The bicuspid (mitral) valve complex of the human heart consists
of functional units which include the valve
leaflets, chordae tendineae and the papillary muscles. The mechanical
properties of these functional units
depend to a large extent on the link between the muscle and the valve.
This link is usually arranged in a
branching network of avascular tendinous chordae composed of collagen
and elastic fibres, which transmit
contractions of the papillary muscle to the valve leaflets. In order
to perform their function efficiently, the
chordae have to possess a high degree of elasticity, as well as
considerable strength and endurance. Human
chordae tendineae originating from the left ventricles were obtained
from 7 embalmed cadavers and 6
postmortem subjects of various ages. Samples washed in saline were
fixed or postfixed in 9% formol saline.
Observations were made by illuminating the chordae along their axes.
The reflected images originating from
the superficial collagenous layers of the relaxed chordae showed a
striped pattern 11 μm in width. Scanning
electron and light microscopy of the chordae confirmed an undulating
pattern of collagen fibrils arranged in
bundles of planar waves in register and around the entire circumference
of the chorda. The dimensions of
the waves correlated with those of the striped reflected pattern.
The observed undulating arrangement
of the collagen fibrils appears to produce an inherent built-in
elasticity which is likely to be of considerable
advantage for a tissue which is under continuous repetitive stress.
The chordae were covered by
endocardium composed of a superficial layer of smooth squamous
endothelial cells and an underlying dense
layer of elastic fibres. It is suggested that the relaxed striped
chordae, consisting of undulating collagen
fibrils, straighten when the chordae become stretched by papillary
muscle contraction, thereby mitigating the
peak stress developed during muscle contraction. On relaxation the
elastic tissue tends to return the collagen
to its wavy configuration. It is also suggested that the regular
wavy pattern of collagen seen in young
individuals gradually changes with age by elongation of the wave
pattern which eventually becomes
randomised. In addition, with increasing age, substantial cushions
of connective tissue appear below
endocardium while the dense collagenous core has a reduced cross-sectional
area which may lead to stretching and eventual rupture of the chordae.