Collagenous skeleton of the human mitral papillary muscle

The papillary muscles (FM) of the heart have been the subject of numerous s tructural and functional studies. However, despite the importance of the co llagenous compartment of the heart in the mechanical and electrical propert ies of the myocardium, little information is available on the structural or ganization of collagen within the PM. We study here the structural organiza tion of collagen within the mitral papillary muscles (PM) of the human hear t. Fragments of human mitral PM from normal and hypertensive subjects were macerated in NaOH to eliminate the cellular components. Macerated and nonma cerated samples were then studied with the scanning electron microscope (SE M). SEM shows that cardiac myocytes and endomysial capillaries are ensheathed i n a layer of collagenous tissue. The myocyte sheath wall is formed by thin collagen fibers oriented at right angles to the main cell axis. These sheat hs are open structures, collagen fibers continuing into adjacent sheaths at the points of lateral communications. Thick perimysial septa do not divide the PM tissue into separate compartments. Hypertensive hearts show perivas cular and interstitial fibrosis. In addition, the lumen of the coronary ves sels is reduced or obliterated, and large areas of the myocardium are subst ituted by densely packed collagen. Endomysial sheaths constitute a continuo us collagenous layer that replicates the myocyte network. The endomysium sh ould play a complex role in myocardial mechanics, assuring the equal distri bution of force during the cardiac cycle. The absence of insulating boundar ies should facilitate lateral propagation of excitation. Fibrosis in hypert ensive hearts appears to be both reactive and reparative. The increase in t he amount of collagen should greatly impair contractile capabilities and el ectrical conductance, severely compromise heart function, and contribute to development of heart failure. Anat. Rec. 252:509-518, 1998. (C) 1998 Wiley -Liss, Inc.