Fat (Adipose Tissue)

Fat, or adipose tissue, is a specialized form of connective tissue that serves as the body’s primary energy storage system. It also plays important roles in thermoregulation, hormone production, cushioning of organs, and maintaining structural support within anatomical spaces. Adipose tissue is widely distributed throughout the body, both superficially and deeply, and varies significantly in form and function depending on its type and location.

Types of Adipose Tissue

1. White Adipose Tissue (WAT)

This is the predominant form of fat in adults. White adipocytes contain a single large lipid droplet (unilocular), which pushes the nucleus and cytoplasm to the periphery. White fat functions as an energy reserve, provides insulation, and cushions organs. It also serves as an endocrine organ by secreting hormones like leptin and adiponectin.

2. Brown Adipose Tissue (BAT)

Brown fat is abundant in newborns and helps in thermoregulation by generating heat through non-shivering thermogenesis. Brown adipocytes are multilocular (contain multiple small lipid droplets) and rich in mitochondria, which gives the tissue its brown appearance. In adults, remnants of brown fat can be found in the neck, supraclavicular region, and around the kidneys.

3. Beige (Brite) Fat

Beige or "brite" (brown-in-white) fat is a type of white fat that can acquire brown-fat-like properties under certain stimuli such as cold exposure. These cells can perform thermogenesis and are functionally plastic, contributing to energy regulation.

Histological Structure

Adipose tissue is composed mainly of adipocytes, supported by a network of collagen fibers, fibroblasts, immune cells, and a rich capillary network. Each adipocyte is enclosed by a thin basement membrane and surrounded by extracellular matrix. The tissue is highly vascular and innervated by sympathetic nerve fibers.

White Adipocytes

• Unilocular lipid droplet
• Peripheral nucleus
• Relatively large in size (50–150 μm)

Brown Adipocytes

• Multilocular lipid droplets
• Central nucleus
• Abundant mitochondria with UCP1 protein

Distribution in the Human Body

Subcutaneous Fat

Located beneath the skin, subcutaneous fat is the most visible fat depot and contributes to body contour. It is abundant in areas like the abdomen, thighs, buttocks, and arms. It provides insulation and mechanical protection and varies by sex and hormonal status.

Visceral (Intra-abdominal) Fat

Visceral fat is found within body cavities surrounding internal organs. It includes the following specific depots:

• Perirenal fat: Surrounds the kidneys, enclosed within the renal fascia
• Omental fat: Located in the greater omentum, draping over the intestines
• Mesenteric fat: Found within the mesentery surrounding the intestines
• Retroperitoneal fat: Posterior to the peritoneum, lining the abdominal wall
• Mediastinal fat: Occupies spaces between thoracic organs, especially in the anterior and superior mediastinum

Other Regional Fat Depots

• Buccal fat pad: Found in the cheeks, prominent in infants
• Orbital fat: Cushions the eyeballs within the orbit
• Ischioanal fat: Located in the ischioanal fossae of the pelvis, helping to support the anal canal
• Pericardial fat: Found around the heart within the pericardial sac
• Bone marrow fat: Adipocytes intermixed with hematopoietic tissue in yellow marrow

Functions of Fat

1. Energy Storage

Fat is the body's most concentrated energy reserve. Triglycerides stored in adipocytes can be broken down into free fatty acids and glycerol during periods of fasting or energy demand.

2. Thermoregulation

Subcutaneous fat helps to insulate the body, while brown adipose tissue generates heat through mitochondrial uncoupling (UCP1-mediated). In infants, brown fat is vital for maintaining body temperature.

3. Mechanical Cushioning

Adipose tissue acts as a shock absorber around organs such as the kidneys, heart, and eyes. It fills potential spaces and prevents mechanical injury from surrounding movements.

4. Structural Support

Fat occupies and maintains space in anatomical compartments, contributing to the shape and position of organs. For example, mediastinal fat keeps soft tissue planes separate and mobile.

5. Endocrine Functions

Adipose tissue functions as an active endocrine organ. Hormones and cytokines secreted by adipocytes include:

• Leptin: Regulates appetite and energy expenditure
• Adiponectin: Enhances insulin sensitivity
• Resistin: Involved in insulin resistance
• TNF-α, IL-6: Pro-inflammatory cytokines in obesity and metabolic regulation

Development and Embryology

Adipose tissue originates from mesenchymal stem cells in the embryonic mesoderm. During fetal development, white fat begins forming in the late second trimester, while brown fat appears earlier (around 23 weeks gestation) and accumulates around the kidneys, neck, and spine in the fetus.

Postnatally, the distribution of fat changes with growth, hormonal influence, and environmental exposure. In infants, brown fat dominates in specific regions, whereas in adults, white adipose tissue becomes the dominant type. Beige fat may appear in adults under certain stimuli such as cold exposure or physical exercise.

Fat in Imaging and Dissection

In imaging modalities such as CT and MRI, fat appears as low-density (CT) or hyperintense (T1-weighted MRI) tissue. It is used as a landmark in anatomical planes and to identify or separate organs. In cadaveric dissection, fat is often the first tissue layer encountered and is removed to expose deeper structures, but it also helps identify fascial boundaries and vascular paths.

Fatty tissue may vary widely in quantity between individuals and often defines body habitus. Despite its simplicity, adipose tissue is an essential component of anatomical systems and plays a foundational role in maintaining physiological stability.