Table 2.

Overview of the context and outcome of a selection of recent studies on in situ TEHVs, with an emphasis on tissue organization

References	Methodology	Follow-up (weeks)	Model system	Cellularization	ECM components	Structure	Functionality
Theodoridis et al. (2015)	Decellularized allograft	52	Sheep pulmonary valve	Spatial variability in cellularization	Collagen observed Collagen content correlated with cellular repopulation	Repopulation more extensive on ventricular than arterial side Poor repopulation in tip Collagen on both ventricular and arterial sides of leaflet Dense collagen in lamina fibrosa No cells in lamina fibrosa	Some calcification No major complications
Tudorache et al. (2016)	Decellularized allograft	87	Sheep aortic & pulmonary valve	Incomplete cellularization and endothelial coverage More cells positive for α-SMA than in native valves	Collagen deposition observed	Stronger repopulation on ventricular side than arterial side No cells in lamina fibrosa Collagen in lamina fibrosa GAGs in lamina spongiosa	Some minor stenosis, retraction, insufficiency and pinpoint thrombosis
Miller et al. (2016)	Decellularized allograft	26	Pig pulmonary valve	Incomplete endothelium		Endothelial coverage only at base of leaflet Incomplete remodeling, initiating at base of leaflets	Significant stenosis in all valves Moderate regurgitation which worsened in all valves 1 case of calcification
Goecke et al. (2018)	Decellularized allograft	26	Sheep pulmonary valve	Incomplete endothelial coverage VICs observed in leaflets		Repopulation stronger on ventricular than pulmonary side More repopulation on proximal side, decreasing toward tip No cells in lamina fibrosa Tri-layered structure maintained Collagen on both sides of leaflet	Satisfactory function in most valves Regurgitation in 2 valves Small thrombus in 2 valves Significant deterioration in 1 valve
Zafar et al. (2015)	Decellularized xenograft	34	Sheep tricuspid valve	Complete endothelial layer Dense cellularization Cells transitioning from remodeling phenotype to homeostasis		Approximation of organized ECM stratification on proximal side of leaflet Lack of ECM organization on distal side of leaflet	One case of severe regurgitation No other major complications Mechanical properties similar to native
Hennessy et al. (2017)	Decellularized xenograft	22	Sheep pulmonary valve	Cellular infiltration observed (remodeling phenotype) VEC monolayer formation	ECM remained intact Signs of collagen deposition	Incomplete endothelium on pulmonary side Cellularization predominant on ventricular side and infiltrated toward pulmonary side	Normal functioning valves Moderate insufficiency due to leaflet retraction Mechanical properties similar to native
Van Rijswijk et al. (2020)	Decellularized xenograft	26	Sheep pulmonary valve	Limited endothelial coverage Cellularization highly variable and lower than native levels	No elastin deposition observed	No signs of remodeling into tri-layered structure	7 of 20 animals died Moderate to severe stenosis Severe regurgitation Signs of calcification
Syedain et al. (2015)	Decellularized TEM	24	Sheep Aortic valve	Endothelium and basement membrane observed Cellularization observed, but DNA content lower than native	Collagen deposition observed but content lower than native Elastin detected in some valves	Cellularization and endothelialization extensive at base and lower belly, but partial toward tip ECM components localized with cellularization	Normal heart function No major complications Unchanged mechanical properties
Reimer et al. (2017)	Decellularized TEM	22	Sheep pulmonary valve	Incomplete cellularization and endothelial coverage	Collagen and elastin deposition Collagen content higher than native, elastin content lower than native No mature elastin observed	Fairly uniform distribution of cells Collagen and elastin on both sides of leaflets	3 premature endings Increasing regurgitation to moderate/severe Leaflet shortening Higher mechanical properties than native
Emmert et al. (2018)	Decellularized TEM (with geometric constraints)	52	Sheep pulmonary valve	Homogeneous endothelial coverage with typical EC morphology Substantial cell infiltration, but lower than native Phenotype cells in leaflets similar to native	Collagen deposition Dense, homogenous, and wavy collagen matrix Elastin present No change in GAG content	Tri-layered structure generally not observed Collagen alignment observed, but less than native Cell repopulation observed throughout leaflet	Generally good functionality without major complications Signs of functional remodeling
Motta et al. (2018)	Decellularized TEM	16	Sheep pulmonary valve	Confluent endothelium DNA content higher than native Moderate expression of α-SMA-positive cells	Collagen content similar to native GAG content less than native Traces of elastin	Densely packed collagen fibers Cells predominantly in wall and hinge regions Elastin on ventricular side	Good acute performance Progressive severity of regurgitation No stenosis or calcification
Kluin et al. (2017)	Synthetic scaffold	52	Sheep pulmonary valve	Confluent endothelium on pulmonary side, near-confluent on ventricular side Extensive colonization by host cells, DNA content similar to native	Collagen content similar to native Sparse elastic fibers	Collagen and elastin on same side of the leaflet Elastic fibers mainly on pulmonary side	Well functioning valves One case of severe regurgitation No other major complications Good mechanical integrity
Fioretta et al. (2020)	Synthetic scaffold (pre-seeded with bone marrow mononuclear cells or unseeded)	24	Sheep pulmonary valve	Incomplete endothelium Cellularization observed	Collagen formation observed Elastic fibers observed	Endothelium absent toward the tip	Variability in organization, cell infiltration and ECM deposition between leaflets Pathological calcification, regurgitation, leaflet fusion, and maladaptive remodeling in pre-seeded scaffolds