2867 The vertebrate heart grows to accommodate increasing circulatory demands by adjusting its mass in response to pressure and volume loading. These adjustments are determined by the principle of Laplace, which states that the major determinants of heart mass are the ventricular radius and blood pressure. Accordingly, the cardiac wall changes in thickness to maintain wall stress within a relatively narrow range of variations, as documented, for instance, in a comparative survey of birds and mammals across a broad range of body size (Seymour and Blaylock, 2000, and references therein). An extensive amount of data from both human and experimental animal cardiology has detailed the main aspects of ventricular remodelling of the mammalian heart in response to volume and pressure loading. Ventricular adaptation to volume loading involves the enlargement of the cavity volume by increasing myocardial fibre length, with a parallel increase in the thickness of the ventricular wall. On the other hand, ventricular adaptation to pressure loading is matched by wall thickening through the parallel addition of myofibrils, without a corresponding increase in luminal volume (Braunwald, 1984). This is illustrated by the paradigm of right and left ventricles of the mammalian heart working as volume and pressure pumps, respectively. The morpho-dynamic design of the right ventricle is well suited for ejecting relatively large volumes of blood against relatively low blood pressure, while that of the left ventricle is better suited for ejecting relatively low blood volumes against higher blood pressure (Rushmer, 1972). The fish heart is capable of impressive morpho-functional rearrangements to match the variable hemodynamic challenges resulting from developmental and eco-physiological changes of the animal, such as changes in body size and shifts in lifestyle patterns. A notable aspect of this cardiac flexibility is evident in the close relationship between the structural organization of the ventricular pump and the mechanical performance of the heart, evaluated in terms of the relative contribution of pressure and volume work to the stroke work (Tota and Gattuso, 1996). In many fish, this relationship allows a distinction between ventricles producing mainly volume work and those producing mainly pressure work (Tota and Gattuso, 1996). This picture provides an insight into how the internal construction of the ventricular chamber is adapted to its functional performance. On the other hand, many fish species experience remarkable The Journal of Experimental Biology 207, 2867-2875 Published by The Company of Biologists 2004 doi:10.1242/jeb.01119 The morphodynamic changes occurring during growth were evaluated in the eel (Anguilla anguilla L.) heart. Using an in vitro working heart preparation, cardiac performance of small (body mass 96.76±27.49·g; mean ± S.D.) and large (body mass 656±12·g; mean ± S.D.) eels was compared under basal conditions and under loading (i.e. preload and afterload) challenges. A parallel morphometric evaluation of the ventricle was made using light and transmission electron microscope images. The small eel hearts show a basal cardiac output lower than their large counterparts (heart rate fH, 38.93±2.82 and 52.7±1.8·beats·min –1 , respectively; stroke volume VS, 0.27±0.017 and 0.37±0.016·ml·kg –1 , respectively; means ± S.E.M.). The two groups show similar responses at increasing preload, but differ remarkably at increasing afterload. Small eel hearts decreased VS at afterload greater than 3·kPa, in contrast to larger hearts, which maintained constant VS up to 6·kPa. These changes in mechanical performance are related to structural differences. Compared with the small eels, the large eels show an increase in the compacta thickness and in the diameter of the trabeculae in the spongiosa, together with reduction of the lacunary spaces. The increased compacta thickness is attained by enlargements of both the muscular and vascular compartments and reduction of the interstitium; consequently, this layer appears more compacted. Both compacta and spongiosa show higher number of myocytes together with reduced cross-sectional area and myofibrillar compartment. The compacta also shows an increased mitochondrial compartment. Our results document a cardiac morphodynamic remodelling in the growing eel. Key words: fish, Anguilla anguilla, myocardial growth, cardiac performance, ventricular ultrastructure, compacta, spongiosa. Summary Introduction Cardiac morphodynamic remodelling in the growing eel (Anguilla anguilla L.) M. C. Cerra 1,2, *, S. Imbrogno 2 , D. Amelio 2 , F. Garofalo 2 , E. Colvee 3 , B. Tota 2 and J. M. Icardo 3 Departments of 1 Pharmaco-Biology and 2 Cellular Biology, University of Calabria, 87030 Arcavacata di Rende, CS, Italy and 3 Department of Anatomy and Cell Biology, University of Cantabria, 39011, Santander, Spain *Author for correspondence (e-mail: cerramc@unical.it) Accepted 28 May 2004