and Jürgen Roth2
Medical University of Vienna, Vienna, Austria
University of Zurich, Zurich, Switzerland
White Adipose Tissue
Adipose tissue exists chiefly in two different forms: white and brown adipose tissue. Both have a highly differing fine structure and their location differs significantly. White adipose tissue is found in the subcutaneous tissue, where it exists mainly as single adipocytes (panel A) and in small groups often along capillaries (inset in panel A), or in the peritoneal cavity where it forms a compact tissue, the visceral fat. White adipocytes contain a single lipid droplet whose size can vary between 40 and 120 μm. In the white adipose tissue, brown-like adipocytes, named beige or brite (brown-in-white) adipocytes, exist, which undergo reversible transition with white adipocytes.
Panel A shows an entire white adipocyte and parts of adjacent white adipocytes of the subcutaneous tissue. The predominant feature is the single, slightly electron dense lipid droplet (L), which gives them the appearance of signet ring. The adipocytes are surrounded by a basement membrane and its dense lamina is well visible in panel B (Lb). The cytoplasm exists only as a small peripheral rim, which contains a flat nucleus, ER, mitochondria (M), and the Golgi apparatus The area marked in panel A is shown at higher magnification in panel B. Along the plasma membrane, numerous caveolae exist (arrows). The lipid droplet is not surrounded by a double-layered membrane (cf. Fig. 83). However, a flat network of intermediate filaments exists between the cytoplasm and the lipid droplet, which can be only fully appreciated in grazing sections.
The white adipose tissue represents an important source for energy. Depending on the energy need of the organism, triglycerides of the lipid droplet will be metabolized or stored. The principal source for the synthesis of the triglycerides of the lipid droplet are free fatty acids. They arrive with chylomicrons from the intestine or with very-low-density lipoproteins from the liver. The lipoprotein synthase of the white adipocytes, after transport to adjacent endothelial cells of capillaries, releases the fatty acids from their protein carriers for subsequent transport to the adipocytes. Here, triglycerides are synthesized from them in the smooth endoplasmic reticulum and subsequently integrated in the lipid droplet. The mobilization of free fatty acids from the lipid droplet triglycerides occurs by the action of a hormone-sensitive lipase. After phosphorylation of the lipase by glucagon, ACTH, or adrenalin, lipolysis occurs. On the other hand, insulin and prostaglandins exhibit an inhibitory effect on the lipase, which stimulates the storage of triglycerides in the white adipose tissue.
In addition to its main function in energy metabolism, white adipose tissue also acts as an insulator and represents a storage site for water. From a mechanical point of view, it is important for the correct positioning of various organs such as the kidneys and eyes and functions as an elastic cushion at sites of high mechanical stress such as the soles and palms as well as joints.
The white adipose tissue is also active as an endocrine organ by secreting various adipocytokines such as leptin, adiponectin, resistin, visfatin, tumor necrosis factor α, and plasminogen activator inhibitor-1. The adipocytokines play important roles in metabolic homeostasis and their dysregulation results in metabolic disease and arteriosclerosis.