Lazare Riviere, a French physician in 1663, identified large caruncle-like masses

obstructing the left ventricular outflow to the aorta in necropsy findings as an earliest description of calcific aortic stenosis [4] and the calcified mass was originally attributed to endocarditis. In 1846, Hass suggested a degenerative process occurred within the valve itself to initiate ossification. In 1904, Moenckeburg found that the aortic valve calcification occurred as a result of calcium deposition on the valve cusps due to degeneration within the layers of valve leaflets or

extension of sclerotic changes of aorta to valve cusps. In 1924, Dr. Carey Coombs described the inflammatory lesion in the rheumatic valve leaflets and the presence of new vessels developing within the valves [5] . In 1931, Christian first described that aortic stenosis with calcification of the cusps was possibly of rheumatic origin [6] . In 1938, Clawson, Noble and Lugkins stated that the “calcified aortic nodular deformity” was predominantly of a rheumatic nature [7] . In 1939, Dry and Willius concluded that the etiologic factor of calcification was rheumatic [8] and Hamburger and Saphir found evidence for both a rheumatic and nonrheumatic origin [9] . In 1961, Edwards discussed the occurrence of calcification in stenotic rheumatic valves [10] .

Rheumatic heart disease is the most common cause of valvular heart disease in tropical nations such as India. Rheumatic valve calcification is not a random passive process, but it is a regulated, inflammatory cellular process associated with the expression of osteoblast markers and neoangiogenesis. Early stage of calcification is in a manner akin to atherosclerosis. Once the osteoblast activity has been established, the disease progression is more likely to be regulated by the mediators of calcium homeostasis [11] . A similar osteoblast-like process that occurs in degenerative calcific valve develops in the calcific process of rheumatic valves [12] . The cellular and molecular mechanisms that involve the valve, circulating and bone marrow derived cells have been implicated in the pathogenesis of calcification. The cellular differences between the rheumatic and degenerative lesions are the intense inflammatory infiltrate and VEGF (vascular endothelial growth factor) expression in areas of neoangiogenesis found in rheumatic valves compared to degenerative valves. The cardiovascular risk factors such as exposure to biochemical (hypercholesterolemia, hyperglycemia) or mechanical (hypertension) may disrupt the normal cellular and matrix architecture of the valve, produces an inflammatory tissue milieu, atherosclerosis like lesions, osteoblastic transformation and calcification process. The key mediators of valve calcification are shown in Table 1.

These mediators promote neoangiogenesis, extracellular matrix remodeling, fibrosis and calcification of valve leaflets.

Calcification of heart valves is an immune-inflammatory disease process. The rheumatic involvement produces an inflammatory tissue environment that disrupts the endothelial layer of valve leaflets in a mechanism similar to that in atherosclerosis. The endothelial disruption may allow the lipids, including low- density lipoproteins, apolipoprotein B, (a) and E₂ to penetrate and accumulate in

