Connective Tissues: Adipose, Cartilage, and Bone

Adipose Tissue

A connective tissue dominated by fat-storing cells (adipocytes) that represents $15\text{--}20\%$ of body weight in men and slightly more in women, primarily functioning as a storage depot for neutral fats (triglycerides).

Triglycerides (Storage Efficiency)

The body's preferred storage molecule because they are insoluble in water (concentrated without osmotic effects) and possess a high caloric density of $9.3\,\text{kcal/g}$, which is twice that of proteins or carbohydrates.

White Adipose Tissue (WAT)

A form of adipose tissue specialized for long-term fat storage, containing unilocular adipocytes with a single large lipid droplet and a nucleus flattened against the periphery.

Brown Adipose Tissue (BAT)

A form of adipose tissue composed of multilocular adipocytes with abundant mitochondria that releases heat rather than storing energy, functioning in thermogenesis in newborns and during cold adaptation.

Unilocular Adipocytes

Cells measuring $50\text{--}150\,\mu\text{m}$ in diameter that are spherical when isolated and polyhedral when packed in tissue, characterized by a single large lipid droplet that displaces the nucleus to the cell membrane.

Stromal Cells

Fibroblasts and macrophages that comprise approximately $\sim 50\%$ of the total cell population in white adipose tissue.

Caveolae

Cell membrane invaginations abundant in white adipocytes (especially immature cells) that are critical for lipid trafficking and the formation of the large central lipid droplet.

Lipoprotein Lipase (LPL)

An enzyme synthesized by adipocytes that hydrolyzes chylomicrons and VLDLs at capillary luminal surfaces to release free fatty acids.

Chylomicrons

Lipoprotein particles up to $1200\,\text{nm}$ in diameter with a triglyceride core surrounded by phospholipids and cholesterol that transport dietary fats from the gut.

Very Low-Density Lipoproteins (VLDLs)

Lipoproteins measuring $30\text{--}80\,\text{nm}$ in diameter, synthesized by the liver, that deliver endogenous triglycerides to adipose tissue.

Anabolic Pathway (Lipid Metabolism)

Metabolic processes that build and store molecules, specifically forming triglycerides from fatty acids and glycerol for storage inside adipocytes.

Catabolic Pathway (Lipid Metabolism)

Metabolic processes that break down stored molecules to make energy available, specifically breaking down stored triglycerides into fatty acids and glycerol.

Hormone-Sensitive Lipase (HSL)

An enzyme in lipid mobilization that catalyzes the breakdown of stored triglycerides into free fatty acids; it is activated by norepinephrine and epinephrine and inhibited by insulin.

Insulin (Lipid Storage Regulation)

A hormone secreted by the pancreas following meals that inhibits HSL, promotes glucose uptake, supplies glycerol phosphate, and stimulates triglyceride conversion in adipocytes.

Leptin

A $16\,\text{kDa}$ satiety hormone synthesized predominantly by white adipocytes that acts on the hypothalamus to decrease appetite and increase energy expenditure.

Leptin Resistance Paradox

A condition in obesity where individuals possess high levels of leptin, yet the hypothalamic signaling pathways are impaired, preventing the brain from responding to the fullness signal.

Visceral Fat

Adipose tissue stored inside the abdomen around internal organs that drains fatty acids directly into portal circulation and is linked to Type II diabetes and cardiovascular disease.

Subcutaneous Fat

Adipose tissue located under the skin that does not carry the same metabolic and cardiovascular risks as visceral fat accumulation.

Starvation-Resistant Fat Locations

Specific white adipose tissue regions—including the palms, soles, and fat pads behind the ears—that remain resistant to uniform lipid mobilization during starvation.

Adipocyte Hypertrophy

An expansion mechanism in adipose tissue characterized by an increase in the size of existing adipocytes, representing the primary feature of adult-onset obesity.

Adipocyte Hyperplasia

An expansion mechanism in adipose tissue characterized by an increase in the total number of adipocytes, prominent during childhood obesity and predisposing individuals to lifelong obesity.

Beige Adipocytes

A population of cells within white adipose tissue with intermediate features between white and brown fat that can adopt a multilocular, thermogenic phenotype upon cold exposure.

Thermogenesis

The primary metabolic function of brown fat in which mitochondrial energy release produces heat instead of ATP to warm circulating blood.

Paraxial Embryonic Mesoderm

The specific embryonic cell lineage from which brown adipocyte preadipocytes differentiate.

Cartilage

An avascular, specialized connective tissue consisting of chondrocytes embedded within a water-rich matrix that provides flexible structural support throughout the body.

Chondrocytes

Mature cartilage cells located within lacunae that synthesize and maintain the surrounding extracellular matrix.

Glycosaminoglycans (GAGs)

Unbranched polysaccharide side chains on proteoglycans within the cartilage matrix that bind large volumes of water to impart compressive resilience and elasticity.

Hyaline Cartilage

The most abundant type of cartilage, having a smooth, translucent matrix; it forms articular surfaces, the laryngeal and bronchial skeleton, and is covered by perichondrium.

Elastic Cartilage

A resilient, flexible cartilage subtype containing a high density of elastic fibers alongside collagen, found in the external ear (auricle), Eustachian tube, and epiglottis.

Fibrocartilage

A tissue combining properties of cartilage and dense connective tissue, rich in collagen fibers for tensile strength, located in intervertebral discs, menisci, and the pubic symphysis.

Chondrogenesis

The process of cartilage formation, progressing through condensation of mesenchymal cells, differentiation into chondroblasts, extracellular matrix production, and maturation of chondrocytes in lacunae.

Chrondocytes

Metabolically active cells. Abundant RER synthesize and secrete both collagen and proteoglycans

Bone

Mineralized connective tissue. Contains bone marrow for blood cell production. Highly avascularized.

Osteoblasts

Bone forming cells. Release matrix vessels

Osteocytes

Mature bone cells. Most abundant cell type

Osteoclasts

Bone resorbing giant cells. Essential for remodeling and mineral homeostasis.

Osteocytes

Unmineralized bone matrix produced by osteoblasts

Collagen Fibers ang Proteoglycans

Provide tensile strength to the bone matrix

Mineral Salts

Provide rigidity

Lacunae

Small spaces occupied by osteocytes

Canaliculi

Microscopic channels that form interconnected network through the bone matrix

Howships Lacunae

Osteoclasts create this resorption lacunae

Osteons / Haversian System

Fundamental and structural unit of compact bone

Intramembranous Ossification

Bone forms from mesenchyme. Direct Bone Formation

Endochondral Ossification

Bine forms from a cartilage. Cartilage as Scaffold