The walls of arteries and veins are composed of three concentric layers (tunicae). The structure and composition of each layer varies along the vascular tree, reflecting the different functions of each vessel type.
Overview of the Three Tunicae
| Layer | Primary Component | Function |
|---|---|---|
| Tunica intima | Endothelium, basement membrane, subendothelial connective tissue | Barrier, selective permeability, hemostasis, vasoregulation |
| Tunica media | Smooth muscle, elastic fibers, collagen | Vasomotion, structural support, vessel caliber regulation |
| Tunica adventitia | Collagen, elastic fibers, fibroblasts, nerves, vasa vasorum | Structural support, vessel attachment, innervation, nutrition |
Tunica Intima
The tunica intima is the innermost layer, in direct contact with the blood.
Components
Endothelium:
- Single layer of squamous epithelial cells
- Aligned with the direction of blood flow
- 0.2-0.5 microns thick (but thicker near the nucleus)
- Surface area: 500-700 m² total
Basement membrane:
- Type IV collagen
- Laminin
- Fibronectin
- Proteoglycans (perlecan)
- Nidogen/entactin
Subendothelial layer:
- Loose connective tissue
- Smooth muscle cells (in larger vessels)
- Collagen (types I, III)
- Elastic fibers
Internal elastic lamina:
- Fenestrated sheet of elastin
- Present in arteries (prominent)
- Absent or thin in veins
- Provides elasticity and acts as a barrier
Endothelial Cell Functions
| Function | Mechanism |
|---|---|
| Barrier | Tight junctions regulate paracellular transport |
| Permeability | Transcytosis (caveolae), fenestrations, paracellular |
| Hemostasis | von Willebrand factor release, thrombomodulin, heparin-like molecules |
| Vasoregulation | NO, endothelin, prostacyclin, EDHF |
| Inflammation | Selectins, ICAM-1, VCAM-1, cytokine production |
| Angiogenesis | VEGF receptors, angiogenic factor production |
| Metabolism | ACE (angiotensin conversion), lipoprotein lipase |
| Immune | Antigen presentation (MHC class II) |
Regional Variations
| Vessel | Intima Characteristics |
|---|---|
| Aorta | Thick subendothelial layer, prominent internal elastic lamina |
| Muscular artery | Thin subendothelial layer, thick internal elastic lamina |
| Arteriole | Very thin, internal elastic lamina present (large arterioles only) |
| Capillary | Endothelium + basement membrane only |
| Postcapillary venule | Thin endothelium, pericytes |
| Medium vein | Thin intima, no internal elastic lamina, valves |
| Large vein | Intima may contain smooth muscle |
Clinical Significance
Endothelial dysfunction:
- Impaired NO production
- Pro-inflammatory state
- Pro-thrombotic state
- Precursor to atherosclerosis
- Associated with: Smoking, diabetes, hypertension, hyperlipidemia
Atherosclerosis begins in the intima:
- Lipid infiltration
- Endothelial activation
- Monocyte adhesion and migration
- Foam cell formation
- Plaque development
Tunica Media
The tunica media is the middle layer and is the thickest layer in arteries.
Components
Smooth muscle cells:
- Spindle-shaped, 20-100 microns long
- Contractile phenotype (normal) or synthetic phenotype (in disease)
- Surrounded by basement membrane
- Connected by gap junctions
Extracellular matrix:
- Elastic fibers (elastin + fibrillin)
- Collagen (types I, III)
- Proteoglycans (versican, decorin, biglycan)
- Fibronectin
Elastic lamellae:
- Concentric sheets of elastin
- Present in elastic arteries (40-70 lamellae)
- Fenestrations allow diffusion
- Each lamella = smooth muscle layer sandwich
Smooth Muscle Phenotypes
Contractile phenotype (normal):
- Abundant myofilaments
- Contractile proteins (smooth muscle actin, myosin)
- Responds to vasoactive stimuli
- Minimal proliferation
Synthetic phenotype (in disease):
- Reduced myofilaments
- Increased rough ER and Golgi
- Active proliferation and migration
- Matrix production
- Seen in atherosclerosis and restenosis
Vasomotion
The smooth muscle of the media regulates vessel diameter:
Contraction (vasoconstriction):
- Calcium influx (voltage-gated calcium channels)
- Calcium release from SR (IP3 pathway)
- Myosin light chain phosphorylation
- Actin-myosin cross-bridge cycling
Relaxation (vasodilation):
- NO -> cGMP -> dephosphorylation of MLC
- Prostacyclin -> cAMP -> reduced calcium
- Hyperpolarization -> reduced calcium influx
Regional Variations
| Vessel | Media Characteristics |
|---|---|
| Elastic artery | Many elastic lamellae, smooth muscle between lamellae |
| Muscular artery | Predominantly smooth muscle, few elastic fibers |
| Arteriole | 1-3 smooth muscle layers |
| Capillary | No smooth muscle |
| Venule | Occasional smooth muscle (larger venules) |
| Medium vein | 2-3 smooth muscle layers |
| Large vein | Thin media relative to lumen |
External Elastic Lamina
- Present in most muscular arteries
- Separates media from adventitia
- More prominent in larger muscular arteries
- Absent or thin in elastic arteries
Tunica Adventitia
The tunica adventitia is the outermost layer of the vessel wall.