areas of inflammation and undergo oxidative modification. These oxidative lipoproteins are highly cytotoxic, capable of stimulating intense inflammatory activity and subsequent mineralization [13] . The VECs (vascular endothelial cells) are activated and the expression of adhesion molecules that facilitate recruitment and transendothelial migration of monocytes which differentiate into macrophages, and infiltration of T-lymphocytes and α-actin expressor cells. The macrophages release matrix metalloproteases and cysteine endoproteases that lead to degradation of collagen and elastin in the valve matrix to disrupt the normal valve architecture [14] . The T-cells release proinflammatory cytokines, including transforming growth factor-β₁, tumor necrosis factor-α and interleukin-1-β which induce myofibroblast activation and cellular proliferation, ultimately help to stimulate and establish subsequent fibrotic and calcification processes that drive increasing valve stiffness [15] [16] . Increased levels of activated CD8 cells (cytotoxic T-cells or natural killer cells) and memory effector cells in patients with calcified valves may indicate a systemic adaptive immune response on valve calcification [17] . Lrp5, a coreceptor of the family of LDL receptors, has been found to regulate cellular proliferation and bone formation in valvular tissues [18] . Genetic bone markers have been found to be important in the development of osteoblast bone formation, a process that precedes calcification [19] . Lp(a) levels mediate the effect of SN rs10455872 in the gene LPA in valvular calcification. The kringle IV type 2 (KIV-2) repeats at LPA locus, which encode apolipoprotein (a) and its variation determines the plasma Lp(a) levels. NOTCH 1 on chromosome 9q34-35 is a susceptibility locus for bicuspid aortic valve and accelerates valvular calcification [20] . At the site of endothelial damage, the endothelial progenitor cells participate in the repair process by transiently establishing residence and secreting factors that facilitate proliferation and migration of resident endothelial cells [21] . In calcified valves, these progenitor cells are dysfunctional and express increased proapoptotic caspne-3 activity and decreased telomeric-repeat binding factor-2 (TERF2) expression, indicating progenitor cell senescence [22] . The VECs (valvular endothelial cells), lose the endothelial cell properties, no longer express endothelial-specific markers such as cadherin, pieracque-like pleomorphism characteristic of mesenchymal cells or myofibroblasts and express α-smooth muscle actin, type I collagen and vimentin [23] . The transforming growth factor-β, transcription factor Msx2 and β-catenin signaling increase the motility and migration of these “transformed” VECs into the surrounding tissues [24] . Once the endothelial-to-mesenchymal transition (EnMT) occurs, the BMP 2 and 4 (bone morphogenic proteins) secreted by the activated endothelial cells stimulate the calcification by activating Smad and Wnt/B-cate- nin signaling and upregulate the expression of osteochondrogenic transcription factor Msx2 and the faster osteoblast transcription factor Runx2 [25] . Once Runx2 is expressed, the cells are committed to an osteoblast lineage, upregulated the expression of calcification-related proteins (osteopontin, bone sialoprotein II, osteocalcin) and undergo calcification [26] . A subpopulation of VICs (valve interstitial cells) become activated in the inflammatory zone of the valve and differentiated into myofibroblasts. These myofibroblasts secrete matrix metalloproteinases which play a role in the restructuring of valve leaflet matrix and responsible for accelerated fibrosis in this process [27] . Under the influence of signaling pathways, Wnt-3-Lrp5-β catenin [28] , OPG/RANK/RANKL pathway (osteoproteogen/receptor activation of nuclear factor kappa B/RANK ligand [29] , Runx-2/NOTCH1, activated by BMPs, the myofibroblasts undergo differentiation into osteoblasts, which subsequently coordinate calcification as a part of a highly regulated process akin to new bone formation and mineralize to form calcific nodules on the valves. The local environment drives the mesenchymal progenitor cells to undergo osteogenesis differentiation, elaborate bone matrix in the valve [30] and capable of contributing to valve calcification.

The transition of valvular cells to an osteoblast-like phenotype is not a mutually exclusive process and these osteoblast-like cells may actively generate vesicles or undergo apoptosis to accumulate calcium and form microcalcifications. Matrix vesicles (100 to 300 nm) are derived from plasma membrane of smooth muscle cells, encapsulate the calcium inhibitors (MGP-vitamin K dependent matrix Gla protein, Fetuin A-a liver derived protein, osteoproteogens), sequester, render them inactive [31] and serve as a nidus for calcium deposition [32] . Calcifying nanoparticles (10 - 500 nm in size) consistent with nanobacteria [33] , [34] which nucleate, crystallize and function as a nidus for matrix calcification [35] . The activated matrix metalloproteinases (1, 2, 3 and 9) and cathepsins (S, K, V and G) degrade the collagen and elastin, compromises valve integrity, augment the inflammatory response and allows for the expansion of calcified nodules [36] . By the end stage of the disease, maturation of valve calcification occurs with lamellar bone and hemopoietic tissue and it is sustained by neoangiogenesis. The new vessels localizing to the region of inflammation and surrounds the calcific deposits. Both intercellular adhesion molecule-1 and vascular cell adhesion molecule-1 expressions are increased in these vessels, suggesting that they act as an important portal of entry of inflammatory cells [37] . The mast cells get degranulated in calcified valves and release tryptase which degrade the angiogenesis inhibitor endostatin [38] and down regulation of local angiogenesis inhibitors such as chondromodulin-1 and pronomodulin-1 have been occurred. Once neovascularization has been established, the network of newly formed vessels, expressing the endothelial markers Tie-2, vascular endothelial growth factor receptor-2, smooth muscle α-actin, facilitate rapid transit of inflammatory cells and procalcific signaling molecules and, these inflammatory lesions in rheumatic valvular disease is associated with angiogenesis and osteogenesis which may contribute disease progression and valvular mineralization.

