SnapShot Part I: Niche Determines Adipocyte Character I

Adipocytes are located throughout the body in discrete adipose depots (Cinti, 2005), as well as singly and in small clusters associated with vascular and lymphatic structures. Although adipocytes throughout the body have overlapping molecular and metabolic characteristics, the degree to which these properties are realized may depend on the specific niches in which these cells reside. The developmental and functional differences between depots are currently under intense study. Here, we present an atlas of rodent adipose depot anatomy to ensure that investigators are uniformly identifying major depots and are able to locate less commonly studied ones. In Part II, we will provide an overview of the properties of the most well-characterized depots.

When the skin of lean mice is pulled back, most visible adipocytes are contained within the anterior (interscapular, subscapular, axillary, and cervical) and posterior subcutaneous (dorsolumbar, inguinal, and gluteal) depots. Brown adipocytes are found within the interscapular depot at room temperature (Kajimura et al., 2015). Opening of the peritoneal cavity reveals the intra-abdominal depots, which surround the inner organs. These depots include mesenteric, a web-like structure surrounding the intestines; perirenal, surrounding the kidneys; and retroperitoneal, located along the dorsal wall of the abdominal cavity. We have also included the omental fat pad, attached to the stomach, because it is important in humans, although rarely seen in rodents. Rodent perigonadal depots include the parametrial depot, associated with the uterus and ovaries in females, and the epididymal depot adjacent to the testes in males. Whereas these depots are commonly studied in rodent models, they are not clinically relevant in humans.

Marrow adipose tissue, contained within bone, can be classified in rodents as constitutive (distal tibia and caudal vertebrae) or regulated (mid-to-proximal tibia, femur, and lumbar vertebrae) (Scheller et al., 2015). Understudied depots that are likely to have important local roles include intramuscular adipocytes interspersed between fibers of skeletal muscles; periarticular adipocytes (e.g., infrapatellar) surrounding or within joints; paracardial and epicardial depots that surround the heart; and retro-orbital, which lies behind the eye. The popliteal depot (Pond, 2005) is located in the popliteal fossa and the dermal depot (Alexander et al., 2015) is a thin layer of adipocytes between the dermis and panniculus carnosus muscle layer (image from Kasza et al., 2014). Investigation of the discrete adipose depots shown here will allow further definition of differences in development, gene expression, and function.

Figure 1.