The remarkable ability of the heart to regenerate has been demonstrated in the zebrafish and giant danio, two fish members of the cyprinid family. Here we use light and electron microscopy to examine the repair response in the heart of another cyprinid, the goldfish (Carassius auretus), following cautery injury to a small portion of its ventricular myocardium. We observed a robust inflammatory response in the first two weeks consisting primarily of infiltrating macrophages, heterophils, and melanomacrophages. These inflammatory cells were identified in the lumen of the spongy heart, within the site of the wound, and attached to endocardial cells adjacent to the site of injury. Marked accumulation of collagen fibers and increased connective tissue were also observed during the first and second week in a transition zone between healthy and injured myocardium as well as in adjacent sub-epicardial regions. The accumulation of collagen and connective tissue however did not persist. The presence of capillaries was also noted in the injured area during repair. The replacement of the cauterized region of the ventricle by myocardial tissue was achieved by 6 weeks. The presence of ethynyl deoxyuridine-positive cardiac myocytes and partially differentiated cardiac myocytes during repair suggest effective cardiac myocyte driven regeneration mechanisms also operate in the injured goldfish heart, and are similar to those observed in zebrafish and giant danio. Our data suggest the ability for cardiac regeneration may be widely conserved among cyprinids.

The Giant danio (GD) and zebrafish (ZF) are small fish belonging to the diverse family of cyprinids; they both display the remarkable ability for heart regeneration. Our lab has recently described the complex junctional region (JR) at the interface of the compact and spongy myocardial layers of the ZF. However, how these two distinct layers are integrated in the GD has not been studied. Using light, scanning, and transmission electron microscopy (TEM), our study reveals that the GD possesses a complex JR with transitional cardiac myocytes (CM) linking compact heart CM and CM of the spongy heart, and that luminal and abluminal contacts are mediated through discrete membrane appositions and adherens junctions. More importantly we also found a complex network of fibroblasts (FB) with extensive cytoplasmic processes occupying the JR and investing the GD compact heart. To ascertain (FB) function, we treated GD with low dose of the antineoplastic agent Doxorubicin (DOX). Using Masson’s trichrome staining, we observed the accumulation of collagen in the compact heart. In addition, TEM shows increased level of collagen in the JR of treated hearts, as well as increase in FB endoplasmic reticulum and cytoplasmic polyribosomes suggesting a state of activation. We conclude that cardiac FB are resident in the complex JR and compact heart of the GD. Moreover our data suggest they are key to the accumulation of collagen that accompany the DOX‐induced injury response. Grant Funding Source: Supported by FDC‐DePauw University