The genesis of calcification in aortic valve is always rheumatic [39] and the calcific extension to the mitral valve resembles J-shaped as shown in Figure 5 and Figure 13. Calcification resembling a string of pearls in both aortic and mitral valves are typical of endocarditis. Calcification of mitral-aortic membrane has been reported as highly suggestive of radiation-induced lesions [40] and a pathologic echo density of anterior mitral leaflet is its characteristic feature [41] . But the dense calcification of anterior mitral leaflet as a bone-like structure was observed due to rheumatic mineralization as shown in Figures 3 along with calcification of aortic valve as in Figure 4 and Figure 5. The calcified aortic valve resembling as “calcium stone” was shown in short axis view in Figure 6.

The degenerative disease, as an aging process frequently involving the aortic and mitral valves and the central fibrous body is called as ‘Lev’s disease. Lenegre’s disease primarily involves the conduction system without affecting the fibrous skeleton of the heart. Both these conditions mostly results in isolated chronic heart blocks in adults. Degenerative calcification of mitral annulus may cause annular mitral stenosis in elderly and the difference between the annular mitral stenosis and rheumatic mitral stenosis [42] was shown in Table 2.

By understanding the cell biology of valvular calcification, gene transfer or cell- based therapies are investigated to modulate the cellular mechanism of calcification. The expressing Runx2 in VICs (valvular interstitial cells) commits these cells to an osteoblast-like phenotype. Gene transfer of small interfering RNA that targets Runx2 using adenovirus or recombinant nonviral vectors have been shown to prevent calcification both in vitro and in vivo [65] [66] . Heterotopic ossification (HO), which was first identified by Dejerne and Ceillier in 1918 [67] and it is characterized by pathologic endochondral bone formation at nonskeletal sites. Angiogenesis and osteogenesis are highly coupled during bone development. In the process of osteoblast differentiation, a set of bone specific genes (ALP-alkaline phosphatase, osteocalcin) are activated. Run-related protein 2 (Runx2) is a key regulator of these genes, cooperate with BMP-specific R-smads and it is considered as a molecular switch (myoblasts to osteoblasts phenotype) in osteoblast biology. Hypoxia is able to upregulate BMP₂ in osteoblasts via the activation of Hif1α signaling pathway. Inhibition of Hif1 α (hypoxia induced factor 1α) leads to reduce vessel formation, which may facilitate migration of host mesenchymal stem cells to the bone regeneration site. Synergistic inhibition of endochondral bone formation by silencing the angiogenic factor (Hif1α) and osteogenic factor (Runx2) by using the RNA interference technology, allow to design more targeted therapies to inhibit angiogenesis and osteogenesis [68] . miRNAs (microRNAs) have essential roles in osteoblast differentiation and heterotopic ossification. miR-203 is involved in these processes by repressing Runx2 expression in the post-transcriptional level. Therapeutic overexpression of miR-203 in osteoblasts may reduce heterotopic ossification and even prevent this disorder [69] . Cotransduction of small or short interfering RNA (siRNA) against Runx2 and Hif1α to inhibit heterotopic ossification have been evaluated. TNP-470 is an antiangiogenic agent that inhibits the neovessel formation and TNP-70 inhibits the biological activity of BMP₂ in early stage of bone induction. A potent delivery of a nanocomposite consisting of serum-stable DOTAP/ siRNA/PPAA-g-1% Jeffamine formulation was shown to halt the osteogenic switch driven by BMP₂ by silencing the Runx2, the downstream mediator of BMP₂ [70] .

Osteoclast-like cells are identified in calcified lesions , which in turn capable of resorbing mineral deposits [71] . Thus, strategies to promote osteoclast recruitment or stimulate differentiation of monocytes and macrophages toward an osteoclast phenotype are the areas of active investigation.

Strategies by delivering stem or progenitor cells via direct injection of small molecule inhibitors, osteoclasts, osteoclast-like cells or molecules that promote the transition of resident monocytes/macrophages towards an osteoclast lineage or viral vectors encoding inhibitory RNA (Runx2, Msx2) in areas of aortic valve lesions or to ischemic myocardium using a NOGA-guided catheter system [72] are under investigation.

