Mitral leaflets
Mitral leaflets (MLs) consist of an extracellular matrix (ECM) layered with valvular endothelial cells (VECs), that adhere each other through a transmembrane protein (vascular endothelial (VE)-cadherin). VECs are anchored basally to the basement membrane, a 50- to 100-nm layer of specialized ECM protein complex which separates cell monolayers from the underlying connective tissue, through integrin adhesion complexes4, that mediates adhesion between the cells and their ECM. VECs can migrate into ECM changing phenotype to replenish the turnover of valvular interstitial cells (VICs), major producers of the valve ECM needed to insure durability and function. This mechanism is known as endothelial to mesenchymal transition (EndMT) (fig. 4).
The ECM is composed of two main classes of macromolecules: proteoglycans (PGs) and fibrous proteins (collagen, elastin, fibronectin and laminin). Collagen is the most abundant fibrous protein within the interstitial ECM and constitutes up to 30% of the total protein mass of a multicellular animal. The bulk of interstitial collagen is transcribed and secreted by VICs that reside or are recruited into the ECM.
The ECM of the MV consists of four histologically distinct layers (fig.4): the atrialis and the ventricularis, both of which are thin elastin-rich layers, the spongiosa, which consists mainly of PGs, and the fibrosa, the main load-bearing layer, composed of circumferentially oriented collagen fibers. In the atrialis hemodynamic shear stresses can result in the deposition of more radially oriented collagen.
VICs, present in the ECM, are responsible for the structural integrity of the valve through protein synthesis and enzymatic degradation.
Quiescent VICs regulate low-grade ECM synthesis and degradation. Quiescent cells, in particular conditions, change phenotype and becomeactivated VICs , that take the features of myofibroblasts, facilitate valvular remodeling, but also play a role in fibrotic changes of the MV. These cells contribute to mitral adaptation, but, if the process becomes dysregulated, permanent scarring and fibrosis occurs.EndMT VICs are VECs undergoing EndMT that enter ECM and change phenotype becoming myofibroblasts. Other VICs can come from circulating blood, directly or from neovascularization, as myeloid cells that take myofibroblasts functionality.
When the integrity of the endothelial layer is reduced due to different stimuli (mechanical stress or stretching or inflammatory response following MI), cells from the circulating blood (mostly leukocytes and/or myeloid cells, which infiltrate valve tissues and differentiate into macrophages5) can be recruited into the valve, After having infiltrated the tissue, macrophages release inflammatory cytokines such as TNF-α, IL-1β, and TGF-β leading to cardiac valve inflammation and inducing EndMT.
Mechanical stimuli that are outside of the normal physiological range induce VICs phenotypic transition into a biosynthetically active myofibroblast-like phenotype (mechanotransduction). This transition leads to increased synthesis of PGs, cytokines, as TFG-β, collagen6, as well as increased proteolytic enzyme expression and activity. This suggests that the in vivo mechanical environment plays a fundamental role in VICs response to external mechanical stimuli caused by pathological factors.