Structure of an artery

Structure of an artery

There are three basic histological layers (‘tunics’) in a vessel:

1 Tunica intima (TI) (innermost layer).

2 Tunica media (TM) (middle layer).

3 Tunica adventitia (TA) (outer layer).

Tunica intima

This is a thin layer consisting of the innermost, single-celled and

physiologically active endothelium housed on a dense connective

tissue basement membrane (internal elastic lamina).

Tunica media

This is the thickest layer of the wall and its content varies according

to arterial subtype, anatomical location and exposure to fluid-mechanical

stress. It is composed principally of vascular smooth muscle cells

(VSMCs) within a connective tissue matrix.

Tunica adventitia

This is a poorly defined, heterogeneous, outermost layer of investing

connective tissue consisting of a variable amount of smooth muscle

cells (SMCs) and fibroblasts along with numerous autonomic nerve

endings and vasa vasora (small, microscopic nutritional vessels traversing the layer). Its thickness varies according to location. Blood vessel nutrition

In large and medium-sized arteries, cells in the innermost media acquire oxygen and nutrition from the blood in the lumen (direct diffusion) while the vasa vasora serve the outer half to two-thirds of the wall.

Arterial subtypes

There are two subtypes:

1 Elastic arteries.

2 Muscular arteries.

These are distinguished according to the histological contents of the

tunica media.

Elastic arteries

These are larger vessels (e.g. the aorta and its major branches) and are

rich in elastic tissue to allow compliant expansion followed by recoil

during the cardiac cycle. This aids prograde blood flow by the conversion

of potential energy into kinetic energy. These vessels appear to

be more susceptible to atherosclerotic degeneration.

Muscular arteries

These are smaller (20–100 μm) vessels rich in SMCs (e.g. renal, coronary).

They branch from the larger elastic arteries and serve to regulate

capillary blood flow (end-organ and peripheries), thereby controlling

peripheral vascular resistance.

Ancillary cells and structures

Endothelium

This is a single-celled (hexagonal-shaped) layer responsible for vessel

tone and structure. It acts as selectively permeable membrane to

control molecular transfer through the vessel wall (e.g. response to

shock, vasoactive substances such as histamine), as well as coordinating

platelet aggregation and coagulation after injury.

Endothelial regulation of coagulation

• Forms a non-thrombogenic blood-tissue interface for flowing blood

by secreting the anticoagulant heparan sulfate (also limits thrombus

formation after activation of coagulation).

• Secretes procoagulants plasminogen activator inhibitor (PAI-1) and

von Willebrand factor (vWF).

• Synthesises various prostaglandins (PGs) including PGI2 (procoagulant,

vasodilator and platelet inhibitor). PGI2 inhibits platelet

aggregation by converting the platelet agonist adenosine diphosphate

(ADP) to adenosine.

• Synthesises tissue plasminogen activator (tPA).

• Expresses the thrombin receptor thrombomodulin, which (after

binding) activates protein C (integral to the coagulation cascade).

Internal elastic lamina (IEL)

This is a thin layer of condensed connective tissue (type IV collagen,

laminin) and complex chemically active macromolecules (e.g.

heparin sulfate proteoglycans [HSPGs]). It regulates and actively prohibits

the movement of molecules and cells through its microscopic

fenestrae.

Vascular smooth muscle cells

These SMCs are the predominant cell type in the tunica media. Under

normal conditions, they exist in a predominantly non-proliferative,

quiescent (but contractile) state responsible for vessel contraction and

relaxation. Under certain conditions (e.g. endothelial injury), they

become activated by growth factors (e.g. platelet-derived growth

factor [PDGF]) and transform to a proliferative, more mobile phenotype

capable of synthesising collagen, elastin and proteoglycans as

well as migration to the intima.

Extracellular matrix

This is a connective tissue matrix giving vessel structure and composition

and providing a medium for cell signalling and interaction within

the vessel wall. It is composed mainly of collagen, elastin, proteoglycans,

glycoproteins (e.g. fibronectin, laminin) and glycosaminoglycans

(GAGs). GAGs are specialised, sulfated proteoglycans of which

there are six primary types (keratin sulfate, hyaluronic acid, chondroitin

sulfate, dermatan sulfate, heparan sulfate and heparin). They have

a diverse role in regulating connective tissue structure and permeability,

as well as cell growth, differentiation, adhesion, proliferation and

morphogenesis, because of their inherent ability to bind to other

ligands.

External elastic lamina (EEL)

This is less developed in comparison with the IEL, but it has a regulatory

role for the passage of molecules and cells.

Other cells

Neutrophils (polymorphonuclear neutrophils [PMNs])

These mainly appear after injury to the vessel wall from the blood,

and adhere to the subendothelial layers via the cell adhesion molecule

P-selectin