Arterial Wall Layers: Tunica Intima, Media, Adventitia

Three-Layer Structure

1. Tunica Intima (Innermost)

• Endothelium: Simple squamous epithelium; anti-thrombotic, regulates tone (NO, prostacyclin), permeability, inflammation
• Basement membrane
• Subendothelial layer: Loose connective tissue, occasional smooth muscle
• Internal elastic lamina: Elastin boundary

2. Tunica Media (Middle — Thickest in Arteries)

• Concentric smooth muscle cells (SMCs) with elastin and collagen lamellae
• Function: Vasoconstriction and dilation → controls diameter, resistance, pressure
• Elastic arteries (aorta): More elastin → Windkessel effect
• Muscular arteries: More SMC → distribution control
• External elastic lamina

3. Tunica Adventitia (Outermost)

• Connective tissue: Collagen (types I, III), fibroblasts, elastin
• Vasa vasorum: Nutrient vessels for the outer wall
• Nervi vasorum: Sympathetic innervation
• Perivascular adipose tissue (PVAT): Paracrine signaling

Clinical Relevance

• Atherosclerosis: Intimal lipid accumulation and inflammation
• Aneurysm: Media degradation (elastin loss, MMPs)
• Dissection: Intimal tear → blood splits the media
• Vasculitis: Inflammatory infiltration by layer