FIG. 6.

Ultrastructural changes of the newt myocardium after mechanical damage. (A-F) Transmission electron microscopy analysis of the regenerating myocardium. (A) The undamaged myocardium of the ventricle shows compact organization of cardiomyocytes inside the trabeculae. (B) At 1 dpi, reduction of cardiomyocytes inside trabeculae is apparent. The majority of damaged cardiomyocytes is removed until 7 dpi (C). (D) At 14 dpi, repatterning of ventricular structures has started as indicated by repopulation of the remaining trabeculae with cardiomyocytes. Gaps in the myocardium and inside trabeculae are still visible. At 49 dpi, regeneration led to continuous improvement of tissue morphology. Most trabeculae are populated by cardiomyocytes with only few exceptions (E). (F) At 200 dpi, regeneration is completed. No differences between the injured and uninjured hearts are visible. (G-J) Morphological alterations of endothelial cells immediately after damage. (G) Endothelial cells in the control hearts are flat with elliptic nuclei. (H) At 1 dpi, the nuclei of endothelial cells in the injured area are arched and spherical. After 14 dpi, the nuclei of endothelial cells regain a normal round (I) to elliptic shape, which is maintained at later stages (J). (K-P) Mechanical injury induces massive damage of myofibrils inside cardiomyocytes. (K) Myofibrils of the hearts without injury display clearly defined sarcomeres as indicated by the typical pattern of Z-lines, I and M bands, as well as H & A zones. (L) Loss of sarcomere integrity and intercalated disks 1 dpi (red arrow). (M) Fusiform myofibrils without intact Z-lines after 7 dpi. (N) Some myofibrils in the border zone contain no Z-lines, although myofibrils are aligned to myofibrils with Z-lines. (O) At 49 dpi, myofibrils show a normal morphology despite some gaps in the myocardium. (P) Regeneration is complete after 200 dpi. (Q-S) Necrosis of different cell types after mechanical injury of the ventricle. (Q) At 1 dpi, cardiomyocytes in the injured area contain necrotic nuclei and (R) swollen mitochondria with fewer and smaller cristae. (S) Membrane integrity of cardiomyocytes and endothelial cells is compromised (indicated by red arrows). (T–V) Changes in collagen deposition during regeneration. (T) The control hearts maintain low levels of collagen fibres in the interstitial space. (U) Increasing deposition of collagen fibres until 35 dpi. After 35 dpi, the collagen level drops until normal concentrations are reached at 200 dpi when regeneration is complete (collagen fibres are indicated by red arrows) (V). Scale bars: A–F: 10 μm; G–H: 2 μm; K–P: 1 μm; Q: 2 μm; R: 200 nm; S: 500 nm; T–V: 1 μm