Adipose Cells: Structure, Function, and Obesity

Introduction to Adipose Tissue

The medical and biological sciences have long been fascinated with the study of cells, and for good reason: within them one can find many clues to the development of the human body. Adipose cells have become quite prominent — especially for medical and clinical researchers — though their prominence is not necessarily a positive one, since a breakdown within the adipose system can lead to obesity, eating disorders, diabetes, and heart failure.

Adipose tissue represents one of the most complex, highly active, and essential parts of the metabolism, and it also functions as a critical endocrine organ. "Adipose tissue is a highly specialized connective tissue found in two forms: white and brown. Both of these forms serve to insulate and cushion the body, but they each have specialized functions as well." Brown adipose tissue, named for its color, is more vascularized, and its primary property is to serve as a heat source in the body. As the organism ages, brown adipose tissue is gradually replaced by white adipose tissue, whose role is to supply energy for the proper functioning of the body (Gomillion & Burg 2006, p. 6053).

Adipocytes, alongside tissue matrix, nerves, stromal vascular cells, and immune cells, create the key components of this integrated unit. Beyond its important endocrine function, adipose tissue responds to signals received from the hormonal and nervous systems. Its secretion factors include "leptin, other cytokines, adiponectin, complement components, plasminogen activator inhibitor-1, proteins of the renin-angiotensin system, and resistin." Another important role of adipose cells lies in the regulation of the metabolism of sex steroids and glucocorticoids, but the major part of the endocrine purpose of adipose tissue is underscored by the adverse metabolic consequences of both adipose tissue excess and deficiency (Kershaw & Flier 2004).

Structure of White Adipose Cells

To better understand how white adipose cells function in relation to the many working networks within each organism, it is important to examine their structure briefly but precisely.

Almost 60 to 85% of the weight of white adipose tissue is lipid, of which 90 to 99% is triglyceride. Fatty acids, diglyceride, cholesterol, phospholipid, and small particles of cholesterol ester and monoglyceride are also present. Within this intricate component of any living organism, six fatty acids make up 90% of the total: myristic, palmitic, palmitoleic, stearic, oleic, and linoleic. Based on each individual's diet, these components can vary in proportion and thus in their metabolic role. The residual mass of white adipose tissue consists of 5 to 30% water and 2 to 3% protein.

While white adipose tissue is not as vascularized as brown adipose tissue, each adipocyte is in contact with at least one capillary, which supplies the metabolism with sufficient blood — every action taking place in the thin rim of cytoplasm surrounding the lipid droplet. Blood flow varies based on body weight and nutritional state; prolonged periods of fasting or dieting are associated with increased flow (Albright & Stern 1998).

Adipose Cell Dysfunction and Obesity

Various conditions are related to the functions of adipose cells, and researchers around the world have worked to understand why these cells cease to function normally. One of the most common such conditions is obesity. Viewed broadly, one might attribute obesity to poor eating habits, lack of physical activity, or hormonal imbalance; but examined at the microscopic level, many answers lie within the adipose cells themselves and the many theories derived from their role and structure.

Wellen and Hotamisligil (2003, p. 1785) state that obesity and associated metabolic pathologies affect over 50% of the adult population, and that "these conditions are associated with a chronic inflammatory response characterized by abnormal cytokine production, increased acute-phase reactants, and activation of inflammatory signaling pathways." During the progression of the disease, many factors trigger the inflammatory response — residing mostly in the adipose tissue — but critical questions about metabolic deregulation and the underlying molecular mechanisms remain unanswered.

Other scientists have proposed that answers to many obesity problems can be found in adipose tissue cellularity and the ability to calculate fat cell number and weight. These conclusions were confirmed by the experiments of Sims et al., which demonstrated that adipose mass increase is directly connected to an increase in fat cell weight. This increase was also shown to be more rapid and more lasting in subjects with a higher number of fat cells (Bosello et al. 1980). Such discoveries are groundbreaking and can lead to better treatments for people with treatment-resistant obesity — a growing concern, due to either genetic malfunctions or the disproportion of white fat cells.

Based on recent findings, the size of the adipose depot in humans and animals is proportionally dependent on the number and size of its constituent cells. Therefore, over the course of an individual's lifetime, the outcome achieved by any organ in the body will be modified by factors that influence cell division and/or cellular enlargement. "It has been demonstrated in other organ systems that the degree to which either of these mechanisms is modulated by nutritional factors depends in part on the age of the animal. The earlier in life that they exert their influence, the greater the likelihood that permanent alterations in body and organ size will occur." Although this theory has strong scientific foundations, few comparative studies of cellular growth exist, and little is known about the factors that determine the number of adipose cells (Knittle & Hirsch 1968, p. 2091).

Obesity is not simply an eating disorder. In addition to its physical and pathological characteristics, a psychological dimension strongly influences how the condition evolves. The way others perceive us is, unfortunately, in today's society closely connected to appearance, and unflattering external remarks can severely undermine the treatment an individual is undertaking. Alongside a well-planned diet and fitness program, a clear and motivated mindset and a rational self-image are crucial elements in treating not only eating disorders, but any illness.

Conclusion

Research on adipose cells is not at its beginning, but has become increasingly intensive in recent years. Obesity is rapidly spreading across the globe, but scientific breakthroughs alone will not produce results in the presence of disinterest and neglect toward the human body — the first step must come from each individual.

References

Albright, AL & Stern, JS 1998, "Adipose Tissue," Encyclopedia of Sports Medicine and Science, vol. 15.

Bosello, O et al. 1980, "Adipose tissue cellularity and weight reduction forecasting," American Journal of Clinical Nutrition, vol. 33, no. 4, pp. 776–782.

Gomillion, CT & Burg, KJL 2006, "Stem cells and adipose tissue engineering," Biomaterials, no. 27, pp. 6052–6060.

Greenberg, AS & Obin, MS 2006, "Obesity and the role of adipose tissue in inflammation and metabolism," American Journal of Clinical Nutrition, vol. 83, no. 2, pp. 461–465.

James, PT, Leach, R, Kalamare, E & Shayegh, M 2001, "The Worldwide Obesity Epidemic," Obesity Research, vol. 9, pp. 228S–233S.

Kershaw, EE & Flier, JS 2004, "Adipose Tissue as an Endocrine Organ," The Journal of Clinical Endocrinology & Metabolism, vol. 89, no. 6, pp. 2548–2556.

Knittle, JL & Hirsch, J 1968, "Effect of early nutrition on the development of rat epididymal fat pads: cellularity and metabolism," The Journal of Clinical Investigation, vol. 47, pp. 2091–2098.

Wellen, KE & Hotamisligil, GS 2003, "Obesity-induced inflammatory changes in adipose tissue," The Journal of Clinical Investigation, vol. 112, no. 12, pp. 1785–1788.