Adipose Tissue

Definition And Meaning Of Adipose Tissue

Adipose tissue, otherwise known as body fat, is a connective tissue comprised primarily of adipocytes. It primarily serves as the fat storage area and energy reserve, but also provides insulation, thermoregulation, and hormone production.

Types Of Adipose Tissue

Adipose tissue is categorised into three types:

White Adipose Tissue

• Composed mainly of large, unilocular adipocytes (cells with a single large fat droplet).

• Storages energy in the form of triglycerides.

• Insulates and cushions internal organs to protect them.

• Secretes hormones like leptin, which works by regulating hunger levels and energy balance.

Brown Adipose Tissue

• Contains adipocytes with many small droplets of lipid and has a high mitochondrial content, giving it a brown colour.

• Specialised in heat production through nonshivering thermogenesis.

• Found mainly in neonates and hibernating mammals; only very small quantities are found in adults.

Beige Adipose Tissue

• Intermediate in nature between White adipose Tissue and Brown Adipose Tissue.

• May be induced through various stimuli, such as cold exposure, to change from white-like to brown-like cells, a process called browning.

• Plays a role in thermoregulation and energy expenditure.

Structure And Composition Of Adipose Tissue

Adipose tissue is very complex in structure, composed of several cellular and extracellular elements

Cellular Composition

Adipose tissue is composed of mainly adipocytes and the stroma vascular fraction

Adipocytes

• Primary fat-storing cells

• Unilocular adipocytes in white adipose tissue store a single large fat droplet.

• Multilocular adipocytes in brown adipose tissue store several small fat droplets.

Stromal Vascular Fraction

• Pre-adipocytes, fibroblasts, vascular endothelial cells, and immune cells.

• Provides support to the growth and maintenance of adipocytes.

• Participates in the regenerative capacity of the tissue.

Extracellular Matrix Components

The ECM provides structural support to the adipose tissue

Collagen

• The major component of ECM.

• Provides tensile strength and structural integrity.

• Involved in the formation of the tissue's supportive network

Fibronectin

• Glycoprotein that binds to collagen and cell surface integrins.

• Facilitates cell adhesion, growth and wound healing.

Adipose Tissue Distribution

Adipose tissue is distributed in different regions of the body with variations in its distribution

Subcutaneous Fat

• Located beneath the skin.

• Provides insulation and energy storage.

• Visible as body contours and can be measured using skinfold callipers.

Visceral Fat

• Surrounds internal organs within the abdominal cavity.

• Linked to metabolic and cardiovascular risks.

• Cannot be seen directly but can be measured using CT or MRI imaging.

Distribution Differences By Gender And Age

• Men have more visceral fat, while women have more subcutaneous fat

• Ageing is characterised by a change in adipose tissue distribution, with a greater increase in visceral fat

Physiological Roles Of Adipose Tissue

Adipose tissue has several crucial roles in the human body

Energy Storage

• Stores energy in excess by converting it into triglycerides.

• Mobilises stored fat during energy deficits

Insulation And Thermoregulation

• Provides thermal insulation to maintain body temperature.

• Brown adipose tissue generates heat through thermogenesis.

Endocrine Functions

• Produces hormones and cytokines which regulate metabolism, appetite and inflammation.

• Hormones Produced: e.g. Leptin, Adiponectin

• Leptin: Mediate's satiety and energy balance

• Adiponectin: Increases insulin sensitivity and exerts anti-inflammatory effects.

Adipose Tissue In Health And Disease

Adipose tissue contributes to a wide array of health disorders and diseases

Obesity

• Results from an imbalance between calorie intake and expenditure

• This leads to increased fat mass and associated health risks

• Body mass index is a measure of body fat based on height and weight.

• High value of BMI—excess adipose tissue, potential health risks

Metabolic Syndrome

• The cluster of conditions including insulin resistance, hypertension, dyslipidemia

• Linked to excess visceral fat

Insulin Resistance And Type 2 Diabetes

• Excess adipose tissue, in particular visceral fat, potentiates insulin resistance.

Cardiovascular Diseases

• Excess adipose tissue, specifically visceral fat, is associated with an increased risk for heart disease and stroke.

• Contributes to the pathogenesis of atherosclerosis.

Adipose Tissue At Various Stages Of Life

Properties and functions of Adipose Tissue change during the lifetime

Adipose tissue development

• Starts in the foetus and continues after birth.

• Growth can refer to hypertrophy, an increase in cell size, and hyperplasia, an increase in cell number.

Puberty Changes

• Changes to body fat distribution occur with changes in hormone levels during puberty.

• Females generally gain more subcutaneous fat while males gain more visceral fat.

Adipose Tissue And The Elderly

• Ageing is associated with changes in body fat distribution and a decrease in the activity of brown adipose tissue.

• The accumulation of visceral fat, and a concomitant loss of subcutaneous fat, contribute to metabolic disease.

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