Structural Alterations in the Hypertrophied and Failing Myocardium. In: Abel FL, Newman WH, editors. Functional Aspects of the Normal, Hypertrophied, and Failing Heart. Boston: Springer US; 1984. pp. 278-300. [DOI: 10.1007/978-1-4613-3825-3_18]

The Myocardial Cell: Normal Growth, Cardiac Hypertrophy and Response to Injury

Development and growth of cardiac muscle tissue is controlled by a variety of intrinsic and extrinsic factors. Fetal growth is by hyperplasia, is strongly governed by inherent factors and is only slightly modified by external environmental factors. The number of cell divisions is species-dependent and normally stops shortly after birth. Developing blood volume and pressure dictate the hypertrophic stage of normal growth, and this may be further modified by abnormal hemodynamic and humoral factors. Connective tissue, neural and vascular components of the myocardium mature along with the myocyte, and alterations in these structures profoundly affect myocyte function. Hypertrophy beyond normal growth is the response of the myocyte by increased protein synthesis to various stimuli including hemodynamic, humoral and ischemic factors. Injury to normal and hypertrophied myocardium may vary due to structural and metabolic adaptations of hypertrophied tissue, such as connective tissue proliferation, vascular supply alterations and glycolytic metabolic potential. Ischemic effects influence not only cell necrosis, but also hypertrophy and congestive myocardial failure.

Role of extracellular matrix proteins in heart function

The cardiac interstitium is populated by nonmyocyte cell types including transcriptionally active cardiac fibroblasts and endothelial cells. Since these cells are the source of many components of the cardiac extracellular matrix, and because changes in cardiac extracellular matrix are suspected of contributing to the genesis of cardiovascular complications in disease states such as diabetes, hypertension, cardiac hypertrophy and congestive heart failure, interest in the mechanisms of activation of fibroblasts and endothelial cells has led to progress in understanding these processes. Recent work provides evidence for the role of the renin-angiotensin-aldosterone system in the pathogenesis of abnormal deposition of extracellular matrix in the cardiac interstitium during the development of inappropriate cardiac hypertrophy and failure. The cardiac extracellular matrix is also known to change in response to altered cardiac performance associated with post-natal aging, and in response to environmental stimuli including intermittent hypoxia and abnormal nutrition. It is becoming clear that the extracellular matrix mainly consists of molecules of collagen types I and III; they form fibrils and provide most of the connective material for typing together myocytes and other structures in the myocardium and thus is involved in the transmission of developed mechanical force. The data available in the literature support the view that the extracellular matrix is a dynamic entity and alterations in this structure result in the development of heart dysfunction.

Morphometric analysis of cardiac hypertrophy during development, maturation, and senescence in spontaneously hypertensive rats

Hypertrophy of the mammalian heart, regardless of the initiating event, results in architectural remodeling of ventricular components that maintain structural and functional characteristics of this organ. Ventricular components that vary their morphology and morphometry in a hypertrophic state are the muscle cells, connective tissue elements, vasculature, or a combination of some or all of the above. Morphologic quantification of the progressive tissue changes occurring throughout the natural life span of the spontaneously hypertensive and normotensive Wistar-Kyoto rats has not been thoroughly documented. Using perfused-fixed tissue from both strains at 1, 6, 12, 18, and 24 months of age, we have determined the morphometric changes that occurred in the subepicardial and midwall regions of the left ventricle. Myocyte cell size, wall thickness, and arterial blood pressure were elevated in 1-month-old spontaneously hypertensive rats, reached significance by 6 months, and remained significantly greater throughout the 24 months examined. Tissue morphometry demonstrated significant tissue component volumetric differences at 6 months in the spontaneously hypertensive rat. Age-related morphometric tissue changes occurred in both strains yet were exacerbated (percent volume of myocytes) or diminished (percent volume interstitial space) in the mature and aging spontaneously hypertensive rat. Capillary density of SHR left ventricle showed a drastic decline so that 6-month-old SHR had the same density as a senescent Wistar-Kyoto. Tissue morphometry and capillary density data strongly support the hypothesis that tissue oxygenation is diminished in the spontaneously hypertensive rat, and as a result, tissue necrosis and myocyte cell death occur.

Age-related changes in ploidy levels and biochemical parameters in cardiac myocytes isolated from spontaneously hypertensive rats

Postnatal growth of the mammalian ventricular myocyte is characterized by a brief period of hyperplasia followed by an extensive period of physiological hypertrophy. Using myocytes isolated from both Wistar-Kyoto and spontaneously hypertensive rats from fetus to 10 months old, we analyzed morphological, biochemical, and ploidy changes. Fetal myocytes from both spontaneously hypertensive and Wistar-Kyoto rats were mononuclear, diploid cells. By 4 weeks, adult binucleation levels (84% binuclear) were found, and myocytes pooled from both ventricles demonstrated nuclear ploidy shifts to tetraploid levels. However, analysis of myocytes isolated from left ventricle, right ventricle, and septum showed that nuclear polyploidation was confined to the right ventricle and septum, with few polyploid nuclei detected in the left ventricle. This pattern remained relatively constant through 10 months of age, although spontaneously hypertensive myocytes showed significantly more polyploidation than Wistar-Kyoto in all regions. Biochemical analysis of isolated myocytes substantiated the nuclear ploidy changes and demonstrated elevated protein and ribonucleic acid content in left ventricular myocytes without extensive polyploidation. Since cardiac hypertrophy, both physiological and pathological, is associated primarily with the left ventricle and occurs in the absence of significant ploidy changes, these findings suggest that a unique pattern of gene regulation may be ongoing in the left ventricle myocytes that is not present in the septum and right ventricle. These variations may be essential for cellular hypertrophy under normal and pathological conditions.