Adipose tissue

Adipose tissue is the anatomical name for body fat found in humans and other mammals. Adipose tissue is composed of a loose collection of adipocytes, or cells specially designed to provide storage for lipids and triglycerides. Lipids and triglycerides are naturally occurring, non–water soluble molecules that store energy for the body's future use. These fat reserves help insulate and protect the body. They provide a source of energy when food is unavailable. When more energy is consumed than the body needs, the quantity of adipose tissue grows, leading to obesity.

For many years, scientists believed that adipose tissue functioned simply as a storage container, holding energy in fat for future use. Twenty-first-century research has shown that it takes an active role in a number of key body functions, including regulating appetite, body weight, immunity, reproduction, and the way the body uses insulin and ingested fats. Although too much body fat can be a bad thing, adipose tissue plays an essential role in overall health.

Background

The two main forms of adipose tissue are white and brown. The center of most white adipocytes is a vacuole, or a sack surrounded by a membrane, filled with lipids. The remainder of the cell components surround the vacuole, giving the adipocyte a ring-like appearance. In brown adipocytes, lipids are dispersed in smaller drops throughout the cell. Brown adipocyte cells have a larger blood supply to help promote the provision of heat to the body. Adipocytes may be as small as 30 microns or larger than 230 microns. Brown cells are generally smaller than white cells. They are loosely held together by collagen fibers to form the third layer of the body under the skin.

White and brown adipose tissue are similar in composition but have some key differences. White adipose tissue stores energy and helps regulate other functions of the body. In addition, white adipose tissue is a poor conductor of heat, making it a very effective form of insulation for the body. This type is far more common and is the type people usually are referring to when they speak of body fat.

Brown adipose tissue is found more commonly in small mammals, such as mice and rats, and in human infants. The main function of brown adipose is to maintain body heat. Therefore, it is more common in small mammals, which are more vulnerable to cold because of their size. Human infants are generally unable to generate body heat by shivering, so the presence of a larger percentage of brown adipose tissue functions as an additional heat source.

As babies grow, brown adipose tissue disappears, and it is rarely present in adults. However, some individuals can develop so-called "beige" fat cells. This type of adipose tissue has been found in mice and in some people who exercise heavily. Exercise prompts skeletal muscles to release a protein called irisin, which in turn acts on white adipose cells to make them function like brown cells. An athlete's lean body can then compensate for the lack of insulating body fat by generating heat from the transformed white adipose tissue as if it were brown adipose tissue.

The number and type of adipose fat cells changes over an individual's lifetime. Each adipocyte has a life span of about ten years, after which it is replaced by a new cell. In humans, brown cells diminish, and white cells become more predominant. People have a tendency to gain weight with age, which is known to be the result of individual adipocytes growing in size. It is unclear, however, whether the body continues to increase the number of adipocytes it produces once an individual reaches adulthood or simply replaces worn out cells.

In addition to helping to regulate the body's temperature through insulation or heat generation, adipose tissue functions as an unofficial part of the endocrine system. The endocrine system regulates most of the body's important functions through the release of hormones from the thyroid, pancreas, pituitary gland, adrenal glands, and other glandular organs. Adipose tissue releases more than fifty adipokines, or substances that act like hormones, even though it is not technically part of the endocrine system.

Two of the more important adipokines produced by adipose tissue are adiponectin and leptin. Adiponectin affects how the body metabolizes fats and sugar; with more adiponectin, fats and sugars are more efficiently processed through the bloodstream. Among the more important results of this are increased glucose tolerance and decreased insulin resistance, leading to lower risk of diabetes. Leptin helps the hypothalamus regulate appetite; when leptin levels are low, individuals are more likely to overeat and become obese. In addition, a relationship exists between the amount of adipose tissue and the effectiveness of the adipokines, and researchers continue to study this relationship as a potential means to reduce the incidence of weight-related illnesses such as diabetes.

Topic Today

The role of adipose tissue in overall health is thought to be related to inflammation. Inflammation is the body's response to something harmful. Researchers have determined that high levels of adipose tissue—obesity—create a low level of persistent inflammation in a body, which affects the immune system and makes the body more susceptible to attack in the form of illnesses, including cancer. Adipose tissue has also been found to affect the function of the reproductive system, including the levels of the hormones necessary for fertility. The presence of either too much or too little adipose tissue can negatively affect fertility and reproduction.

Scientists continue to work to understand the complex interactions between adipose tissue and other body systems in the hopes of reducing or eliminating diseases such as type 2 diabetes and other weight-related conditions. They also are considering the unique aspects of adipose tissue to help find treatments for other conditions. For example, among the materials that make up adipocytes are stem cells, which are generally more abundant in and more easily harvested from adipose tissue than stem cells from bone marrow and other sources. These cells may help to treat bones that resist healing after breaks and other conditions.

Bibliography

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