*2.1.2. Myofibroblasts: functions and origins*

Myofibroblasts, or activated fibroblasts, are contractile, resistant to apoptosis and have an upregulated rate of matrix deposition. They also express different cytokine and chemokine receptors that enable fiber regulation and wound contraction at injury sites [21–24]. In this sense, activated fibroblasts not only favor wound healing, but can also cause injuries when their activation is uncontrolled, producing a pathological fibrotic response [25, 26]. The precursor of myofibroblast is not precisely known, but many cells can differentiate into myofibroblasts through different signaling pathways or gene regulation. Some of these precursors include epithelial cells, ECs, pericytes, multipotent monocytes, and fibroblasts.

#### **2.2. Adipose tissue–derived stem cells**

Fat is an abundant and accessible source of stem cells. Adipose tissue–derived stem cells (ASCs) include preadipocytes, and a subpopulation of stromal cells able to differentiate into multilineages, including neuronal cells, chondrocytes, and osteoblasts [27, 28]. Moreover, these MSCs are able to secrete cytokines and growth factors promoting regenerative processes because they can influence cell recruitment, proliferation rates, or inhibit apoptosis [29, 30]. ASCs extracted from liposuctions can be expanded in culture and used as building blocks for tissue engineering. Both connective and adipose tissues were engineered in vitro using ASCs [31–33]. Both allogeneic and xenogeneic ASCs can be transplanted in patients regardless of their immunocompatibility and without the need of immunosuppression therapy, making them an unlimited source for regenerative medicine applications [34].
