Adipose tissue, or fat tissue, is considered a connective tissue even though it does not have fibroblasts or a real matrix and only has a few fibers. Adipose tissue is made up of cells called adipocytes that collect and store fat in the form of triglycerides, for energy metabolism. Adipose tissues additionally serve as insulation to help maintain body temperatures, allowing animals to be endothermic, and they function as cushioning against damage to body organs. Under a microscope, adipose tissue cells appear empty due to the extraction of fat during the processing of the material for viewing, as seen in Figure. The thin lines in the image are the cell membranes, and the nuclei are the small, black dots at the edges of the cells.

Fibroblasts are present in all connective tissue proper (Figure). Fibrocytes, adipocytes, and mesenchymal cells are fixed cells, which means they remain within the connective tissue. Other cells move in and out of the connective tissue in response to chemical signals. Macrophages, mast cells, lymphocytes, plasma cells, and phagocytic cells are found in connective tissue proper but are actually part of the immune system protecting the body.

Cell Types

The most abundant cell in connective tissue proper is the fibroblast. Polysaccharides and proteins secreted by fibroblasts combine with extra-cellular fluids to produce a viscous ground substance that, with embedded fibrous proteins, forms the extra-cellular matrix. As you might expect, a fibrocyte, a less active form of fibroblast, is the second most common cell type in connective tissue proper.

Adipocytes are cells that store lipids as droplets that fill most of the cytoplasm. There are two basic types of adipocytes: white and brown. The brown adipocytes store lipids as many droplets, and have high metabolic activity. In contrast, white fat adipocytes store lipids as a single large drop and are metabolically less active. Their effectiveness at storing large amounts of fat is witnessed in obese individuals. The number and type of adipocytes depends on the tissue and location, and vary among individuals in the population.

Loose Connective Tissue

Loose connective tissue is found between many organs where it acts both to absorb shock and bind tissues together. It allows water, salts, and various nutrients to diffuse through to adjacent or imbedded cells and tissues.

Adipose tissue consists mostly of fat storage cells, with little extracellular matrix (Figure). A large number of capillaries allow rapid storage and mobilization of lipid molecules. White adipose tissue is most abundant. It can appear yellow and owes its color to carotene and related pigments from plant food. White fat contributes mostly to lipid storage and can serve as insulation from cold temperatures and mechanical injuries. White adipose tissue can be found protecting the kidneys and cushioning the back of the eye. Brown adipose tissue is more common in infants, hence the term “baby fat.” In adults, there is a reduced amount of brown fat and it is found mainly in the neck and clavicular regions of the body. The many mitochondria in the cytoplasm of brown adipose tissue help explain its efficiency at metabolizing stored fat. Brown adipose tissue is thermogenic, meaning that as it breaks down fats, it releases metabolic heat, rather than producing adenosine triphosphate (ATP), a key molecule used in metabolism.

Fatty tissue composed of white adipocytes and generally found directly under the skin (subcutaneous fat) and around the internal organs (abdominal fat). It has less vascularization and less coloration than the brown fat. White fat provides heat insulation, mechanical cushion, and source of energy.

Brown fat is a special kind of fat that is found in only very small amounts in adults. Some researchers think it is more like muscle tissue than adipose tissue. Brown fat is found in the newborn animals of many species, as well as animals that hibernate. In a full-term human newborn, brown fat can account up to 7% of the infant's weight, and is concentrated in the back and nape of the neck, as well as around the heart, lungs, esophagus, liver and kidneys.

Brown fat, which was once thought to be useless tissue in adults, has a specialized function: to generate heat. The reason that babies have brown fat is that they can't shiver. As babies mature, they are able to rely more on physical methods of heat generation such as shivering and increasing their activity. With that development, brown fat gradually disappears.

Compared to a baby, an adult has only a tiny amount of brown fat. In recent years, however, researchers have discovered that adults have more of this special fat than was once thought. And furthermore, it appears that an adult's brown fat can still play a significant role in energy consumption and weight regulation.

More than 2 in 3 adults nationwide are considered overweight or obese. In recent years, researchers have gained insights into the roles of different types of fat tissue in weight control. Adults have mostly white fat, in which excess calories are stored as chemicals called fatty acids. Too much white fat increases the risk of several metabolic disorders.

Brown fat, in contrast, burns energy to create heat and help maintain body temperature. Infants have brown fat when they're born, but we lose it as we age. Brown fat was only recently proven to exist in adults. Since brown fat cells are able to burn calories, they could potentially play a role in strategies to reduce obesity and its associated disorders.