AT types: structures and cellular composition
AT subtypes can be organized by their anatomical localization in mammals. SAT, found beneath the skin, contrasts with visceral adipose tissue (VAT), which lines internal organs (12). In addition, structural and functional features can be used to divide AT into three subtypes: white adipose tissue (WAT), brown adipose tissue (BAT), and beige adipose tissue (Fig 1a).
WAT is mainly known for its role in energy storage and immune regulation (13). It is the main constituent of visceral, and ectopic subcutaneous AT (SAT). In rodents, a striated muscle layer, the panniculus carnosus, subdivides SAT into two functionally distinct compartments, namely subcutaneous and dermal SAT (14). Such a barrier is missing in human SAT (14, 15).
In contrast to WAT, BAT is highly specialized for thermogenesis, capable of dissipating stored energy as heat to maintain optimal body temperature (16). BAT is abundant and broadly distributed in newborns. In adults, BAT is limited to cervical, supraclavicular, paravertebral, mesenteric, and pericardial areas. Beige AT emerges through “browning” of WAT, induced by external stimuli, such as low temperature or exercise (17, 18).
In all AT types, approximately one-third of the cellular content consists of adipocytes. The remaining two-thirds constitute the stromal vascular fraction (SVF) (Figure 1b). The stromal component of AT contains adipose stem cells (ASCs), preadipocytes, fibroblasts, endothelial cells, and immune cells. ASCs serve as precursor cells for preadipocytes (19, 20), specialized progenitors committed to becoming adipocytes and residing in a unique perivascular tissue niche (21-23). Fibroblasts in the SVF provide support to preadipocytes and help to maintain the adipose tissue homeostasis (24). In SAT, immune cells include macrophages (Mac), helper, cytotoxic, and regulatory T cells, natural killer cells, and B-lymphocytes (25). In healthy individuals, T cells and macrophages in SAT tend to favor a type 2 and regulatory phenotype (26-28). While T cells in epidermis and dermis generally adopt a T-helper 1 (Th1) phenotype, acting as the primary defense line in homeostatic conditions (26), T cells in SAT may function as counter-regulators (Fig 1c).