Thursday, August 11, 2022
Cold Thermogenesis

White adipocyte vs brown fat

white adipocyte : WA is white to yellowish white in colour
brown adipocyte :BA is brown in colour due to presence of large number of mitochondria
mitochondria :
WA contain poorly developed and less number of mitochondria
BA contain well developed and large number of mitochondria
WA : thin cytoplasmic rim contain a Golgi complex, few poorly developed mitochondria, poorly developed cisternae of the rough endoplasmic reticulum, and free polyribosomes,  the lipid droplet contains cisternae of smooth endoplasmic reticulum and numerous pinocytotic vesicles
BA: The multilocular adipocyte cytoplasm contains numerous mitochondria, a small Golgi apparatus, and only small amounts of rER and sER. The mitochondria contain large amounts of cytochrome oxidase, which imparts the brown color to the cells.
Predominently found in
WA: in adult life
BA: in fetal life
WA Unilocular adipocytes are large cells, 100 µm or more
BA brown (multilocular) adipose tissue are
Smaller diameter (10–25 µm)
Location/ distribution
subcutaneous tissue, breast, greater omentum, mesenteries, visceral pericardium, orbital cavity , retroperitoneal space, bone marrow 
BA: It is located on the back, along the upper half of the spine, & toward the shoulders.
In adult it is gradually decreases.
At  first decade of life it is situated in the cervical, axillary, paravertebral, mediastinal, sternal, and abdominal regions of the body.
It then disappears from most sites except for regions around the kidney, adrenal glands, large vessels (aorta), & regions of the neck (deep cervical and supraclavicular), regions of the back (interscapular and paravertebral), & thorax (mediastinum).
Fat droplet
WA contains unilocular fat
BAmultilocular adipose tissue contain numerous fat droplets.
Histological feature : HE stain
white adipocytes are spherical, but they may appear polyhedral or oval when crowded together, Unilocular, spherical, flatten nucleus, rim of cytoplasm. cell look like a ring due to large single lipid droplet which washout during histological slide preparation and leave a large white empty space .
BA: Multilocular, spherical, round eccentric nucleus, cytoplasm sponge like due to multiple lipid droplet
amount of Capillary

WA It contains less capillary because it consume less oxygen
BA contains more capillary because it consume more oxygen
WA Early lipoblasts look like fibroblasts also known as preadipocyte but develop small lipid inclusions and a thin external lamina.Growth and differentiation
Throughout entire life from stromal-vascular cells
BA Most brown fat cells originate from precursor cells in the embryonic mesoderm that also give rise to skeletal muscle cells and a subpopulation of white adipocytes. Only during fetal period
Decreases in adult life (exception: individuals
with pheochromocytoma and hibernoma)
UCP-1 genes expression The uncoupling protein (UCP) genes belong to the superfamily of electron transport carriers of the mitochondrial inner membrane.
WA not present
BA Yes (unique to brown fat)
unmyelinated sympathetic fibres density
WA less
BA more
percentage of body fat
White adipose tissue make up about 15-20% of the body mass in men of normal weight, it represents 20-25% of body mass in women of normal weight
brown adipose tissue makes up about 5% of the total body mass

Lipolysis and thermogenesis in white and brown adipocytes respectively regulated by sympathetic innervation.
WA: In white adipocytes, sympathetic innervation regulates a cascade of lipolysis to convert stored triglycerides (TG) to free fatty acids (FFA) and glycerol that can be used as fuels of other tissues.
BA: In brown adipocytes, sympathetic innervation regulates non-shivering thermogenesis. FFAs are transferred into mitochondria, primarily by carnitine palmitoyltransferase 1 (CPT1), and serve as fuel for β-oxidation. UCP1 is activated to disassociate respiratory chain from ATP production, and ultimately dissipates energy as heat.
Response to environmental stress (cold exposure) :
WA Decreased lipogenesis
Increased lipoprotein lipase activity
BA Increased lipogenesis
Decreased lipoprotein lipase activity
WA : acts as metabolic energy storage, insulation, cushioning of vital organs, and secretion of hormones, source of metabolic water.

BA is present in large amounts in the newborn, which  prevent  the extensive heat loss by heat production (thermogenesis) that results from the newborn’s high surface-to-mass ratio and avoid lethal hypothermia (a major risk of death for premature babies).
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