Body Fat

Adipose tissue

In histology, adipose tissue or fat is loose connective tissue composed of adipocytes. Adipose tissue is derived from lipoblasts. Its main role is to store energy in the form of fat, although it also cushions and insulates the body. Obesity or being overweight in humans and most animals does not depend on body weight but on the amount of body fat—specifically, adipose tissue. Two types of adipose tissue exist: white adipose tissue (WAT) and brown adipose tissue (BAT). Adipose tissue also serves as an important endocrine organ by producing recently-discovered hormones such as leptin, resistin and the cytokine TNFα. The formation of adipose tissue appears to be controlled by the adipose gene.

Anatomical features

In humans, adipose tissue is located beneath the skin, and is also found around internal organs. Adipose tissue is found in specific locations which are referred to as 'adipose depots'. Adipose tissue contains several cell types, with the highest percentage of cells being adipocytes, which contain fat droplets. Other cell types include fibroblasts, macrophages and endothelial cells. Adipose tissue contains many small blood vessels. In the integumentary system, which includes the skin, it accumulates in the deepest level, the subcutaneous layer, providing insulation from heat and cold. Around organs, it provides protective padding. However, its main function is to be a reserve of lipids, which can be burned to meet the energy needs of the body. Adipose depots in different parts of the body have different biochemical profiles.

In a severely obese person, excess adipose tissue hanging downward from the abdomen is referred to as a panniculus (or pannus). A panniculus complicates surgery of the morbidly obese. The panniculus may remain as a literal "apron of skin" if a severely obese person quickly loses large amounts of fat (a common result of gastric bypass surgery). This condition cannot be effectively corrected through diet and exercise alone, as the panniculus consists of adipocytes and other supporting cell types shrunken to their minimum volume and diameter. Reconstructive surgery is one way to fix the problem.

In mice, there are eight major adipose depots, four of which are within the abdominal cavity: the paired gonadal depots are attached to the uterus and ovaries in females and the epididymis and testes in males, the paired retroperitoneal depots are found along the dorsal wall of the abdomen, surrounding the kidney, and when massive extend into the pelvis. The mesenteric depot forms a glue-like web that supports the intestines, and the omental depot, which originates near the stomach and spleen and when massive extends into the ventral abdomen. Both the mesenteric and omental depots incorporate much lymphoid tissue as lymph nodes and milky spots respectively. The two superficial depots are the paired inguinal depots, which are found anterior to the upper segment of the hind limbs (underneath the skin) and the subscapular depots, paired medial mixtures of brown adipose tissue adjacent to regions of white adipose tissue, which are found under the skin between the dorsal crests of the scapulae. The layer of brown adipose tissue in this depot is often covered by a “frosting” of white adipose tissue, sometimes these two types of fat (brown and white) are hard to distinguish. The inguinal depots enclose the inguinal group of lymph nodes. Minor depots include the pericardial which surrounds the heart, and the paired popliteal depots, between the major muscles behind the knees, each containing one large lymph node(Pond 1998). Of all the depots in the mouse, the gonadal depots are the largest and the most easily dissected (Cinti, 1999), comprising about 30% of dissectible fat, e.g., (Bachmanov et al. 2001).