Untitled Notes

Adipose Tissue Overview

Dr. ÖĞR. ÜYESİ ESİN AK

Adipose tissue is characterized as tissues where adipocytes (fat-storing cells) are the primary cell type.
It is classified as a specialized connective tissue with a crucial role in energy homeostasis.
Energy reserves are stored within lipid droplets of adipocytes specifically in the form of triglycerides.
Triglycerides are acknowledged as the most concentrated form of metabolic energy storage for humans.
- Energy Density of Triglycerides: They lack water, providing them with about twice the energy density compared to carbohydrates and proteins.

White Adipose Tissue (WAT)
- Predominant type in adult humans.
- Specialized for fat storage.
- Predominantly composed of unilocular cells, each containing one large cytoplasmic droplet of whitish-yellow fat.
Brown Adipose Tissue (BAT)
- Contains multilocular cells with multiple lipid droplets, combined with abundant mitochondria, contributing to a darker appearance.
- Present in large quantities during fetal life and diminishes during the first decade after birth, but retains presence around internal organs.

Mesenchymal stem cells differentiate into progenitor cells for various connective tissue types, including preadipocytes.
Types of Preadipocytes:
- Large production of white adipocytes and a smaller number of beige adipocytes.
- White Adipocytes: Unilocular, typically featuring one large lipid droplet.
- Beige Adipocytes: Intermediate features between white and brown adipocytes; they adapt to cold temperatures by forming smaller lipid droplets and changing the gene expression profiles to mimic brown fat.
- Brown Adipocytes: Derived from another population of preadipocytes, they maintain a multilocular structure with multiple small lipid droplets and numerous mitochondria.

Represents at least 10% of the body weight in a normal healthy individual.
- Shape: Is spherical when isolated, potentially appearing polyhedral or oval when densely packed.
- Large size can be over 100 µm in diameter due to lipid accumulation.
- The nucleus is flattened and displaced to one side, with the cytoplasm forming a thin rim around lipid droplets.
- Cytoplasmic organelles are situated near the peripheral nucleus, including mitochondria, a small Golgi apparatus, and few cisternae of rough endoplasmic reticulum (RER) and free polyribosomes.

White adipocytes appear empty in standard light microscopy due to lipid removal during histological preparation.
Adipocytes exhibit a signet-ring morphology where the lipid droplet displaces and flattens the nucleus against the membrane.
They are subdivided into incomplete lobules by partitions of connective tissue that include vascular and nerve networks.
- Collagen: Adipocytes are surrounded by reticular fibers (type III collagen) secreted by the adipocytes themselves.

White adipocytes store triglycerides derived from three primary sources:

Dietary fats transported via circulation as chylomicrons.
Lipids synthesized in the liver and transported in plasma as very low-density lipoproteins (VLDLs).
Free fatty acids and glycerol synthesized by the adipocytes directly.

Transport: Triglycerides travel through blood and lymph from the intestine and liver in lipoprotein complexes (chylomicrons and VLDLs).
In capillaries of adipose tissue, lipoprotein lipase partially breaks down these complexes, releasing free fatty acids and glycerol.
Free fatty acids diffuse into the adipocyte, where they are reesterified to glycerol phosphate to form triglycerides stored in lipid droplets until required for energy.

Norepinephrine released from nerve endings activates the cyclic AMP (cAMP) system, stimulating hormone-sensitive lipase.
Hormone-sensitive lipase hydrolyzes stored triglycerides into free fatty acids and glycerol.
These substances then diffuse into the capillary blood, with fatty acids binding to albumin for transport throughout the body to be used as energy sources.

Humans are born with stores of white adipose tissue, beginning to accumulate by the 14th week of gestation.
Proliferation of progenitor cells decreases by late gestation; adipose tissue expansion occurs primarily through the filling of existing adipocytes until approximately age 10.
Although adipocytes can differentiate from mesenchymal stem cells throughout life, adult-onset obesity primarily involves hypertrophy (increase in cell size).
Childhood obesity influences both adipocyte size and number owing to differentiation of more preadipocytes from mesenchymal cells (hyperplasia).

Energy Storage: Stores energy in the form of triglycerides within lipid droplets.
Insulation: Forms the panniculus adiposus (fatty layer in subcutaneous fascia) that provides thermal insulation to reduce heat loss.
Cushioning: Acts as a cushion for vital organs, including locations such as the palms, soles, and around the heart and eyeballs.
Hormonal Secretion: White adipose tissue secretes various adipokines, including hormones, growth factors, and cytokines.

Brown adipocytes are smaller and polygonal compared to white adipocytes.
Contains many small lipid droplets in contrast to white adipose tissue's larger droplet.
Nuclear Structure: The nucleus in brown adipocytes is eccentric and not flattened like in white adipocytes.
Contains numerous large mitochondria rich in cytochrome oxidase, contributing to the tissue's color.

Rich vascularization accompanies the brown adipocyte structure, often closely packed around capillaries.
Tissue is subdivided by connective tissue partitions into lobules more distinct than those found in white adipose tissue.

Accounts for 2%-5% of newborn body weight, predominantly positioned in the back, neck, and shoulders.
Significant in newborns to prevent hypothermia due to high surface-to-mass ratio.
In adults, it exists only in scattered locations, primarily around organs like the kidneys and adrenal glands.

Lipid metabolism in brown adipose tissue generates heat through thermogenesis.
Beneficial anatomical features (small lipid droplets, plentiful mitochondria, rich vasculature) enhance heat production and blood warming.

In routine H&E (Hemotoxylin and Eosin) stained sections, the cytoplasm appears to consist largely of empty vacuoles due to lipid loss during preparation.

Metabolic activity is regulated by norepinephrine from sympathetic nerves and is influenced by environmental temperature (more active in colder conditions).
Expression of uncoupling protein (UCP-1), also known as thermogenin, is crucial for the metabolism in brown adipocytes, being involved in thermogenesis.
Lipid oxidation in brown adipose tissue is uncoupled from ATP production, generating heat, identified as nonshivering thermogenesis.
Notably present in hibernating animals where brown fat serves to warm blood post-hibernation and assist in temperature maintenance in cold conditions.