Components
Extracellular matrix:
- Dense collagen (type I, III)
- Elastic fibers
- Proteoglycans
Fibroblasts:
- Produce matrix components
- Maintain adventitial structure
Vasa vasorum:
- Network of small vessels that supply the vessel wall
- Penetrate from adventitia into the outer media
- Present in vessels > 1 mm diameter
- Supply oxygen and nutrients to the vessel wall
Nerves (nervi vasorum):
- Sympathetic (vasoconstrictor)
- Sensory (pain fibers)
- Parasympathetic (limited)
Lymphatics:
- Present in adventitia of larger vessels
- Drain interstitial fluid
Functions
| Function | Description |
|---|---|
| Structural support | Collagen provides tensile strength, prevents overdistension |
| Vessel attachment | Connects the vessel to surrounding tissues |
| Nutrition | Vasa vasorum supply the outer vessel wall |
| Innervation | Autonomic regulation of vascular tone |
| Remodeling | Adventitial fibroblasts contribute to vascular remodeling |
Vasa Vasorum
Distribution:
- Present in vessels > 1 mm diameter
- More extensive in veins than arteries
- In arteries: Penetrate to the outer media (inner media nourished by luminal diffusion)
- In veins: Penetrate closer to the intima
Clinical significance:
- Neovascularization in atherosclerotic plaques (intraplaque hemorrhage)
- Vasa vasorum rupture -> intraplaque hemorrhage -> plaque instability
- Vasa vasorum in aortic vasa -> aortic dissection risk
The Adventitia as an Active Layer
Traditionally considered a passive support layer, the adventitia is now recognized as:
- A signaling center: Adventitial fibroblasts respond to mechanical stress
- An inflammatory site: Lymphocytes and macrophages in adventitial inflammation
- A source of progenitor cells: Adventitial progenitor cells contribute to vascular repair
- A remodeling participant: Adventitial fibrosis in hypertension
Comparative Histology
Artery vs. Vein
| Feature | Artery | Vein |
|---|---|---|
| Wall thickness | Thick | Thin |
| Lumen shape | Round (patent) | Collapsed (when empty) |
| Intima | Thick, internal elastic lamina | Thin, no internal elastic lamina |
| Media | Thick (many muscle layers) | Thin (2-3 muscle layers) |
| Adventitia | Thin (relative to total wall) | Thick (often the thickest layer) |
| Valves | Absent | Present (many) |
| Vasa vasorum | Limited to adventitia/outer media | More extensive |
Elastic vs. Muscular Artery
| Feature | Elastic Artery | Muscular Artery |
|---|---|---|
| Location | Near heart | Distal circulation |
| Media composition | Elastic > smooth muscle | Smooth muscle > elastic |
| Elastic lamellae | 40-70 | 0-5 |
| Internal elastic lamina | Not distinct (part of lamellar unit) | Prominent, undulating |
| External elastic lamina | Absent | Present |
| Vasa vasorum | Yes | In larger muscular arteries |
Vessel Type Summary
| Vessel Type | Intima | Media | Adventitia | Key Features |
|---|---|---|---|---|
| Elastic artery | Thick, IEL prominent | Many elastic lamellae | Thin | Windkessel function |
| Muscular artery | Thin, IEL prominent | Thick, circular smooth muscle | Moderate | Active vasomotion |
| Arteriole | Very thin, IEL (large) | 1-3 smooth muscle layers | Thin | Resistance vessels |
| Capillary | Endothelium + basement membrane | None | None (pericytes) | Exchange |
| Postcapillary venule | Thin endothelium | Pericytes | Thin | WBC migration |
| Medium vein | Thin, no IEL, valves | 2-3 smooth muscle layers | Thickest layer | Capacitance |
| Large vein | Thin, may have muscle | Thin | Thick | Conduit, low resistance |
Age-Related Changes
| Layer | Change |
|---|---|
| Intima | Thickening, endothelial dysfunction, senescence |
| Internal elastic lamina | Fragmentation, calcification |
| Media | Smooth muscle loss, collagen increase, elastin fragmentation |
| Adventitia | Fibrosis, increased collagen crosslinking |
| Vasa vasorum | Increased density, neovascularization |