Adipose Tissue

Adipose tissue, also known as fat tissue or fatty tissue, is a connective tissue that is mainly composed of fat cells called adipocytes. Adipocytes are energy storing cells that contain large globules of fat known as lipid droplets surrounded by a structural network of fibers. 

Adipose tissue is a specialized type of connective tissue that arises from the differentiation of mesenchymal stem cells into adipocytes during fetal development. Mesenchymal stem cells are pluripotent cells that can transform into various cell types, including fat cells, bone cells, cartilage cells, and muscle cells, among others.

Adipocytes are categorized into three different cell types—white, brown, and beige adipocytes—based on their origin, location, and function. White adipocytes are the most abundant adipocytes in the human body. They are filled with a large, single lipid droplet and contain few cellular organelles. Brown adipocytes are very metabolically active cells that contain multiple lipid droplets, and a high concentration of mitochondria (i.e., a cellular organelle that allows brown adipocytes to generate heat). Beige adipocytes, also known as Brite adipocytes, are generally found scattered amongst pockets of white adipocytes and have the potential to generate heat under certain conditions, such as cold exposure and during stimulation of specific nervous adrenergic receptors. 

Based on the type of adipocytes, adipose tissue can be classified into two functionally different tissues: white adipose tissue, composed primarily of white and beige adipocytes, and brown adipose tissue, composed of brown adipocytes. The increased concentration of iron-containing mitochondria in brown adipocytes gives brown adipose tissue its characteristic dark color.

Adipose tissue can be found in a number of different places throughout the body. White adipose tissue is the most abundant type of fat in humans. It is distributed within subcutaneous fat, visceral fat, and bone marrow fat. Subcutaneous fat is found throughout the whole body, in the spaces between the skin and underlying muscles. Visceral fat is predominantly found around the organs in the abdominal cavity, such as the liver, intestines and kidneys, as well as in the peritoneum (i.e., a serous membrane that lines the outside of the abdominal organs). White adipose tissue is also present in the bone marrow (i.e., a sponge-like tissue present in the central cavity of bones). In addition, white adipose tissue can be found in the pericardium surrounding the heart, or cushioning other parts of the body, like the soles of the feet, eyeballs, and certain blood vessels.

Unlike white adipose tissue, brown adipose tissue, also known as brown fat, is mostly present during fetal life and in infants. In newborns, brown adipose tissue is mainly located on the back, along the upper half of the spine, in between the shoulders, and surrounding the kidneys. With age, the amount of brown fat decreases progressively. In adults, remaining brown fat deposits can be found surrounding the vertebrae, above the clavicles, in the upper back, and in the mediastinum (the central compartment of the thoracic cavity).

In healthy adults, adipose tissue usually comprises 20 to 25% of the total body weight. Nonetheless, the specific body fat percentage varies considerably amongst individuals, ranging from less than 10% to over 40% of the total body weight. Increased levels of adipose tissue has been associated with several health problems, including obesity, diabetes mellitus, and heart disease, among others.

The main function of white adipocytes is to store excess energy in the form of fatty molecules, mainly triglycerides. Fat storage is regulated by several hormones, including insulin, glucagon, catecholamines (e.g., adrenaline and noradrenaline), and cortisol. Depending on the body’s immediate energy requirements, these hormones can either stimulate adipose tissue formation and storage (i.e., lipogenesis) or initiate the release of fat from adipose tissue (i.e., lipolysis). Under the influence of insulin, for instance, adipocytes can increase the uptake of blood glucose and transform it into fatty molecules, thereby increasing fat storage.

In addition to being an energy storing reservoir, white adipose tissue performs important endocrine and metabolic roles by secreting several biologically-active factors known as adipokines. These molecules contribute to a variety of different functions, including regulation of energy balance, food intake and satiety, inflammatory response, and metabolism of steroid hormones. Finally, white adipose tissue also helps cushion and protect parts of the body, as well as insulate the body from extreme temperatures.

Conversely, the main role of brown fat is to use energy to generate heat through a process called non-shivering thermogenesis, which serves as an important defense mechanism to protect newborns against hypothermia. In adults, non-shivering thermogenesis becomes secondary to shivering thermogenesis, which is achieved by the contraction of skeletal muscles.

Adipose tissue is a specialized connective tissue mainly composed of fat cells known as adipocytes. Adipocytes can be subdivided into three cell types: white, brown and beige adipocytes, which differ in their structure, location, and function. Accordingly, adipose tissue can be classified as white adipose tissue, composed primarily of white and beige adipocytes, and brown adipose tissue, composed of brown adipocytes. White adipose tissue is the predominant type of fat in the human body. It can be found beneath the skin (i.e., subcutaneous fat), around internal organs (i.e., visceral fat), and in the central cavity of bones (i.e., bone marrow fat), as well as cushioning various parts of the body. Its main role is to serve as an energy storing reservoir, but it also insulates the body from extreme temperatures, cushions vital organs, and secretes hormones and biological factors. On the other hand, brown adipose tissue is mostly present during fetal life and in infants. It is mainly located in the upper back, above the clavicles, around the vertebrae, and in the mediastinum. The main role of brown adipose tissue is to generate heat through non-shivering thermogenesis; a process that’s especially important to prevent hypothermia in newborns.