Adipose tissue

Adipose tissue is a special type of connective tissue in which adipose cells predominate. These cells can be found isolated or in small groups within the connective tissue itself; most are found in large aggregates, making up the adipose tissue spread throughout the body. In men of normal weight, adipose tissue represents 15-20% of the body weight; in women of normal weight, 20-25% of body weight.

FUNCTIONS:

• Adipose tissue is the largest repository of energy (in the form of triglycerides) in the body. Since eating is a periodic activity and the supply of glycogen is limited, there must be a large store of calories that can be mobilized between meals. Because triglycerides are of lower density than glycogen and have a higher caloric value, adipose tissue is a very efficient storage tissue;

• Subcutaneous layers of adipose tissue help to shape the surface of the body, while deposits in the form of pads act as shock absorbers, chiefly in the soles and palms;

• Since fat is a poor heat conductor, it contributes to the thermal insulation of the body;

• Adipose tissue also fills up spaces between other tissues and helps to keep some organs in position;

• Fat cells produce hormones: leptin, estrogens.

There are 2 types of adipose tissue; they present different localizations, structures, colors, and pathologies.

• Unilocular (common or yellow) adipose tissue is composed of cells that when completely developed contain one large central droplet of yellow fat in their cytoplasm;

• Multilocular (or brown) adipose tissue is composed of cells that contain numerous lipid droplets and abundant, brown mitochondria.

Both types of adipose tissue have a rich blood supply.

Unilocular adipose tissue. Cells of this tissue have only one large fat vacuole; they are the main energy depot for the organism. The color of unilocular adipose tissue varies from white to dark yellow, depending on the diet; it is due mainly to the presence of carotenoids dissolved in fat droplets of the cells. Almost all adipose tissue in adults is of this type. The distribution and density of adipose deposits are determined by age and sex.

Unilocular adipose cells are spherical when isolated but are polyhedral in adipose tissue, where they are closely packed. Each cell is between 50 and 150 μm in diameter. Since lipid droplets are removed by the alcohol and xylol used in routine histologic techniques, each cell appears in standard microscope preparations as a thin ring of cytoplasm surrounding the vacuole left by the dissolved lipid droplet – the signet ring cell (Fig.11).Consequently, these cells have eccentric and flattened nuclei. The thickest portion of the cytoplasm surrounds the nucleus of these cells and contains a Golgi complex, filamentous and ovoid mitochondria, poorly developed cisternae of the RER, and free polyribosomes. The rim of the cytoplasm surrounding the lipid droplet contains vesicles of SER, and numerous pinocytotic vesicles.

Unilocular adipose tissue is subdivided into incomplete lobules by a partition of connective tissue containing a rich vascular bed and network of nerves. Reticular fibers form a fine interwoven network that supports individual fat cells and binds them together.

Adipose cells develop from mesenchymally derived lipoblasts. These cells have the appearance of fibroblasts but are able to accumulate fat in their cytoplasm. Lipid accumulations are at first isolated from one another but soon fuse to form the single larger droplet that is characteristic of unilocular tissue cells.

Fig.14. Origin of adipocytes.

Obesity in adults may result from an excessive accumulation of fat in unilocular tissue cells that become larger than usual (hypertrophic obesity). An increase in the number of adipocytes causes hyperplastic obesity.

Multilocular adipose tissue. Cells of multilocular adipose tissue have several fat vacuoles and transform stored chemical energy to heat when stimulated.

Multilocular adipose tissue is also called brown fat because of its color, which is due to both the large number of blood capillaries in this tissue and the numerous mitochondria (containing colored cytochromes) in the cells. Unlike unilocular tissue, which is present throughout the body, brown adipose tissue has a more limited distribution because it is more abundant in hibernating animals.

In the human embryo and the newborn, multilocular adipose tissue is encountered in several areas and remains restricted to these locations after birth. It is greatly reduced in adulthood. The function of this tissue in humans appears to be of importance mainly in the first months of postnatal life, when it produces heat and thus protects the newborn against cold.

Дата добавления: 2015-10-31