BLOOD VESSEL REGENERATION - 3D Bioprinting and Nanotechnology in Tissue Engineering and Regenerative Medicine - page 148

the tissue; and capillaries, which provide for the efficient exchange of waste and nutrients in the tissue
( Figure 7.1 ). Arteries and veins have larger inner diameters, which range from 2 to 5 mm and have a
wall thickness on the order of 1 mm for arteries and 0.5 mm for veins. Additionally, there are struc-
tural differences between arteries and veins even though they have the same three basic layers. Blood
vessels can be thought of as a concentric structure with each concentric ring comprised of different
cells and matrices. Endothelial cells form the smallest-diameter layer (tunica intima), followed by
smooth muscle cells (tunica media), and then fibroblasts and primarily collagen farther out forming the
outermost layer (tunica adventitia) ( Junqueira, 1995; Iaizzo, 2005 ).
The cells are embedded inside ECMs that are primarily comprised of proteins, glycoproteins, gly-
cosaminoglycans, and proteoglycans. In between these layers there can be a layer of elastin. However,
the thickness of each layer and the composition of those layers can vary for arteries and veins. Arteries
have more smooth muscle cells surrounding the endothelial cells, whereas the veins have a decreased
presence of smooth muscles cells resulting in the disparity in their wall thicknesses, as well as provid-
ing different mechanical properties and functionality. It is especially important for arteries to have a
thicker layer of smooth muscle cells and elastin layers due to the active role arteries play in the circula-
tory system. Veins on the other hand have a diminished need for smooth muscle cells due to their role
in the vascular system not requiring strong contractions. As a result, the tunica media is not the thick-
est layer. However, veins do have valves periodically along their length that prevent the back-flow of
blood. These mechanical properties and functionality need to be taken into account when designing an
implant for regenerative medicine ( Junqueira, 1995; Iaizzo, 2005 ).
The vascular system is a very dynamic environment. Typically, the average adult human heart pumps
5 l of blood per minute throughout the body. Different organs and tissues receive varied quantities of
flow depending on their nutrient requirements. These levels can also be dynamically modulated in or-
der to account for various activities such as digestion, physical exertion, and basal metabolic activities.
FIGURE 7.1 Vasculature.
A cross-sectional view showing the basic layers of veins and arteries. The three main layers are labeled with black
arrows. The tunica media and the tunica adventitia are of different thicknesses for veins and arteries based upon
their respective mechanical requirements. In between the tunica media and the inner and outer layers, there are thin
layers of elastin matrix. The thickness of the elastin matrices is thicker in arteries.