Mutations in laminin A gene results in the production of a mutant lamin A protein, progerin, a component of nuclear envelope which becomes unstable, leading to cell death prematurally. The syndromes associated with these defects are called as “laminopathies” (Hutchinson Gilford Progeria Syndrome (HGPS), Wiedemann-Rautenstrauch syndrome, Restrictive dermopathy) [73] . In progeria, calcific aortic stenosis represents a degenerative premature aging process and principally involving the anterior mitral leaflet and proximal aortic valve cusps [74] . Systemic hypertension is common and appears before the child reaches the age of 5, and high mortality due to myocardial infarction and heart failure may occur.

Farnesylation is essential for the function of both mutant and nonmutant lamin A protein, including progerin. Lonafarnib is a farnesyl transferase inhibitor that blocks the posttranslational farnesylation of prelamin A, an experimental drug under trials [75] . Another treatment attempted is a combination of two molecules-Zoledronic acid and pravastatin.

Balloon mitral valvotomy (BMV) is the procedure of choice for severe mitral stenosis when the valve morphology is feasible. However, BMV is contraindicated in bicommissural calcification since it results a high probability of leaflet tear with acute severe mitral regurgitation. BMV can be performed safely and successfully in unicommissuraly calcified mitral valves [76] .

An emergency percutaneous transatrial balloon mitral commissurotomy is a life-saving procedure in a valve with unfavourable morphology, presented with cardiogenic shock in critical calcific mitral stenosis [77] [78] .

TAVR (transcatheter aortic valve replacement) was approved by FDA (Food and drug administration) for high risk surgical candidates in June 2012. There is an increase in risk of stroke due to thrombotic embolic material from the calcified valve tissue and vascular damage results in severe paravalvular regurgitation, which is poorly tolerated in hypertrophied left ventricle leading to increase in mortality [79] . The surgical risk for mitral valve replacement is very high due to the comorbidities and technical challenges related to calcium burden. Guerrero, et al reported the first-in-human percutaneous implantation of a balloon-ex- pandable transcatheter valve in a native mitral valve [80] . The use of newer aortic THV technologies such as Lotus (Boston Scientific Corporation) and direct flow valves (direct flow medical) in the mitral position for severe MAC (mitral annular calcification) [81] with improved outcomes were observed [82] .

Aortic valve replacement surgery remains the only established treatment for symptomatic aortic stenosis patients [83] . Hemodynamically significant asymptomatic patients will develop symptoms within 5 years, the issues of medical management and surgical correction may pay a close attention to the researchers [84] . Presence of moderate or severe valvular calcification identifies the patients with poor prognosis who may benefit from early valve replacement before the development of symptoms [85] and it is to be determined whether this benefit overweighs the risk of surgery [86] .

Calcific mitral stenosis with severe mitral annular calcification may present a different high risk to the surgeon. Extensive debridgement of calcium may increase the risk of paravalvular leakage and bleeding. Bypass of the mitral valve with a valved conduit is another option for the treatment of severe calcific mitral stenosis [87] .

Bioprosthetic valves are generally recommended for patients > 65 years and there is also greater usage in younger patients due to lack of necessity of chronic anticoagulation. The Ross operation (replacement of aortic valve with a pulmonary autograft and replacement of the pulmonary valve with a homograft) is an alternate for adolescents, young adults, including woman who wish to become pregnant and it is not favoured by the majority of surgeons due to complexity of the procedure, but excellent outcomes occurred in centers with expertise in this procedure [88] .

Aortic valve calcification extending to anterior mitral leaflet was shown in Figure 9 in a 64-year-old male. A bicuspid aortic valve calcification extending to anterior mitral leaflet was shown in Figure 10 and it is “mushroom” shaped [89] or bulbous (clublike or circle) configuration [90] as in Figure 11 in a 34-year-old male. J-shaped [91] mitral-aortic calcification was shown in Figure 5 in a 52- year-old male and in Figure 13 in a 60-year-old male. Aortic valve calcification

resembling as “calcified amorphous tumor” [92] as in Figure 12 in a 58-year-old male. Calcification of aortic valve and anterior mitral leaflet was shown in Figure 15 in a 5-year-old child having phenotypic features of Hutchinson-Gilford Progeria syndrome as shown in Figure 14 with bicuspid aortic valve regurgitation as shown in Figure 16 and Figure 17 and AML (anterior mitral leaflet) calcification in a newborn as in Figure 18 and Figure 19 due to rheumatic etiology rather than degenerative change of premature aging.