Blood vessels participate in homeostasis on a moment-to-moment basis and contribute to the pathophysiology of diseases of virtually every organ system.

 

Hence, an understanding of the fundamentals of vascular biology furnishes a foundation for understanding the normal function of all organ systems and many diseases.

 

The smallest blood vessels-capillaries-consist of a monolayer of endothelial cells apposed to a basement membrane, adjacent to occasional smooth-muscle-like cells known as pericytes.

 

Arteries typically have a trilaminar structure:

  • The intima consists of a monolayer of endothelial cells continuous with those of the capillaries
  • The middle layer, or tunica media, consists of layers of smooth-muscle cells; in veins, the media can contain just a few layers of smooth-muscle cells
  • The outer layer, the adventitia, consists of looser extracellular matrix with occasional fibroblasts, mast cells, and nerve terminals.

Larger arteries have their own vasculature, the vasa vasorum, which nourishes the outer aspects of the tunica media.  The adventitia of many veins surpasses the intima in thickness.

 

ORIGINS OF VASCULAR CELLS

The intima in human arteries often contains occasional resident smooth-muscle cells beneath the monolayer of vascular endothelial cells. The embryonic origin of smooth-muscle cells in various types of artery differs.

 

Some upper-body arterial smooth-muscle cells derive from the neural crest, whereas lower-body arteries generally recruit smooth-muscle cells from neighboring mesodermal structures during development.

 

Derivatives of the proepicardial organ, which gives rise to the epicardial layer of the heart, contribute to the vascular smooth-muscle cells of the coronary arteries. Bone marrow-derived endothelial progenitors may aid repair of damaged or aging arteries.

 

In addition, multipotent vascular stem cells resident in vessel walls may give rise to the smooth-muscle cells that accumulate in injured or atheromatous arteries.