great progress in recent years, the following problems remain unresolved: (1) so far, there is no perfect solution for obtaining and cultivating expanded MSCs, most laboratories are isolated according to the adherence of MSCs reported by Friedenstein et al. although many laboratories have reported a more uniform cell population approach, none have been widely accepted. (2) The result of some laboratories cannot be repeated by others because of the different batch numbers used for serum, therefore, seeking stable culture condition for MSCs remains to be further studied. (3) MSCs have multilineage differentiation potential, but the efficiency of differentiation into multiple lines is not ideal, especially whether the differentiation in vitro will cause the genetic characteristics of MSCs to be further confirmed. (4) what kind of signal induces the differentiation of MSCs into multiple lines, and the molecular mechanism of its differentiation is still unknown. Despite the above problems, there are still many advantages compared with embryonic stem cell transplantation. For example, there is no immune rejection problem, and there is no ethical and political problem. In addition, MSCs have multiple-directional differentiation potential and high proliferation characteristic, which are easy to obtain and facilitate autologous transplantation.