中文摘要：

间充质干细胞由于其独特的生物学特性和临床应用潜能，一直是干细胞研究的热点。这里，我首次提出间充质干细胞系统新概念：自胚胎发育早期，经历整个生长发育全过程的不同胚层、多个发育阶段，将亚全能间充质干细胞到不同等级分化潜能间充质干／祖细胞群体统称为MSC系统。MSC系统的确立具有重要意义：（1）赋予了MSC三大重要生物学特征，即干细胞自身特性、组织微环境功能及免疫调控功能；（2）平衡机体免疫与组织代谢；（3）提供高效转化为临床安全有效的组织特异性干细胞。总之，该系统很好的解释了间充质干细胞的异质性，同时丰富了我们之前提出的亚全能干细胞理论，为研究人员更好的理解间充质干细胞提供了深厚的理论平台。

英文摘要：

Mesenchymal stem cells have generated great interest among researchers and physicians due to their unique biological characteristics and potential clinical applications. Here, I propose for the first time the concept of a hierarchical system which is composed of all mesenchymal stem cells from post-embryonic subtotipotent stem cells to MSC progenitors. Post-embryonic subtotipotent stem cells are left-over ceils during embryonic development and are on the top of the hierarchy. MSC system is a combination of cells that are derived from different stages of embryonic development, possess different differentiation potential and ultimately give rise to cells that share a similar set of phenotypic markers. The concept of MSC system has important implications ： （i） it entirely explains the three important biological characteristics of MSC： stem cell properties of MSC, MSC as components of tissue microenvironment and immunomodulatory functions of MSC. （~）It balances immune responses and tissue metabolism. （~）It could provide tissue- specific stem cells for clinical application with high efficiency and safety. In a word, this concept constitutes an important part of the biological properties of MSC and will help researchers gain better insight into MSC.