White adipose tissue, a. Photomicrograph of white adipose tissue, showing its characteristic meshwork in a H&E-stained paraffin preparation. Each space represents a single large drop of lipid before its dissolution from the cell during tissue preparation. The surrounding eosin-stained material represents the cytoplasm of the adjoining cells and some intervening connective tissue. X320. b. High-power photomicrograph of a glutaraldehyde-preserved, plastic-embedded specimen of white adipose tissue. The cytoplasm of the individual adipose
cells is recognizable in some areas, and part of the nucleus of one of the cells is included in the plane of section. A second nucleus (arrow), which appears intimately related to one of the adipose cells, may actually belong to a fibroblast; it is difficult to tell with assurance. Because of the large size of adipose cells, the nucleus is infrequently observed in a given cell. A capillary and a small venule are also evident in the photomicrograph. x950.
Deposition and mobilization of lipid are influenced by neural and hormonal factors
One of the major metabolic functions of adipose tissue involves the uptake of fatty acids from the blood and their conversion to triglyceride within the adipocyte. Triglyceride is then stored within the cell's lipid droplet. When adipose tissue is stimulated by neural or hormonal mechanisms, triglycerides are broken down into glycerol and fatty acids, a process called mobilization. The fatty acids pass through the adipocyte cell membrane to enter a capillary. Here, they are bound to the carrier protein albumin and transported to other cells, which use fatty acids as metabolic fuel.
Neural mobilization is particularly important during periods of fasting and exposure to severe cold. During early stages of experimental starvation in rodents, adipose cells in a denervated fat pad continue to deposit fat. Adipose cells in the intact contralateral fat pad mobilize fat. It is now known that norepinephrine (which is liberated by the endings of nerve cells of the sympathetic nervous system) initiates a series of metabolic steps that lead to the activation of lipase. This enzyme splits triglycerides (neutral fats), which constitute over 90% of the lipid in the fat of the adipocyte. The enzymatic activity is an early step in the mobilization of the lipid.
An unusual form of obesity is due to injury of the hypothalamus, the portion of the base of the brain where neurosecretions that control pituitary function are synthesized. Although the clinical manifestation of hypothalamic obesity is associated with increased food consumption, experiments with laboratory animals with hypothalamic injury show that caloric intake is not the only factor leading
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