Adipose Tissue

Adipose Tissue: Definition and Meaning

The adipose tissue, also known as body fat or simply fat, are connective tissues that are made up of adipocytes. The adipocytes are storage reservoirs that store fat globules in a structural network of fibres. Apart from storage, it also serves as a cushion and insulator to protect our bodies. Fats can be found subcutaneously (under the skin), between muscles, in the folds of the intestine, around the heart, in the bone marrow and in breast tissues.

Recently, the adipose tissue has been acknowledged as an endocrine gland because it produces hormones, such as oestrogen, leptin, resistin and cytokines. Adipose tissue is also associated with the release of pro-inflammatory markers called adipokines in the development of metabolic syndromes, such as type-2 diabetes, atherosclerosis and cardiovascular diseases.

Classification of Adipose Tissue

The adipose tissue arises from the differentiation of mesenchymal stem cells into adipocytes at the time of foetal development. The adipocytes can be classified into three different cell types based on their function, origin, and location. The three types are white, beige and brown adipocytes.

White adipocytes are the most common in the human body; they are filled with a single, large lipid droplet and some cellular organelles. Brown adipocytes are filled with multiple lipid droplets and have a large number of mitochondria that helps them to generate heat. Beige adipocytes are found scattered between the white adipocytes. They have the ability to generate heat under conditions, such as exposure to cold or stimulation of the adrenergic receptors. They are also known as Brite adipocytes.

The adipose tissue can be divided into two on the basis of types of adipocytes: white adipose tissue and brown adipose tissue. White adipose tissue is composed of white adipocytes that are scattered with beige adipocytes and are abundantly found. Brown adipose tissue is composed of brown adipocytes and is mostly found in newborn organisms.

Function of Adipose Tissue

1. White Adipose Tissue: White adipose tissue is the most abundant type that not only serves as an insulator but also as an energy reservoir at the time of starvation and exertion. At the time of energy requirement by muscles or tissues, hormones, such as epinephrine, glucagon, norepinephrine and adrenocorticotropin, bind to the adipocytes that induces hydrolysis of triacylglycerol such that energy-rich fatty acids and glycerols are formed. The process is known as lipolysis and is catalysed by the enzyme lipase.

The white adipose tissue also releases a number of hormones, such as leptin, resistin and adiponectin, that play various roles in metabolism.

2. Brown Adipose Tissue: The brown adipose tissue commonly occurs in newborn organisms and starts to decrease as the organism ages. They generate heat and consume energy. However, in animals that hibernate, brown adipose tissue persists even in adolescence because these tissues help in their survival at the time of hibernation.

During hibernation, the animal experiences a rapid drop in temperature and the metabolic rate is also decreased; thus, there is high conservation of energy in their body. At the time of breaking dormancy, brown adipose tissues produce heat and consume energy to make the animal active again.