**1. Introduction**

Excess fat accumulation in adipose tissue causes obesity, which increases the risks of metabolic syndrome, diabetes, cardiovascular disease, and cancer. White adipose tissue (WAT) includes subcutaneous and visceral adipose tissue (SAT and VAT) with different metabolic features. SAT protects from metabolic disorders, while VAT promotes them [1].

SAT is the most important adipose tissue deposit and is characterized by its capacity to expand in reponse to surplus of energy. However, in the context of obesity, when the storage capacity of SAT is exceeded, fat is stored in other undesirable sites such as visceral depot or non-adipose organs (liver, skeletal muscle, myocardium, and pancreas). Impaired adipocyte development is associated with insulin resistance, so hypertrophic SAT is an important link with obesity-induced metabolic dysfunctions [2].

Adipocytes come from mesenchymal stem cells in the stroma of adipose tissue. These mesenchymal stem cells become preadipocytes when they lose their ability to differentiate into other mesenchymal lines and intervene in the adipocyte line. The second phase of adipogenesis is terminal differentiation, through which preadipocytes acquire the characteristics of mature adipocytes, acquiring lipid droplets and the ability to respond to hormones such as insulin. Terminal differentiation consists of a cascade of transcriptional events [3].

The number of mature adipocytes present in adipose tissue is largely determined by the ability of the limited number of preadipocytes to undergo the process of differentiation and the availability of mesenchymal cells to be differentiated into new preadipocytes when necessary [3]. Because new adipocytes are considered protective against metabolic dysfunction, it is plausible that the maladaptive adipogenesis could be involved in the pathogenesis of metabolic syndrome and insulin resistance associated with obesity [4]. In vitro studies have confirmed a decrease in the ability of adipogenic differentiation of ASCs in obese people.

The individual "set point" and the ability to expand the SAT depends on both the individual's genetic background and lifestyle. Studies have shown that obese people, metabolically healthy, have preservation of the architecture and functionality of adipose tissue. Women can recruit new fat cells in the femural or gluteal region at maturity. This ability to expand lower-body fat may reduce the abdominal subcutaneous adipocyte hypertrophy and the accumulation of ectopic visceral fat in obese women. By contrast, the reduced ability to expand SAT in lower-body region is observed in men and this is accompanied by the accumulation of fat in subcutaneous abdominal and visceral adipose tissues [5].

Adipocyte expansion, which involves adipose tissue-derived mesenchymal stem cells (ASCs), is a critical process with implications in the pathogenesis of metabolic syndrome and insulin resistance associated with obesity. Impaired subcutaneous adipogenesis leads to dysfunctional, hypertrophic adipocytes, chronic low-grade inflammation, and peripheric insulin resistance. Alternatively, it has also been proposed that the preservation of the functionality of subcutaneous adipocyte precursors could contribute to some obese individuals remaining metabolically healthy. Very few studies evaluated the changes in the adipogenic differentiation for human subcutaneous ASCs following bariatric surgery. Weight loss after bariatric surgery involves extensive adipose tissue remodeling, implicating mechanisms underlying adipose tissue plasticity, and the adipogenic potential.

### **2. Subcutaneous adipose stem cells**

#### **Isolation of subcutaneous human adipose stem cells**

SAT consists predominantly of adipocytes, but also contains other cell populations generally referred to as the stromal vascular fraction (SVF). Studies from the 1970s first revealed that fibroblast-like cells from the cultures of the SVF [stromal vascular cultures (SVCs)] could be propagated and differentiated into mature adipocytes in vitro. These in vitro stromal vascular-derived adipocytes, named adipose stem cells (ASCs), molecularly resemble the adipocytes found in their depot of origin [6]. After the isolation and proliferation of these ASCs, they can be used for the experimental study of the molecular processes in regulating adipocyte differentiation [7].

SAT can be isolated by a minimally invasive liposuction procedure. Tissue separation studies have involved the adipose stromal and vascular compartment as the site of origin of adipose stem cells. The SVF is operationally defined as a

heterogeneous mixture of cells, isolated by enzymatic dissociation and densitybased separation, a procedure designed to remove the group of cells that were in the deposits around the floating adipocytes. These stromal-vascular cells represent a rich potential resource for examining a variety of ambiguities relevant to adipogenesis as well as regenerative medicine [8].
