Myocardial fibrosis in nonhuman primate with pressure overload hypertrophy

Characteristics of pressure overload hypertrophy are known to include an accumulation of collagen (or fibrosis) and a biochemical remodeling of fibrillar type I and III collagens. The corresponding structural nature of myocardial fibrosis is less clear. This light morphologic study was undertaken to address this issue in the hypertrophied left ventricle of the nonhuman primate with experimental hypertension. For this purpose, the picrosirius red technique and polarization microscopy were used to examine the myocardium during the evolutionary, early, and late phases of established hypertrophy corresponding to 4, 35, and 88 weeks of experimental hypertension. Evidence of increased thin perimysial fiber formation, together with collagen fiber disruption and edema at 4 weeks of hypertrophy, was found when the chamber volume to left ventricular mass ratio was reduced. After 35 weeks, when this ratio was again normal, a greater number of intermuscular spaces contained both thick and thin perimysial fibers. In addition to this interstitial fibrosis, a reactive fibrosis consisting of a meshwork of thick and thin perimysial fibers was seen extending over muscle fibers. Finally, at 88 weeks, this fibrous meshwork had encircled muscle and there now was evidence of cell necrosis. The accompanying replacement fibrosis consisted of yet another distinctive orthogonal grid of thick and thin collagen fibers. Thus, a continuum of fibrillar collagen remodeling was seen in pressure overload hypertrophy in the nonhuman primate myocardium. Structurally distinct patterns of myocardial fibrosis were recognized based on the alignment of perimysial fibers with muscle that may explain the cellular remodeling and altered mechanical behavior of the concentrically hypertrophied myocardium.