Adipose Tissue

Two kinds of adipose tissue are found in mammals:

white adipose tissue (WAT) and
brown adipose tissue (BAT).

White adipose tissue is the most common and is the fat that so many of us complain of acquiring.

Brown adipose tissue is present in small mammals (e.g., mice) and in newborn humans. Most of it disappears in adult humans.

The cells in both types of fat are called adipocytes although they differ in origin, structure, and function in the two types of tissue.

WAT Adipocytes	BAT Adipocytes
a narrow rim of cytoplasm with its nucleus pressed near the margin of the cell surrounding	Cytoplasm throughout the cell with a central nucleus and
a single large membrane-enclosed lipid droplet [View]	many small lipid droplets
few mitochondria	many mitochondria (providing the brown color)
modest blood supply	rich blood supply
serves as a depot of stored energy	function is to generate heat

New adipocytes in white adipose tissue are formed throughout life from a pool of precursor cells. These are needed to replace those that die (after an average life span of 10 years). They are probably also at least partially responsible for the increase in the volume of fat tissue e.g., in obesity. However, an increase in the size of individual adipocytes as they become filled with oil also contributes.

The adipocytes of white adipose tissue secrete several hormones, including:

leptin
asprosin (which, during fasting, causes a rapid release of blood sugar (glucose) from the liver).
adiponectin.

In addition to serving as a major source of energy reserves, white adipose tissue also provides some mechanical protection and insulation to the body. Obesity is the excessive accumulation of white adipose tissue.

Brown adipose tissue provides a vital source of heat to maintain body temperature in small mammals (with their high surface to volume ratio) and infants (who usually cannot shiver when they are cold).

Brown adipose tissue is activated when the body temperature drops. This is the mechanism:

Cold activates the sympathetic nervous system.
Noradrenaline is released by the postganglionic neurons.
The noradrenaline binds to G-protein-coupled receptors on the adipocytes surface.
The second messenger cAMP is generated and moves into the nucleus where
it binds to the promoter of the gene encoding an enzyme that converts thyroxine (T₄) to triiodothyronine (T₃) [Link].
T₃ enters the nucleus and bind to the promoter of the gene encoding uncoupling protein1 (UCP1).
UCP1 inserts into the inner membrane of the mitochondria where
it allows the protons that have been pumped out into the intermembrane space by the electron transport chain
to return to the matrix without having to pass through ATP synthase.

So instead of cellular respiration (of fatty acids and glucose) generating ATP, it generates heat instead.

WAT Can Acquire the Properties of BAT

Skeletal muscles that have undergone a period of vigorous exercise secrete a protein hormone called irisin. Irisin acts on white adipose tissue to give it the properties of brown adipose tissue:

an increase in the number of mitochondria and lipid droplets;
a marked increase in the synthesis of UCP1;
an increase in the rate of cellular respiration but with the energy released as heat rather than fueling the synthesis of ATP.

These brown-like fat cells derived from white fat cells have been called "beige" or "brite" cells.

Lean adult humans have deposits of beige cells in the neck and upper chest regions. When they are exposed to cold, their beige cells are activated. Obese people have few or no beige cells. Probably their layers of white adipose tissue provide such good insulation that they are in less danger of heat loss.

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6 May 2019