Histology
Adipose tissue - White

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Adipose Tissue: Histology and Histopathology

Adipose tissue, commonly known as body fat, is a specialised form of loose connective tissue primarily composed of adipocytes. It plays a essential role in energy storage, insulation, and cushioning of organs. Recent studies have also recognised adipose tissue as a significant endocrine organ, producing various hormones and cytokines.

Cellular Composition

Adipose tissue consists of several cell types, including adipocytes, preadipocytes, fibroblasts, macrophages, and vascular endothelial cells. These cells together form the stromal vascular fraction (SVF), which supports the adipocytes functionally and structurally.

The Cells Include:

• Adipocytes: The primary cells responsible for lipid storage.
• Preadipocytes: Precursors to adipocytes, playing a very important role in adipogenesis.
• Fibroblasts: Cells that secrete collagen proteins, maintaining the structural integrity of the tissue.
• Macrophages: Immune cells involved in phagocytosis, removing dead cells and microorganisms.
• Vascular Endothelial Cells: Form the lining of blood vessels and contribute to the vascularisation of the tissue.

Types of Adipose Tissue

White Adipose Tissue (WAT)

White adipose tissue (WAT) stores energy in the form of triglycerides and provides insulation and mechanical cushioning. Histologically, WAT is characterised by large, unilocular adipocytes, each containing a single large lipid droplet that occupies most of the cell, pushing the nucleus to the cell periphery.

Brown Adipose Tissue (BAT)

Brown adipose tissue (BAT) is specialised for thermogenesis and is abundant in mitochondria. BAT adipocytes are multilocular, containing multiple small lipid droplets and a central nucleus. The high mitochondrial content gives these cells a brown appearance.

Distribution and Anatomical Features

In humans, adipose tissue is distributed in specific anatomical depots:

• Subcutaneous fat: Beneath the skin, providing insulation.
• Visceral fat: Surrounding internal organs, offering protection and support.
• Bone marrow (yellow bone marrow): Involved in haematopoiesis.
• Intermuscular fat: Scattered among muscle fibres.
• Breast tissue: Part of the mammary gland structure.

Histopathological Features

Obesity

Obesity is characterised by an excessive accumulation of adipose tissue. In histopathological examination, obese adipose tissue shows hypertrophy of adipocytes and an increased number of macrophages, often forming crown-like structures around dead or dying adipocytes. This increased macrophage infiltration is associated with chronic inflammation and the release of pro-inflammatory cytokines, contributing to metabolic syndrome.

Visceral Fat

Visceral fat is located within the abdominal cavity and is more metabolically active compared to subcutaneous fat. It is associated with a higher risk of metabolic diseases such as type 2 diabetes and cardiovascular disease. Histologically, visceral fat shows a higher degree of vascularisation and infiltrating immune cells, contributing to systemic inflammation.

Subcutaneous Fat

Subcutaneous fat is found just below the skin and is less associated with metabolic diseases compared to visceral fat. It serves as an energy reserve and insulation. Histologically, subcutaneous fat is characterised by a homogenous distribution of adipocytes with fewer infiltrating immune cells compared to visceral fat.

Marrow Fat

Marrow adipose tissue (MAT) is found within the bone marrow and increases in conditions like anorexia and calorie restriction. MAT adipocytes are similar to white adipocytes but are interspersed with haematopoietic cells. Exercise is known to reduce MAT quantity and size, indicating its dynamic nature.

Ectopic Fat

Ectopic fat refers to triglyceride storage in tissues not typically associated with fat storage, such as the liver, skeletal muscle, heart, and pancreas. This abnormal fat deposition can disrupt cellular functions and is linked to insulin resistance and metabolic disorders.

Physiology and Hormonal Functions

Adipose tissue acts as an endocrine organ, secreting various hormones and cytokines such as leptin, adiponectin, resistin, and TNFα. These adipokines play roles in metabolism, insulin sensitivity, and inflammation. The balance of these secretions is very important for maintaining metabolic homeostasis.

Brown Fat and WAT Browning

Brown fat generates heat through non-shivering thermogenesis, mediated by uncoupling protein 1 (UCP1) in mitochondria. The "browning" of white adipose tissue (WAT) involves the transformation of white adipocytes into beige adipocytes, which exhibit features of brown adipocytes and contribute to thermogenesis. This process is regulated by environmental factors like cold exposure and exercise, as well as various molecular pathways.

Genetic Factors

Genetic factors play a significant role in adipose tissue function and distribution. Mutations in genes like leptin and those involved in melanocortin signalling can lead to obesity and metabolic disorders. The thrifty gene hypothesis suggests that certain populations have evolved to store fat more efficiently, providing a survival advantage during periods of food scarcity.

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Self-assessment MCQs (single best answer)

What is the primary cell type responsible for lipid storage in adipose tissue?

Which type of adipose tissue is specialised for thermogenesis?

Which cell type in adipose tissue is involved in phagocytosis?

Where is subcutaneous fat primarily located?

Which hormone is produced by adipose tissue and regulates hunger?

What histological feature is characteristic of white adipose tissue?

What is a common histopathological feature of obese adipose tissue?

Which type of adipose tissue is associated with a higher risk of metabolic diseases?

What process involves the transformation of white adipocytes into beige adipocytes?

What genetic factor is associated with obesity and metabolic disorders?