Formation of leptofibrils is associated with remodelling of muscle cells and myofibrillogenesis in the border zone of myocardial infarction Eduard I. Dedkov a , Alexander A. Stadnikov b , Mark W. Russell c , Andrei B. Borisov c, * a Department of Anatomy and Cell Biology, University of Iowa, Iowa City, USA b Orenburg State Medical Academy, Orenburg, Russia c Division of Pediatric Cardiology, Department of Pediatrics, University of Michigan Medical School, Ann Arbor, USA Received 19 April 2006; accepted 31 August 2006 Abstract Leptofibrils, or leptomeres, remain the least studied cytoskeletal structures in muscle cells, and their function and mechanism of assembly are still poorly understood. Our ultrastructural study of the surviving cardiac myocytes located in the perinecrotic border zone of the infarcted left ventricle in rats revealed intense formation of leptofibrils and leptofibrillar clusters during 4–15 days following experimental myocardial infarction. In the perinecrotic myocytes, leptofibrils developed predominantly in the subsarcolemmal areas, near disassembled intercalated discs and at the sites of intense myofibrillogenesis in the peripheral zones of the sarcoplasm. We found that the development of these structures occurred before or at the time of assembly of myofibrils. In our material, leptofibrils consisted of longitudinally oriented filamentous bundles inserted in electron dense Z-band-like material and periodically crossed by 3–8 bands of this material with the period of cross-striation of 120–210 nm. The presence of leptofibrils in growing cytoplasmic processes and ruffles developing in the border zone in the areas of lost intercellular contacts indicates their formation de novo during post-infarction period. We observed four major morphological types of localization of these structures: (1) direct contact of one end of leptofibrils with Z bands of nascent, mature or disassembling myofibrils; (2) direct contact with the sarcolemma: (a) multifocal attachment of leptofibrils to the sarcolemma through the lateral surfaces of their minute Z band-like structures; (b) attachment of one or both ends of leptofibrils to the sarcolemma without contacts or in contact with myofibrils; (3) attachment of leptofibrils to subsarcolemmal accumulations of electron dense Z-band material in newly formed fasciae adherentes of the remodeled intercalated disks; (4) clustering and contacts of leptofibrils with one another predominantly at the level of their Z bands. Interestingly, most leptofibrils of all four types were topographically associated with the system of T-tubules, the sarcoplasmic reticulum and subsarcolemmal vesicles. Serial sections through the areas containing leptofibrils indicate their spindle-like or nearly cylindrical shape. Thus, we found that leptofibrils assemble in terminally differentiated cardiac myocytes following destabilization of their differentiated state and partial dedifferentiation induced by myocardial infarction. The results of this study demonstrate that formation of leptofibrils, earlier described mainly in the developing and malignant muscle, is temporally associated with adaptive structural remodelling and the activation of myofibrillogenesis in functionally overloaded cardiac myocytes of adult animals. Our findings suggest that re- expression of some structural characteristics of the embryonic muscle appear to represent one of the mechanisms that underlie adaptive plasticity of the myocardium following injury and under conditions of hyperfunction. # 2006 Elsevier Ltd. All rights reserved. Keywords: Myocardium; Injury; Cardiac myocytes; Leptomeres; Myofibrillogenesis; Leptofibrils; Myocardial infarction; Heart 1. Introduction Leptofibrils (from Greek lepto ´s meaning small, thin, fine and Latin fibra, fiber), are still one of the least studied structural elements associated with the contractile system of cardiac and skeletal muscle cells. Due to the very small size of these structures, electron microscopy remains the only technique that permits their reliable detection and characterization (for literature, see Martynova and Borisov, 1987; Ghadially, 1997). Electron microscopy identifies leptofibrils as tiny cross-striated myofibril-like structures that consist of long- itudinally aligned bundles of 4–10 nm filaments periodically crossed by 3–20 electron-dense bands. These dense bands www.elsevier.com/locate/micron Micron 38 (2007) 659–667 * Corresponding author at: Room 8303A, MSRB III, Division of Pediatric Cardiology, Department of Pediatrics and Communicable Diseases, University of Michigan Medical School, Ann Arbor, MI 48109, USA. Tel.: +734 647 9429; fax: +734 615 1386. E-mail address: aborisov@med.umich.edu (A.B. Borisov). 0968-4328/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.micron.2006.08.006