Adipose Tissue: Types and Function

• Adipose tissue is a specialized connective tissue that plays an important role in energy homeostasis.
• Individual fat cells, called adipocytes, are found throughout loose connective tissue.
• Tissues where adipocytes are the main cell type are known as adipose tissue.
• Adipocytes have a key role in energy homeostasis.

Energy Storage

• The body cannot store unlimited carbohydrates and proteins so energy reserves are stored within lipid droplets of adipocytes as triglycerides.
• Stored energy in adipocytes can be rapidly released for use in other parts of the body.

Types of Adipose Tissue

• There are two types of adipose tissue: white (unilocular) and brown (multilocular).
• The names refer to their color in the living state.
• White adipose tissue is most common in adult humans.
• Brown adipose tissue is present during fetal life but diminishes after the first decade after birth.

White Adipose Tissue

• White adipose tissue functions in energy storage, insulation, cushioning of vital organs, and hormone secretion.
• It forms a layer called the panniculus adiposus (or hypodermis) in the connective tissue under the skin.
• Adipose tissue has half the thermal conductivity of skeletal muscle, and the subcutaneous layer of connective tissue provides thermal insulation against cold by reducing heat loss.
• Concentrations of white adipose tissue are found under the skin of the abdomen, buttocks, axilla, and thigh.
• Sex differences in the fatty layer thickness in the skin cause body contour differences between females and males.
• The mammary fat pad is a primary site for adipose tissue accumulation in both sexes, and the nonlactating female breast is mainly composed of this tissue.
• The mammary fat pad supports breast function in lactating females.
• It provides lipids and energy for milk production and is the site for growth factor synthesis, which modulates responses to steroid, protein, and hormones for mammary gland function.
• White adipose tissue is areolar, but very sparse.
• Closely packed adipocytes, or fat cells, have a nucleus pushed to the side by a large fat droplet.
• White adipose tissue is located under the skin in the hypodermis, around kidneys and eyeballs, within the abdomen, and in breasts.
• Sex differences occur in this fatty layer's thickness in different body parts.

Differentiation of Adipocytes

• White adipocytes differentiate from mesenchymal stem cells controlled by PPAR /RXR transcription factors.
• Adipocytes are a specific cell type from undifferentiated mesenchymal stem cells associated with small venule adventitia.
• A transcription factor called peroxisome proliferator-activated receptor gamma (PPAR) with retinoid X receptor (RXR) controls adipocyte differentiation and lipid metabolism initiation.
• PPAR induces the maturation of early lipoblasts (adipoblasts) or preadipocytes into fat cells of white adipose tissue.

Lipoblasts

• Early lipoblasts look like fibroblasts but develop small lipid inclusions and a thin external lamina.
• TEM studies show early lipoblasts have an elongated configuration, multiple cytoplasmic processes, and abundant endoplasmic reticulum and Golgi apparatus.
• Midstage lipoblasts become ovoid as lipid accumulation changes cell dimensions.
• Early lipoblasts assume an oval configuration.
• The most characteristic feature is an extensive concentration of vesicles and small lipid droplets near the nucleus extending to both poles of the cell.
• Mature adipocytes have a single, large lipid inclusion surrounded by a thin rim of cytoplasm.
• Cells increase in size and become more spherical in the late stage of differentiation.
• Small lipid droplets coalesce to form single, large lipid droplets that occupy the cytoplasm's central portion.
• Abundant Smooth-surfaced endoplasmic reticulum (sER) , whereas rER is less prominent.
• These cells are considered late lipoblasts.
• The lipid mass compresses the nucleus to an eccentric position, producing a signet-ring appearance in hematoxylin-and-eosin (H&E) preparations.
• Unilocular adipocytes or mature lipocytes, because the cells have a single lipid droplet [Lat. unus, single; loculus, a little place].

Brown Adipose Tissue

• Brown adipose tissue has a special function related to heat production..
• There are several small droplets in the cytoplasm, and the nucleus is located in the center.
• Newborns benefit particularly from the tissue.
• Brown adipose tissue is also present in nonhibernating animals and again serves as a source of heat, especially in winter sleep.
• Adipocytes of brown, multilocular adipose tissue contain numerous fat droplets.
• Brown (multilocular) adipose tissue cells are smaller than white adipose tissue cells.
• The nucleus of a mature multilocular adipocyte is typically in an eccentric position within the cell, but not flattened like the nucleus in a unilocular adipocyte.
• The cytoplasm of the multilocular adipocyte consists largely of empty vacuoles because the lipid that ordinarily occupies the vacuolated spaces are lost during preparation.
• Multilocular adipocytes without lipid look similar to epithelial cells rather than connective tissue cells.
• Multilocular adipocytes contain numerous mitochondria, a small Golgi apparatus, and only small amounts of rER and sER.
• The mitochondria contain large amounts of cytochrome oxidase that give cells their brown color.
• Brown adipose tissue is abundant in newborns, but markedly reduced in adults.
• Brown adipose tissue is present in large amounts in the newborn.
• Brown adipose tissue helps offset the extensive heat loss that results from the high surface-to-mass ratio of newborns and avoids lethal hypothermia.
• Brown adipose tissue makes up about 5% of total body mass in newborns.
• It is located on the back, along the upper half of the spine, and toward the shoulders.
• Brown adipose tissue decreases as the body grows, but it remains widely distributed throughout the first decade of life in the cervical, axillary, paravertebral, mediastinal, sternal, and abdominal regions of the body.
• It then disappears from most sites except for regions around the kidney, adrenal glands, large vessels (i.e., aorta), regions of the neck (deep cervical and supraclavicular), regions of the back (interscapular and paravertebral), and thorax (mediastinum).
• Brown adipose tissue is subdivided into lobules by connective tissue partitions, and the connective tissue stroma between individual cells within the lobules is sparse.
• This tissue has a rich supply of capillaries that enhance its color.
• Numerous unmyelinated nerve fibers are among the fat cells.
• Metabolism of lipid in brown adipose tissue generates heat (thermogenesis).
• Hibernating animals have large amounts of brown adipose tissue.
• The tissue serves as a ready source of lipid.
• When oxidized, it produces heat to warm the blood flowing through the brown fat on arousal during hibernation and maintains body temperature in the cold.
• Brown adipocytes differentiate from mesenchymal stem cells controlled by PRDM16/PGC-1 transcription factors in the presence of catecholamines.

Clinical Correlation

• Studying benign and malignant adipose tissue tumors gives insight into and confirms the sequence of adipose tissue differentiation described above.
• The most common benign tumor of adipose tissue of adulthood is the lipoma.
• Well-differentiated liposarcoma is characterized by a predominance of mature adipocytes that vary in size and shape.
• These cells are spread between broad fibrous septa of connective tissue containing cells (mostly fibroblasts) with atypical hyperchromatic nuclei.