Adipose tissue
It is a specialized connective tissue which is composed of fat cells,
reticular fibres and rich vascular supply.
In men of normal weight, adipose
tissue represents 15-20% of the body weight;
in women of normal weight, it
represents 20-25% of body weight.
Adipose tissue is a very efficient storage
tissue. It is in a state of continuous turnover and is sensitive to both
nervous and hormonal stimuli.
The sympathetic division of the autonomic
nervous system richly innervates both uni-locular and multi-locular adipose
tissues. In uni-locular adipose tissue, nerve endings are found only in the
walls of blood vessels; the adipocytes are not directly innervated. Release of
the neurotransmitter norepinephrine activates the hormone-sensitive lipase
described above. This innervation plays an important role in the mobilization
of fats.
Types:
1.
White (unilocular) and
2.
Beige adipocyte
3.
Brown (multilocular).
White
adipocyte :
These types of
adipocytes are predominantly found in adult. White adipocytes are spherical, but they
may appear polyhedral or oval when crowded together. It contains
unilocular fat which store triglyceride, flatten nucleus, rim of cytoplasm.
Adipocyte looks like signet ring
so it also known as signet ring cell. Early
lipoblasts look like fibroblasts but develop small lipid inclusions and a thin
external lamina.
Unilocular adipocytes are large cells, sometimes 100 µm or more in diameter. The lipid mass in the adipocyte is
not membrane bounded.
It
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. Each adipose cell is surrounded by a basal lamina.
Innervation:
Few
sympathetic nerve fibers
Secretions: White
adipose tissue produces a variety of hormones, growth factors, and cytokines.
ASP(Acylation-stimulating protein ) Influences the rate
of triglyceride synthesis in adipose tissue
Insulinlike
growth factor I (IGF-I) : Stimulates proliferation of a wide variety of cells
and mediates many of the effects of growth hormone
Leptin Regulates
appetite and body energy expenditure
Plasminogen
activator inhibitor-1 (PAI-1): Inhibits
the fibrinolytic system
Elevated levels
are associated with increased formation of blood clots
Prostaglandins
I2 F2_, (PGI2 and PGF2_) Helps regulate inflammation, blood clotting,
ovulation, menstruation,and acid secretion
Resistin: Increases insulin resistance ,Linked to
obesity and to type 2 diabetes
Distribution:
subcutaneous tissue, breast, greater omentum, mesenteries, visceral
pericardium, orbital cavity, retroperitoneal space, bone marrow
Metabolism:
It hydrolyzes the stored lipids
into fatty acids and glycerol, which are released from the cell.
Functions of white adipose tissue include
1.
metabolic energy storage,
2.
insulation,
3.
cushioning of vital organs, and
4.
secretion of hormones,
5. Source of
metabolic water.
6. Subcutaneous
layers of adipose tissue help to shape the surface of the body, whereas
deposits in the form of pads act as shock absorbers, chiefly in the soles and
palms.
Beige
adipocytes: Beige fat” is a new kind of
energy-burning cell that prevent obesity and help weight loss for adults.
Distribution:
This
new fat cell is found underneath the skin along the spine and in the proximal
area of the collarbone in deposits resembling the size of a pea.
Microscopic
features: Like brown fat, beige fat cells also have a plentiful amount of
mitochondria and iron, giving them their darker color.
Function:
Prevent obesity
Increase insulin receptor sensivity
How this cell is formed? :
These beige fat cells
are converted from white fat cells while exercising when muscles release a
hormone called irisin.
Brown adipocyte :
Brown adipose
tissue is present in humans during fetal life but diminishes during the first
decade after birth. Brown adipose tissue makes up about 5% of the total
body mass.
Microscopic
features: Multilocular adipose tissue
contains numerous fat droplets. Brown (multilocular) adipocytes are smaller in diameter (10–25 µm) than those of white adipose tissue.
Multilocular,
spherical, round eccentric nucleus. The multilocular adipocyte 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.
Function: Brown adipose tissue
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).
Distribution
:
It is located on the back, along the upper half of the spine, and 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
(i.e., aorta), and regions of the neck (deep cervical and supraclavicular),
regions of the back (interscapular and paravertebral), and thorax
(mediastinum).
Metabolism:
brown fat contain numerous mitochondria which are capable of uncoupling proton
leakage and instead of producing adenosine triphosphate (ATP), they release heat,
thus arousing the animal from hibernation.
