## 3 Undifferentiated Cells (Stem Cells)
### Adult Stem Cells
#### Adipose Tissue-Derived Mesenchymal Stem Cells (ASCs)  
##### 3.4.2.1 General Information  
The origin of adipose tissue (AT) is the mesodermal layer of the embryo and develops during pre-natal and post-natal growth. Histologically, two types of adipose tissue were identified including white adipose tissue (WAT) and brown adipose tissue (BAT). There are some morphological and functional differences between the two AT types. Compared to BAT, WAT is more abundant in the human body [184]. One of the first reports on the cell isolation from AT belongs to [PERSON] et al. in the early 1960s [185]. Cellular components in AT contain adipocytes and a heterogeneous set of cell populations, i.e., the stromal vascular fraction (SVF) (Figure 6). The SVF acts as a supporting cell population for adipocytes and comprises adipose-derived stem cells (ASCs), vascular endothelial cells, vascular smooth muscle cells (VSMCs), and leukocytes. ASCs as the most important cells of AT have been isolated from both white and brown tissues with different characteristics. These cells were first formally recognized in human lipoapspirate by [PERSON] et al. in 2001 [186]. Among different methods, the liposuction technique is widely used to harvest ASCs from the human body due to its relative ease of execution. Furthermore, as a good point, the function of ASCs is not affected by the lipo-aspiration procedure. Apart from ASCs, a variety of terms have been applied to describe these cells including adipose-derived adult stem cells (ADAS), adipose-derived stromal cells (ADSCs), adipose-derived mesenchymal stem cells (Ad-MSCs), lipoblasts, pericytes, pre-adipocytes, and processed lipoapspirate (PLA) cells [187].  
According to the International Fat Applied Technology Society (IFATS), the term of ASCs is the standard name for the isolated, plastic-adherent, multipotent cell population. The isolation procedure of ASCs is a simple process in which lipoapspirate adipose tissue is digested by collagenase (type IV), which is followed by centrifugation to separate the stromal population from the adipocytes [188]. Similar to BM-MSCs, ASCs can be purified by using well-established approaches such as FACS and MACS techniques. In the culture situation, these cells show the primary characteristics of MSCs including attachment to plastics, fibroblast-like morphology, and differentiation to different cell lineages including osteoblasts [189], chondrocytes [190], endothelial cells (ECs) [191], cardiomyocytes [192], and neural cells [193]. Similar to other MSCS, ASCs express some cell surface markers, which can be used for their characterization. Like other MSCs, ASCs serve as a \"secretome\" and secret regulatory proteins and growth factors in the extracellular environment [194]. Therefore, these cells contribute to angiogenesis and suppression of inflammation and, thereby, induce cell migration and proliferation in the body [195; 196]. Moreover, the ease of availability, a high proliferation rate, and less discomfort and morbidities with the ASCs isolation are considered as the ratiocinations for increasing use of these cells for TE applications [187]. From an immunological point of view, ASCs are classified as immunosuppressive cells due to the lack of MHC class II antigens on their surface [197].