Chemical contribution
Glut crosslinking make tissues biocompatible and nontrombogenic, while
maintaining anatomic integrity, leaflet strength and flexibility (45).
Its preference among aldehydes is related to its availability, low
price, quick action and capacity to react with a large number of amino
acids (45).
Glut is responsible for the effective crosslinking of collagen, the most
common structural component of the valves. It forms covalent bonds by
the formation of Schiff bases (reaction of an aldehyde group with an
amino group of lysine or hydroxylysine) and/ or by aldol condensation
between two adjacent aldehydes (46). Cross-linking increases tissues
durability, reducing resistance to proteolysis of the cross-linked
proteins. However, after Glut fixation residual aldehydes remain
expressed on the tissue surface and may act as calcification locations.
Although its action is essential for valve preservation and to eliminate
cellular components to reduce tissue immunogenicity, the induced
chemical reactions are probably the most important part of bioprosthetic
valve degeneration. After Glut fixation, valvular interstitial cells
lose their viability (42). However, some studies have showed that Glut
retains many of the viscoelastic proprieties of the collagen (45), with
haemodynamic proprieties of the prostheses similar to those of living
tissues (45).
Additionally, as part of the fixation and fabrication process, the
cellular content of the tissues is modified, with loss of endothelial
cells, loss of interstitial cell viability and interstitial cell
degeneration (41). Indeed, bioprosthetic heart valves show several
histological differences from native heart valves, with flattening of
the cuspal corrugations, loss of the endothelium or mesothelium
surfaces, disruption of interstitial cells and loss of GAG (42). With
the loss of interstitial cells viability, the mechanical proprieties and
durability of the valve depends primarily on the quality of the collagen
and the remaining viscoelastic proprieties are not enough to avoid valve
tissue degeneration.
Schoen et al described the calcification process as having two phases:
nucleation (or initiation) and propagation (41). One important change to
initiate calcification is the abnormal extrusion of calcium ions from
the nonviable cells. Cross-linking to proteins of the cellular membrane
alters their proprieties, resulting in a different permeability in the
non-viable cells. Additionally, there is a the reduction of the
functional transmembrane ion pumps and an increased permeability to
calcium ions that contribute to the onset of calcifications (47).
Usually calcium concentration is 1000 to 10.000 times lower in the
cytoplasm due to the healthy ion pumps that carry calcium out of the
cells. With deregulated calcium levels inside the cells, cellular
membranes and other intracellular structures bind calcium and serve as
nucleators for calcifications. Indeed, calcification seems to start
predominantly at the cell membranes and other intracellular structures
rich in phospholipids, while the loss of proteoglycans may enhance this
phenomenon by removing calcification inhibitors (48). Glut also reacts
directly with intracellular structures, predisposing to calcification in
the presence of high intracellular calcium levels (49). The debris of
interstitial cells that remain in valve tissue also serves as initiation
sites for calcification.
Collagen and elastic fibers can also serve as nucleation sites,
independent of cellular components. One important difference is that
calcification of collagen requires cross-linking alterations, while
calcification of elastin occur independently of cross-linking (50).
After the nucleation, calcification is influenced by all the metabolic
and pathologic changes in calcium and phosphorus metabolism, with
calcium-enriched crystals growing to eventually culminate in prosthesis
malfunction (propagation phase).
Another important characteristic of native heart valves is their
remodeling and reparation capacity. In biological prosthesis the fixed
and nonviable tissue is incapable to maintain the ongoing repair, and
every damage do the extracellular matrix is cumulative. Moreover,
endothelial is denuded or absent and adjacent smooth muscle cells might
proliferate and migrate freely to the non-endothelized valve surface
(51), also contributing to valve dysfunction.
The dynamic role of native valve cells’ and their importance for the
durability of bioprosthesis is now recognized, and new strategies of
repopulation and regeneration have been proposed to minimize the
problem. Repopulation defines the process of using a clean connective
tissue matrix valve, repopulated with the recipient cells’, before or
after prosthesis implantation (52). A completely “self-populated”
prosthesis would maintain tissues invisible, avoiding host reaction to
the graft. Although this technology reached clinical practice, the
results were not as good as expected. Regeneration involves the
implantation of a remodeling matrix with the proteins and cells of the
recipient that can resemble the dynamic changes of native heart valves
(52), and remains in the pre-clinical development.
Storage is another important issue regarding bioprosthetic valve
dysfunction. Actually, several bioprosthesis are stored in liquids
containing aldehydes, which are toxic and a source for calcification, as
previously described in this chapter. Regardless fixation and production
procedures with a reduced aldehyde content, tissues are exposed once
again to deleterious free aldehydes when they are stored in aldehyde
enriched solutions. Even pre-implant rinsing does not guarantee complete
removal of toxic aldehydes with such storage solutions, and new
technologies regarding storage are also an active field of research.