Adipose tissue
Adipocysts or fat cells are derived from undifferentiated fibroblast-like
mesenchymal cells. Adipose cells are fully differentiated and do not undergo cell
division. Adipose tissue are basically an aggregation of fats cell also called
adipocytes. It is found subcutaneously throughout the body except over the
eyelid, auricle, penis and scrotum. It is also present in the superficial fascia over
most of the body. This subcutaneous layer of fat is called the panniculus
adiposus. The distribution of subcutaneous fat in different parts of the body is
also different in the male and female and is responsible (to a great extent) for the
differences in body contours in the two sexes. In women it forms a thicker and
more even layer. Subcutaneous fat is not present in animals that have a thick coat
of fur. In the adult much of the space in marrow cavities of long bones is filled by
fat in the form of yellow bone marrow. Much fat is also present in synovial folds
of many joints filling spaces that would otherwise have been empty during certain
phases of movement.
Structurally, Fat cells may be scattered singly in some situations, but they are
usually aggregated into groups that form lobules of adipose tissue. The cells are
supported by reticular fibers, and the lobules are held together by areolar tissue.
Adipose tissue is richly supplied with blood, and is rich in enzyme systems.
Functions:
1. It acts as a store house of nutrition, fats are deposited when available in
excess and being removed when deficient in diet.
2. As insulation against heat loss, and performs this function when the
subcutaneous fat layer is thick.
3. It performs a mechanical function in many situations example the kidney
where it keeps it in position [floating kidney is when there is depletion of
this fat]. Also seen around the eyeball where it supports and allows the
eyeball to move smoothly. In palms, soles, buttocks where it has a
cushioning effect protecting underlying tissues from pressure.
4. It is also believed that adipose tissue contributes to warmth, by serving as a
heat generator. The heat generated can be rapidly passed on to
neighboring tissues because of the rich blood supply of adipose tissue.
Development:
It is believed that fats cells are derived from specific cells [lipoblasts] arising
during development from undifferentiated mesenchymal cells. Some authors
believes that adipose tissue are specialized tissues because of the following
reasons:
When an animal puts on fat it is because of an increase in the size of fat
cells rather than an increase in their number.
Mature adipose tissue does not appear to have any capacity for
regeneration. If a pad of fat is partially excised compensatory hypertrophy
cannot be observed in the remaining part.
In man the fat in fats cells is in the form of triglyceride: it consists mainly of
oleic acid, and of smaller amounts of linoleic and palmitic acids. The
composition of fat differs from species to species and is influenced by diet.
The esterification of triglyceride results in the liberation of large amounts of
heat.
Types of adipose tissues
i. Yellow (white) or unilocular adipose tissue (adult type)
ii. Brown or multilocular adipose tissue (embryonic type)
Unilocular adipocytes are cells with a single, large lipid droplets, they are
large spherical cells, up to 120 µm in diameter, that become
polyhedral when crowded into adipose tissue. Unilocular fat cells
continually store fat in the form of a single droplet, which
enlarges so much that the cytoplasm and nucleus are displaced
peripherally against the plasma membrane, thus giving these cells
a “signet ring” profile when viewed by light microscopy. Electron
micrographs reveal a small Golgi complex situated adjacent to the
nucleus, only a few mitochondria, and sparse rough endoplasmic
reticulum (RER) but an abundance of free ribosomes. That the fat
droplet is not bound by a membrane is clear in electron
micrographs but unclear in light micrographs. The external
surfaces of the plasma membranes are enveloped by a basal
lamina–like substance and possess receptors for glucocorticoids,
growth hormone, insulin, and norepinephrine whose function is to
regulate free fatty acid and glycerol transport into and out of
the cell. Minute pinocytotic vesicles, whose function is unclear,
have been noted on the surface of the plasma membrane.
During fasting, the cell surface becomes irregular, having
pseudopod-like projections.
Multilocular adipocytes contrast with unilocular adipocytes in
several ways in that they are cells with multiple, small lipids droplets.
Brown fat cells are smaller and more polygonal than white fat
cells. Moreover, because the brown fat cell stores fat in several
small droplets rather than a single droplet, the spherical nucleus
is not squeezed up against the plasma membrane. Multilocular
fat cells contain many more mitochondria but fewer free
ribosomes than unilocular fat cells. Although brown fat cells lack
RER, they do have smooth endoplasmic reticulum (SER).
Unilocular fat cells are found throughout the body in loose connective
tissue and are concentrated along blood vessels. Brown adipose tissue is
abundant in the newborn, but most of it is lost during childhood.
Brown fat is also abundant in hibernating animals in whom it serves mainly as a
heat generator when the animal comes out of hibernation. They may also
accumulate into masses, forming adipose tissue. Also white fat is much
more abundant than brown fats.
(A) Unilocular adipose tissue (white adipose tissue cell); (B) Multilocular
adipose tissue (brown adipose tissue cell) (Schematic representation)
White adipose tissue
CLINICAL POINT
Lipomas—the most common mesenchymal soft tissue tumor in adults—are
benign, slow-growing neoplasms found mostly in the proximal limbs, back,
shoulder, and neck. They typically originate in subcutaneous and submucosal
tissues, occur singly or in groups, and are only rarely found in internal organs or
intramuscular sites. Histologic examination shows encapsulated lobules of mature
unilocular adipocytes resembling those in normal white fat with minimal variation
in cell size. There usually are no mitotic figures, but some areas of fat necrosis
with macrophages and focal sites of infarction occur. Lipomas are typically around
5 cm in diameter but can grow larger. They can be removed by liposuction or local
surgical excision, usually without complications and with a low recurrence rate.
