BOX 4.1. Clinical Correlations: Adipose Tissue Tumors 162

s? overview of adipose tissue

Adipose tissue is a specialized connective tissue consisting of fat-storing cells (adipocytes) associated with a rich blood supply

Individual fat-storing cells, or adipocytes, and groups of adipocytes are found throughout loose connective tissue. Tissues in which adipocytes are the primary cell type are designated adipose tissue. Adipocytes function as fat-storage containers. The body has a limited capacity to store carbohydrate and protein; therefore, the fat contained within adipocytes represents the storage of excess nutritional calories that are not immediately used in metabolism or other activity. Fat is an efficient form of calorie storage because it has about twice the calorie density of carbohydrate and protein. The metabolism of fat can also be an essential source of water and energy for the body in the event of food deprivation. For example, the hump of a camel consists largely of fat and is a source of both energy and water for this desert animal.

There are two types of adipose tissue: white (unilocular) and brown (multilocular)

The two types of adipose tissue, white adipose tissue and brown adipose tissue, are so named because of their color in the living state.

• White adipose tissue is the predominant type in adult humans.

• Brown adipose tissue is present in humans during fetal life but diminishes during the first decade after birth.

v white adipose tissue Function of White Adipose Tissue

Functions of white adipose tissue include energy storage, insulation, and cushioning of vital organs

Unilocular adipose tissue forms a layer called the pan-niculus adiposus or hypodermis in the connective tissue under the skin. This subcutaneous layer of connective tissue has a significant insulating function. Concentrations of adipose tissue are found in the connective tissue under the skin of the abdomen, buttocks, axilla, and thigh. Sex differences in the thickness of this fatty layer in the skin of different parts of the body account, in part, for the differences in body contour between females and males. In both sexes, the breast is a preferential site for accumulation of adipose tissue; the nonlactating female breast is composed primarily of this tissue.

Internally, adipose tissue is preferentially located in the greater omentum, mesentery, and retroperitoneal space and is usually abundant around the kidneys. It is also found in bone marrow and between other tissues, where it fills in spaces. In the palms of the hands and the soles of the feet, beneath the visceral pericardium (around the outside of the heart), and in the orbits around the eyeballs, adipose tissue functions as a cushion. It retains this structural function even during reduced caloric intake; when adipose tissue elsewhere becomes depleted of lipid, this structural adipose tissue remains undiminished.

White adipose tissue produces the hormone leptin

Adipose tissue is exclusively responsible for the synthesis and secretion of leptin [Gr. leptos, thin], a 16-kDa peptide hormone involved in the regulation of energy homeostasis. Generally accepted biologic effects of leptin are inhibition of food intake, loss of body weight, and stimulation of the metabolic rate. Thus, leptin fulfills the criteria for a circulating satiety factor that controls food intake when the body's store of energy is sufficient. Leptin most likely participates in an endocrine signaling pathway that communicates the energy state of adipose tissue to centers that regulate energy uptake. It acts on the central nervous system by binding to specific receptors, mainly in the hypothalamus. In addition, leptin communicates the fuel state of adipocytes from fat storage sites to other metabolically active tissues (i.e., from adipose tissue to muscle at a different site).

Histogenesis of Fat Cells

Early histologists debated whether adipose tissue was a specific tissue, distinct from connective tissue, or whether it was ordinary connective tissue in which fibroblasts stored fat globules. The current consensus is that adipocytes are a specific cell type and that they are derived from undifferentiated mesenchymal cells associated with the adventitia of small venules (Eig. 6.1). Therefore, adipocytes originate from the same stem cell population as fibroblasts and myofibroblasts in healing wounds. Even with the transmission electron microscope (TEM), it is still nearly impossible to distinguish early lipoblasts or preadipocytes from fibroblasts. Thus, many investigators describe the early lipoblast as a cell that is committed to differentiate into an adipocyte but is morphologically indistinguishable from a fibroblast.

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