The fine structure of the Necturus (amphibia) heart

Reconstitution of the myocardium in regenerating newt hearts is preceded by transient deposition of extracellular matrix components

Adult newts efficiently regenerate the heart after injury in a process that involves proliferation of cardiac muscle and nonmuscle cells and repatterning of the myocardium. To analyze the processes that underlie heart regeneration in newts, we characterized the structural changes in the myocardium that allow regeneration after mechanical injury. We found that cardiomyocytes in the damaged ventricle mainly die by necrosis and are removed during the first week after injury, paving the way for the extension of thin myocardial trabeculae, which initially contain only very few cardiomyocytes. During the following 200 days, these thin trabeculae fill up with new cardiomyocytes until the myocardium is fully reconstituted. Interestingly, reconstruction of the newly formed trabeculated network is accompanied by transient deposition of extracellular matrix (ECM) components such as collagen III. We conclude that the ECM is a critical guidance cue for outgrowing and branching trabeculae to reconstruct the trabeculated network, which represents a hallmark of uninjured cardiac tissue in newts.

The effects of hypoxic stress on the fine structure of the flounder heart (Platichthys flesus)

1. Flounder hearts were examined by conventional transmission electron microscopy. Hearts showed clear evidence of a coronary circulation but no intrinsic conduction network or innervation was detected. 2. The box-like cells showed many surface inpocketings and many scattered glycogen granules and membrane bound liposomes. The nucleus position was variable and the cells contained numerous small ovoid mitochondria. 3. Dark staining intercalated discs with clear nexal and desmosomal areas separated individual cells. 4. No organized T-tubular system or sarcoplasmic reticulum was present but the cells displayed abundant surface vesicles which may perform the physiological role of the latter. 5. Hearts of flounder subjected to 3 weeks hypoxia showed striking changes to the mitochondria which were smaller than in controls and there was evidence of increased mitochondrial budding and also evidence of mitochondrial necrosis. 6. Hypoxically-stressed hearts exhibited normal liposome populations but increased glycogen granule deposits. These hearts also showed evidence of myofibril degeneration with torn Z discs and with a build up of fibrous material. 7. It is concluded that the cellular damage seen in hypoxic cells may be due to excessive calcium accumulation or increased catecholamine release.

Morphometrics and ultrastructure of myocardial tissue in Notothenioid fishes

Antarctic fish of the family Channichthyidae (Icefishes) lack the respiratory pigments haemoglobin and myoglobin. The morphometrics and ultrastructure of the ventricular myocardium of a benthic icefish,Chaenocephalus aceratus has been compared with that of a red-blooded Notothenioid fish,Notothenia neglecta, of similar habit.The mass of ventricular muscle as a percentage of bodyweight is 3 times greater in adultC. aceratus (0.32%) thanN. neglecta (0.11%). Myoglobin concentration in the ventricle ofN. neglecta, 20 nmoles/g, is comparable to that of temperate teleosts with similar activity patterns. The volume and surface densities of mitochondria are 41.5% and 0.32 μm(-1) for Icefish and 25% and 0.15 μm(-1) forN. neglecta, Cytochrome oxidase activities are similar in the two tissues whilst the volume density of myofibrils is higher forN. neglecta (47%) thanC. aceratus (29.9%).The proliferation of mitochondria in the myocardium of Icefish will reduce the diffusion path-length for oxygen between ventricular lumen and the outer mitochondrial membrane and may compensate for the absence of myoglobin.

Isolation of cardiac myocytes from the adult newt, Notophthalmus viridescens. an electron microscopic and quantitative light microscopic analysis

This report describes a technique which permits a high yield of viable adult cardiac myocytes from the adult newt using enzymatic separation techniques at low temperature and high enzyme concentrations. Observations by light microscopy showed the isolated myocytes to have a distinctively slender morphology which consisted of a variable number of arm-like appendages radiating from the center of cells which were predominantly mononucleated. Atrial myocytes were typically observed to have two to three arm-like appendages while ventricular myocytes typically had three to six appendages. The majority of myocytes displayed normal fine structure when examined by transmission electron microscopy. Computerized image analysis revealed that atrial cells were significantly greater in cell length (192.9 +/- 53.4 microns) and in nuclear length (25 +/- 5.3 microns) and perimeter (59.2 +/- 10.7 microns) than were ventricular cells (162.8 +/- 39 microns, 23.6 +/- 5.1 microns and 57.4 +/- 11.1 microns, respectively), while cell widths and areas were greater in ventricular cells (16.5 +/- 4.7 microns and 1839.8 +/- 585.0 microns, respectively) than in atrial cells (13.2 +/- 3.1 microns and 1520.3 +/- 527.6 microns, respectively). Comparison of these data with previous descriptions of isolated amphibian and mammalian cardiac myocytes emphasizes species-related differences.

Autoradiographic and electron microscopic studies of minced cardiac muscle regeneration in the adult newt, notophthalmus viridescens

The regenerative response of minced cardiac muscle grafts in the adult newt was studied using autoradiography and electron microscopy. One-sixteenth to one-eighth of the newt ventricle was amputated, minced, and returned to the wounded ventricle. At five days after grafting, no reorganization of graft msucle pieces was apparent and there was degeneration of much of the muscle graft. Another, smaller population of 5-day myocytes had euchromatic nuclei and intact sarcolemmae. In 10- and 16-day grafts, continuity between ventricular and graft lumina was established and coalescence of graft pieces was apparent. Ultrastructurally, 10- and 16-day graft myocytes appeared to have fewer myofibrillae and increased amounts of rough endoplasmic reticulum, polyribosomes, Golig complexes, and dense bodies when compared to uninjured ventricular myocytes. The peak of proliferative activity of graft cells was observed at 16 days. Electron microscopic autoradiography revealed breadkdown of myofibrillar structure in labeled myocytes, whereas in myocytes in the later stages of mitosis only scattered myofilaments and no Z bands were present. By 30 days, grafts appeared as an integrated structure composed primarily of cardiac muscle. Myocytes of 30-day grafts were observed in various stages of myofibrillogenesis and contained numberous 10-nm filaments. Seventy-day graft mycoytes had numberous well organized myofibrillae and intercellular junctions similar to those seen in uninjured ventricular myocytes.

Spheroidal bodies in the junctional sarcoplasmic reticulum of lizard myocardial cells

The sarcoplasmic reticulum (SR) of lizard (Anolis carolinensis) myocardial cells has been examined, with particular attention being paid to the structural details of the peripheral couplings (junctional SR). Spheroidal bodies are present within the opaque core of junctional SR; these can be seen both in sections made en face and in sections cut to show the apposition of the junctional SR with the sarcolemma. Opaque junctional processes extend between the sarcolemma and the peripheral junctional SR. The myocardial cells in addition contain some SR cisternae deep within the cells which also possess opaque cores composed of spheroids. Although the significance of the junctional SR spheroidal bodies is unknown, it is thought that they could act as a matrix on which enzymes such as calcium-specific ATPase may be located.