Cellular Pathology - Adipose Tissue Study Notes

Cellular Pathology - Adipose Tissue
Instructor Information

Dr. Adrian O'Hara

a.w.ohara@ljmu.ac.uk

Learning Outcomes

This unit focuses on identifying key features of the three types of adipose tissue:

  • Brown Adipose Tissue (BAT)

  • White Adipose Tissue (WAT)

  • Beige Adipose Tissue

The Adipose Organ

Adipose tissues play a vital, multi-faceted role in the body, functioning as an active organ known as the adipose organ. It is pivotal not only for storing energy but also for regulating metabolism, thermogenesis, and other physiological processes. It acts as a multi-depot organ characterized by a sophisticated and complex structure, contributing fundamentally to homeostasis and energy balance in the body.

Depot Types

The adipose organ is classified into two primary types of depots:

  • Subcutaneous Depot: Located directly beneath the skin, this depot serves as an energy reserve and provides insulation, protecting the body from external temperature variations.

  • Visceral Depot: Surrounding internal organs, this depot is closely associated with metabolic processes and is involved in the regulation of energy homeostasis and metabolic syndrome risks.

Additionally, adipose tissue can be classified based on the types of adipocytes present, resulting in either the white or brown coloration, each serving distinct physiological functions.

Anatomical Characteristics

The adipose organ of an adult Sv129 mouse, maintained at 29 °C for 10 days, serves as a reference illustration of its intricate anatomical composition. Detailed dissection reveals various depots that can be precisely located on the mouse profile. These depots consist of:

  • Subcutaneous Depots:

    • Deep cervical

    • Anterior subcutaneous (comprises interscapular, subscapular, axillo-toracic, and superficial cervical layers)

    • Posterior subcutaneous (comprising dorso-lumbar, inguinal, and gluteal depots)

  • Visceral Depots:

    • Mediastinic

    • Mesenteric

    • Retroperitoneal

    • Perirenal, periovaric, parametrial, and perivescical depots

The diagrammatic representation of these areas highlights the composition of white adipose tissue (white areas) and brown adipose tissue (brown areas). Cinti S. (2005) provides additional context relevant to the complex morphologies of the adipose organ.

Brown Adipose Tissue (BAT)

Functions

The core function of brown adipose tissue is thermogenesis. Unlike white adipose tissue, BAT is specialized for oxidizing lipids and glucose to produce heat, which is essential for maintaining body temperature in cold environments and assisting in overall energy expenditure.

  • Activation of Uncoupling Protein 1 (UCP1):

    • The thermogenic activity of BAT depends on UCP1, which disrupts the typical mitochondrial process of ATP production by allowing protons to flow back into the mitochondrial matrix instead of through ATP synthase, thus generating heat instead of energy.

    • UCP1 activation is facilitated by noradrenaline, released by the sympathetic nervous system in response to cold exposure or energy needs.

Vascular Supply and Innervation

The vascular system of BAT is remarkably developed, featuring an extensive network of capillaries that enable swift heat dissipation and nutrient delivery. The tissue is heavily innervated by noradrenergic fibers from the sympathetic nervous system, establishing crucial synapses with the plasma membranes of brown adipocytes, contributing to the regulation of thermogenic activities. The development of BAT in humans begins around the 20th week of gestation and continues to evolve until shortly after birth.

Histological Features

Histologically, brown adipose tissue is characterized by:

  • A multilocular arrangement of lipid droplets

  • Centrally positioned nuclei that reflect the metabolic activity of the cells

  • A high concentration of mitochondria, abundant with cristae that facilitate the energy conversion necessary for thermogenesis

Histology Examples:

  • Hematoxylin and Eosin (H&E) Staining demonstrates the centrally placed nuclei and multilocular lipid droplets.

  • Immunohistochemistry (IHC) for UCP1 viewed at 50 μm magnification, alongside Oil Red O Staining, provides visual evidence of lipid content and thermogenic activity within the tissue.

White Adipose Tissue (WAT)

Functions

White adipose tissue serves numerous physiological roles:

  • Utilizes triacylglycerols (TAGs) as an extensive long-term fuel reservoir.

  • Provides thermal insulation, thereby enhancing body temperature regulation during cold exposure.

  • Offers a cushioning effect to protect other organs from mechanical trauma.

  • Recognized as an endocrine organ since the discovery of Leptin in 1994, WAT significantly influences metabolic regulation, appetite control, and energy homeostasis.

Endocrine Functions

WAT plays a crucial role in the endocrine system by releasing fatty acids (FAs) and various other lipid moieties, including:

  • Cholesterol and its derivatives

  • Retinol (Vitamin A)

  • Prostanoids, which include prostaglandins that are vital mediators in various physiological processes

  • Steroid hormones and metabolic enzymes such as lipoprotein lipase, which participates in lipid metabolism.

Adipokines

Adipokines are specialized proteins secreted by WAT that contribute to various physiological functions. Key aspects of adipokines include:

  • Formerly known as adipocytokines, emphasizing their role in cellular signaling

  • Must be expressed at the mRNA level to effectively serve their locally and systemically pivotal roles

  • Specifically secreted by white adipocytes, indicating a targeted function in metabolic regulation and homeostasis.

Histological Features

White adipose tissue is histologically diverse, mainly composed of:

  • White adipocytes, which are typically spherical and vary in diameter (30 μm to 70 μm), possibly reaching up to 100 μm depending on specific depot characteristics.

  • Storage of lipids is predominantly organized in a single, large unilocular lipid droplet, leading to the compression of the cytoplasm and nucleus into a thin, peripheral rim that is distinctive in cell morphology.

Histology Examples:

  • H&E Staining reveals a characteristic “signet-ring” cell morphology with large triglyceride droplets in adipocytes.

  • Oil Red O Staining and IHC for Perilipin 1 (PLN1) assist in better visualization of the cytoplasmic lipid contents and structural operating mechanisms of WAT.

Beige Adipocytes

Characteristics

WAT can undergo a transformation known as “browning,” during which beige adipocytes, displaying characteristics of both white and brown adipocytes, emerge at sites traditionally associated with white adipose tissue. It is crucial to note that:

  • Beige adipocytes originate from a distinct lineage that is different from that of brown adipocytes, highlighting their unique developmental pathways and regulatory mechanisms.

Histological Features

Histological examination of beige adipose tissue reveals:

  • The presence of multilocular lipid droplets indicative of browning patterns.

  • H&E Staining provides evidence of both multilocular and unilocular lipid droplets, illustrating the phenotypic diversity among various adipocyte types.

Summary of Adipose Tissue Types

White Adipose Tissue (WAT)

  • Contains predominantly unilocular lipid droplets;

  • Plays a vital role as an energy reservoir alongside endocrine signaling capabilities.

Brown Adipose Tissue (BAT)

  • Characterized by multilocular lipid droplets;

  • Exhibits a highly vascularized and richly innervated structure;

  • Integral to thermogenesis by dissipating stored chemical energy as heat in response to cold exposure or energetic demand.

Beige (Brite) Adipocytes

  • Exhibit significant phenotypic plasticity;

  • Functionally characterized to store and release energy, potentially contributing to metabolic processes, especially when activated under specific stimuli, such as cold exposure or dietary changes.