Adipose tissue

It is a dynamic tissue considered to be the largest endocrine organ in the human body. It is of two types - brown and white adipose tissue which show anatomical and functional differences. White adipose tissue is seen in adults while brown fat is seen mostly in newborns and behind the kidneys in adults. White adipose tissue is made up of adipocytes or fat cells which are larger and unilocular meaning have a single droplet of fat in the cytoplasm, nucleus is flat and peripheral and have fewer mitochondria. It looks yellow due to the presence of carotene. It is distributed in the subcutaneous tissues, abdominal cavity in the peritoneal folds, mediastinum, between the muscles, bone marrow and around visceral organs like the kidneys. Comparatively, brown adipocytes are smaller, multilocular, with multiple fat droplets in the cytoplasm, rich in mitochondria and have a centrally placed nucleus. Brown fat specializes in thermogenesis, helping to keep the body warm. It participates in non-shivering thermogenesis by the oxidation of fatty acids and by the action of uncoupling protein (UCP 1). The color of brown fat is from rich circulation and high number of mitochondria with the heme containing cytochrome c.

Apart from adipocytes, adipose tissue also contains a stroma comprised of blood vessels, pericytes and precursor cells called pre-adipocytes. In the presence of excess fat for storage, the existing adipocytes undergo hypertrophy while the pre-adipocytes differentiate to adipocytes. White adipose tissue plays a key role in everyday metabolism by secreting many hormones, growth factors and cytokines with endocrine, paracrine and autocrine functions. After menopause, androstenedione, secreted by the adrenal gland, is converted into oestrone in the adipose tissue. Estrone is then eventually converted to oestradiol in peripheral tissues.

Important molecules secreted by adipose tissue and their effects on the body