MSCs (mesenchymal stem cells) should have the following basic characteristics: (1) adnexal growth; (2) specific surface markers, such as not expressing CD14, CD34, CD45, HLA-II, but expressing CD29, CD73 and CD105; (3) capable of self-renewal and also differentiating into bone, cartilage and adipose in vitro multiple cell lines. Although different research groups have given various names to cells isolated from human umbilical cords, such as stromal cells, stromal stem cells and MSCs, they usually share the above basic characteristics, and this paper intends to review the biological characteristics of MSCs of this origin. 1. Genetic analysis Genetic analysis of human umbilical cord MSCs shows that the cells are similar to hematopoietic stem cells (HSCs) and embryonic stem cells (ESCs) in that the common genes that are highly expressed include those expressed by undifferentiated ESCs, morphogenesis-related proteins, the extracellular adhesion molecules, neurotrophic factors, and three embryonic layer-derived zygotic cell markers [1]. In addition, RT-PCR analysis revealed that human umbilical cord stromal stem cells also express a variety of undifferentiated cell markers, 3 germ layer and trophoblast-associated genes and a series of pluripotent stem cell markers such as Nanog, Oct-4, Sox-2, Rex-1, SSEA-3, SSEA-4, Tra-1-60 and Tra-1-81 [2]. 2. expression of cellular markers Numerous studies analyzing the expression of surface markers of umbilical cord stromal cells by flow cytometry, PCR, microdotting methods and immunohistochemistry have shown that these cells, similar to MSCs of other origins, express CD10, CD13, CD29, CD44, CD49 b, CD49 c, CD49 d, CD49e CD51, CD73, CD90, CD105, CD146, CD166, HLA-1, and HLA-A,B,C; but not CD14, CD31, CD33, CD34, CD38, CD45, CD56, CD123, CD133, CD235a, HLA-G, HLA-DP, HLA-DQ HLA-DR and Strol-1 [21. 3. Telomerase activity Telomerase activity plays an important role in the proliferative capacity of stem cells. It has been found that telomerase activity in umbilical cord stromal cells is 10% of that in tumor cell lines [3] and that the reverse transcriptase gene for telomerase is expressed at a high level [1], and the continuous expression of telomerase has been further confirmed by β-galactosidase staining [4]. However, it has also been suggested that telomerase activity is more stable and higher than normal at the beginning of culture and gradually decreases below the level of HeLa cell lines thereafter [2]. 4. In vitro differentiation potential 4.1. Differentiation to adipocytes, osteoblasts and chondrocytes The ability to differentiate to various mature cells derived from the mesoderm is one of the basic characteristics of MSCs. Studies have shown that human umbilical cord MSCs can differentiate into cells with the structure and function of mature adipocytes [5], as well as into osteoblasts and express markers such as bone bridging proteins, salivary proteins, osteocalcin and osteocalcin [2, 6], and can also form spherical bone needles of 1 to 2 mm in diameter similar to articular cartilage on a mucopolysaccharide matrix [2]. 4.2, Differentiation to cardiac and skeletal muscle cells 5-azacytidine, an analogue of cytosine nucleoside, is a key substance for inducing differentiation of stem cells to cardiac myocytes. It has been shown that either 5-azacytidine or cardiomyocyte co-culture systems can induce umbilical cord MSCs into cardiomyocyte-like cells expressing calmodulin and cardiac troponin [6], and also form myotubular structures characteristic of cardiomyocytes with spontaneous beating [7]. In addition, CD105+ cord MSCs obtained by immunosorting can be induced into skeletal myocytes expressing Myf5 and MyoD [8]. 4.3, Differentiation to neuronal cells The differentiation potential to neuronal cells is one of the hot spots of MSCs research. Pretreatment with basic fibroblast growth factor (bFGF) overnight followed by induction with various chemical reagents can transform umbilical cord MSCs into β-III microtubulin and neurofibrillary protein M neuron-like cells [3]; if induced with neuronal conditioned medium can yield mature neurons expressing glutamate-induced inward currents [9]; if then added with sonic hedgehog and bFGF, dopaminergic neurons expressing tyrosine hydroxylase can be obtained [10]. 4.4 Differentiation to hepatocyte-like and islet cell-like cells In vitro-expanded umbilical cord MSCs express a variety of hepatocyte markers, such as albumin, methemoglobin, cytokeratin 19, connexin-32, and dipeptidyl peptidase. The induced cells not only upregulate the expression levels of these markers, but also store glycogen and produce urea [11]. In addition, human umbilical cord MSCs can also be induced to differentiate into an islet cell-like cell mass and adjust insulin release according to the glucose concentration in the culture fluid, as well as synthesize and secrete C-peptides [12]. This suggests that this cell population is expected to be an alternative source of hepatocytes and islet cells. 5. Support functions 5.1 , Support for the expansion of HSCs Support for hematopoiesis is one of the characteristics of MSCs. Cord MSCs are able to support CD34+ cord blood HSCs for a long time and effectively [13], and their ability to expand HSCs is similar to that of bone marrow MSCs, which is expected to replace bone marrow MSCs as a new source of cells [14]. 5.2, Maintenance of islet-like cell mass survival and function Human umbilical cord MSCs can secrete a variety of cytokines, such as interleukin-6, metalloproteinase-1/2 tissue inhibitory factor, monocyte chemotactic protein-1, growth-associated oncogene, hepatocyte growth factor, insulin-like growth factor binding protein 4 and interleukin-8, thus maintaining the survival of islet-like cell mass and increasing its insulin expression level [15]. 5.3. Expansion of cord blood-derived natural killer cells Natural killer (NK) cells are important for overt immunotherapy. The combination of human umbilical cord MSCs with cytokines (IL-2, IL-5, IL-3 and FTL-3L) significantly amplifies cord blood CD56(+)/CD3(-) NK cells [16], laying the foundation for obtaining sufficient NK cells to meet clinical needs. 5.4 , Support for ESCs A recent researcher found that ESCs with human umbilical cord MSCs as trophectoderm can differentiate into cells of the inner, middle and outer germ layers in vivo and also differentiate into hematopoietic cells in vitro [17]. This suggests that human umbilical cord MSCs can serve as trophoblast cells for ESCs. 6. immunogenicity Immunosuppression and immune immunity are among the characteristics of MSCs. The immunosuppressive effect of human umbilical cord stromal cells is specific, which may be related to the expression of immunomodulatory molecules (vascular endothelial growth factor and interleukin-6), co-stimulatory surface antigens (CD40, CD80 and CD86) and HLA-G6 [18]. However, human umbilical cord MSCs can be activated by interferon-γ to increase the expression level of MHC class I molecules and express MHC class II molecules, which can induce an immune response if they are injected multiple times into the inflamed area or if interferon-γ is applied before injection [19]. Therefore, its immunogenicity needs to be fully evaluated before cell therapy for clinical use. 7, Summary In summary, human umbilical cord-derived MSCs have the following advantages: (1) The acquisition process is free from ethical and moral constraints, etc. (2) High content and proliferative capacity. (3) High acquisition success rate. (4) Its acquisition process is a non-invasive operation. (5) Low risk of bacterial and viral infection. (6) It has multi-differentiation potential without the risk of teratoma formation. (7) Low immunogenicity. Therefore, it is reasonable to believe that human umbilical cord MSCs will have a wide range of applications in cell therapy and tissue engineering, etc.