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Adipose tissue
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Special type of connective tissue with predominant adipose cells
Located isolated or in groups within loose/irregular connective tissue
15-20% of weight in men, 20-25% in women
Largest repository of energy in form of triacylglycerides (TAGs)
o Lower density than glycogen (mostly stored in muscle and liver)
and higher caloric value
o Thus efficient storage tissue
Adipose under constant metabolic turnover and also release hormones
and other factors
Poor heat conductor – thermal insulation
Padding between organs
Subcutaneous layers shape surface of body and act as shock
absorbers (mostly in soles and palms)
Two types that differ in
o Location
o Structure
o Colour
o Pathologic characteristics
White adipose tissue is the most common form and comprise one large
central droplet of fat in the cytoplasm
Brown adipose tissue contain cells with multiple lipid droplets
interspersed amongst numerous mitochondria (hence dark
appearance)

White adipose tissue
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Specialised for long-term energy storage
Large cells with one huge lipid droplet comprising 85% cell weight
Known as unilocular as triglycerides stored in single locus
Large droplet causes them to have eccentric and flattened nuclei
o Thickest portion of cytoplasm surrounds the nuclei
Adipose cells usually possess minute droplets as well as the large
central droplet – not enveloped but are surrounded by intermediate
filaments
Surrounded by thin basal lamina
Subdivided into incomplete lobules by portion of connective tissue with
rich vascular network and nerves; richly vascularised
Rich in fibroblasts and macrophages
Colour varies from white to yellow, depending on presence of
carotenoids

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Most adipose in adults is this type – age and gender determine
distribution and density
Uniform distribution of white adipose in the new-born – as they mature,
levels increase in some areas and decrease in others
o Partly regulated by sex hormones (disposition in breast and thigh)

Storage and mobilisation of lipids
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Lipids stored are mostly TAGs
TAGs originate in dietary fat and brought to adipose by circulating
chylomicrons and TAGs synthesised in the liver and released in VLDLs
Chylomicrons are particles formed in intestinal epithelial cells and
transported initially in lymph and then blood plasma
o Central core of TAGs and cholesterol esters
o Monolayer of apolipoproteins, cholesterol and phospholipids
Very low density lipoproteins (VLDLs) smaller than chylomicrons with
proportionally more lipid in their surface layer; also different
apolipoproteins and more cholesterol esters: TAGs vs chylomicrons
Chylomicrons and VLDLs are both hydrolysed at the surface of
endothelial cells by lipoprotein lipase (synth by adipose)
Free fatty acids (FFAs) enter the adipocyte by active transport and
diffusion
Within the adipocyte, FFAs combine with glycerol phosphate (supplied
by glucose metabolism) to form TAGs
TAGs then deposited in fat droplets
Insulin stimulates the following in adipose cells
o Glucose uptake
o Synthesise fatty acids from glucose
o Synthesis of lipoprotein lipase
Stimulation of adipocytes by nerves or hormones results in mobilisation
of lipid stores and release of FFAs and glycerol into the blood
o Noradrenaline triggers activation of hormone sensitive lipase
which breaks down TAGs at the surface of lipid droplets
o FFAs diffuse across the cell membrane to bind albumin in the
blood
o Glycerol is more water soluble and travels freely to the liver
Hormones important in regulation of lipid mobilisation and synthesis
o Insulin – increases lipid synthesis and inhibits hormone sensitive
lipase
o Glucagon and growth hormone promote TAG breakdown and
FFA release
Adipose is also an important endocrine organ

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o Release leptin, which targets the hypothalamus and other cells
to regulate appetite
Although white adipose appears histologically and physiologically
similar throughout the body, some differences in gene expression
between visceral deposits and subcutaneous deposits
o Increased visceral increases cardiovascular disease and
diabetes risk while subcutaneous does not
Lipids are mobilised uniformly except for the palms, soles and
retroorbital pads resist long periods of starvation
After starvation, adipocytes lose nearly all fat to become spindle
shaped with few droplets

Image taken from Mescher, Junqueira’s Basic Histology: Text and Atlas, 12th Edition
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Clinical note
 White adipose secretes a number of hormones and factors in addition to
leptin
o Paracrine and autocrine activity
o Not clear if produced by adipose or other cells within the tissue e
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macrophages and fibroblasts
 As obesity has increased adipose, it is associated with chronic inflammation

Brown adipose tissue
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Colour due to numerous mitochondria and blood capillaries
Contain many small lipid inclusions – multilocular
Small lipid droplets, mitochondria and vasculature all assist in primary
function of heat production
Limited distribution compared with white adipose

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Cells are smaller than white adipose but cytoplasm has more lipid
droplets of varying size
Spherical and central nuclei, mitochondria have elongated cristae
Tissue subdivided by connective tissue into lobules
Receive direct SNS innervation

Function of brown adipocytes
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Main function is heat production by non-shivering thermogenesis
More abundant in hibernating animals
Sympathetic nervous system (SNS) stimulation releases noradrenaline to
brown adipose – activates hormone-sensitive lipase promoting TAG
hydrolysis
o However, FFAs metabolised quicker than white fat, increasing
oxygen consumption and heat production
o Increases temperature and warms blood passing through it
Heat production increased in brown adipose as the mitochondria have
a transmembrane protein called thermogenin/uncoupling protein I
(UCP-1) in the inner cell membrane
Thermogenin permits backflow of proteins from the intermembrane
space into the mitochondrial matrix without passing through ATPsynthase
o Thus not used to synthesise ATP and is dissipated as heat
SNS stimulation also increases differentiation and inhibits apoptosis

Histogenesis of brown adipose
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Also develops from embryonic mesenchyme, emerges earlier than
white fat during embryonic development
Mesenchymal cells and lipoblasts resemble epithelium before
accumulating much fat
Brown fat mostly restricted to new-born, when non-shivering
thermogenesis most needed
Tissue largely disappears or converts to white fat with age; only in
scattered areas in adult (kidneys, adrenals, aorta, mediastinum)
Number of adipocytes increased during cold adaptation – usually
appears as clusters within white adipose
o Likely differentiation of mesenchymal stem cells

Clinical note
 Common benign tumours can form from unilocular adipocytes (lipomas)
 Malignant tumours originating from adipocytes (liposarcomas) are rare

Reference
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Junqueira’s Basic Histology: Text and Atlas
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