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Human mitral valve chordae tendineae in which elastic response curves are nonlinear have also been found to exhibit extensibility that increases with chordal size and decreases with chordal age. We used selective enzymatic digestion and scanning transmission electron microscopy to explain these observations. Removal of the outer elastin sheath by enzymatic digestion did not significantly affect the elastic response of this tissue. Scanning electron microscopy revealed that the collagen fibers in the central core of young chordae exhibited a very wavy pattern but the pattern in adult specimens was relatively straight. The increased waviness accounted for the greater extensibility of the young specimens. The collagen fibers from young and old chordae consisted of a network of collagen fibrils that became more collapsed when the tissue was fixed under tension. This network arrangement of the fibrils explains the nonlinearity in the elastic response of the tissue. Transmission electron micrographs showed that the density of collagen fibrils decreased as chordal size increased. The number of fibrils per 10(-8) cm2 of the central core decreased from 182.4 (SE = 1.3) to 131.3 (SE = 1.6) as average chordal cross-sectional area increased from 0.0016 cm2 to 0.0268 cm2. This difference in fibril density provides an explanation for the greater extensibility shown by the thicker chordae. The collagen fibril diameters ranged from 516 A to 552 A.

Morphology and relationship to extensibility curves of human mitral valve chordae tendineae.