With aging, rabbit tendon tissue undergoes a series of morphological and
biochemical changes which involve both the cells and the extracellular matrix.
The extracellular matrix increases in volume, causing a relative decrease of
the number of cells per unit of tissue surface. The tenoblasts become longer
and more slender, while their cytoplasmic processes increase in number and
become thinner and more elongated, forming a dense network. In addition,
tendon cells show a marked decrease in the intracytoplasmic organelles
responsible for protein synthesis, while their intracellular content of
contractile proteins does not change.
With aging, collagen fibers increase in diameter and vary more in
thickness. These morphological changes correspond to biochemical changes that
include an increase in collagen, a decrease in mucopolysaccharides, and a
decrease in water content. During aging parallel changes occur in the elastic
fibers, which decrease in number and show structural alterations.
Clinical Relevance: Ultrastructural and biochemical
studies of tendon diseases need a normal comparison. Our ultrastructural and
biochemical findings in aging tendon may be useful in that regard. The
presence of actin and myosin in tendon cells could be related to some aspects
of tendon physiology and pathology.