Adipose tissue is a type of connective tissue that is composed mainly of adipocytes. It is found throughout the body and fulfills a number of important functions: it provides structural support and protective padding for major organs (e.g., kidneys), it serves as an insulating layer that prevents cutaneous heat loss, and it stores energy for longer periods of fasting. While the insulating and energy storage functions of adipose tissue provide decisive evolutionary advantages, their roles have diminished in modern times. Today, humans also rely on clothing for insulation and, in industrialized societies where food is readily available and affordable, its positive role as an energy reservoir has reversed to become a major health concern (obesity). However, in infants, adipose tissue continues to play a central role in the maintenance of body heat, and in individuals with severe illness, energy stores within the body make an essential contribution to sustaining the body.
There are two types of adipose tissue. Brown adipose tissue is abundant in neonates but recedes in adulthood. Its primary function is to prevent hypothermia through thermogenesis. White adipose tissue remains present throughout a lifetime and serves as the body's most important energy reservoir and also produces hormones.
- Energy storage: energy reservoir (triglycerides)
- Insulation: provides thermal insulation
- Structural support
- Estrogens: adipocytes express cytochrome P-450 aromatase, which catalyzes the conversion of steroids to estrogens (i.e., it converts to estrone).
- Reservoir for lipophilic biomolecules (e.g., )
Blocking aromatase activity is the mechanism of treatment of estrogen-dependent diseases such as breast cancer, endometriosis, and endometrial cancer. Obese women are more prone to develop these conditions due to higher circulating levels of estrogens synthesized by their adipose tissue.
- Depot fat: subcutaneous tissue, abdominal region
- Insulation fat: subcutaneous tissue
- Structural fat
A persistently high-calorie intake that exceeds daily needs can lead to a pathological increase of white adipose tissue, causing obesity.
Nonshivering thermogenesis (heat production)
- Mechanism: uncoupling of the respiratory chain in mitochondria with the help of to generate heat instead of ATP
Procedure: cold stimulus → increased activity of sympathetic nervous system → release of norepinephrine → stimulation of β3-adrenergic receptors of adipocytes for fat burning (beta oxidation)
- In beta oxidation, many reducing equivalents (NADH + H+, FADH2) accumulate and are introduced into the respiratory chain
- The , a protein channel, is incorporated into the inner mitochondrial membrane → increased permeability of mitochondrial membrane → influx of H+ into the matrix of the mitochondrion, bypassing the ATP synthase channel → electron transfer continues but ATP synthesis stops → the energy from the proton motive force is released in form of heat instead of producing ATP from ADP
- Importance: heat production of the newborn and hibernating animals.
- Infants: especially in the neck and thorax
- Adults: only surrounding the vertebra, vessels (along large arteries), mediastinum, and clavicle
- Embryonic: mesoderm germ layer → mesenchymal stem cells → brown adipose tissue
- In adult organism: regression over the course of a lifetime