The growth of succinoxidase activity in the hearts of rat and chick embryos

Differentiation of the atrioventricular conducting system of the heart

The structure and function of the atrioventricular conducting system of the heart, and its relationship to the myocardium, are examined from a developmental point of view. On the basis of information derived from electron micrographic, electrophysiologic, and developmental studies of heart tissue, it is concluded that: (1) The idea of the syncytial nature of the heart lacks a sound anatomic basis. (2) Cytodifferentiation during embryonic cardiogenesis results in the development of at least 2 distinct populations of cells: those comprising the bulk of the myocardium and a second type, the specialized cells of the conductive tissue, which differs in histology, biochemistry, and physiology. (3) The common view of the myocardium as a spontaneously active tissue may require revision, since several lines of evidence appear to indicate that myocardial cells are quiescent until stimulated by an extrinsic source. Under normal circumstances, this stimulus source is the conductive tissue.

Alterations in mechanical response of aged rat myocardium. Effects of dexamethasone

1. Mechanical responses of young and old rat myocardium to increasing rates of stimulation were compared. As the animals aged, we found a significant enhancement of the negative force-frequency response and a decline in the velocities of contraction and of relaxation. 2. At 6 months of age, there were no differences between rats obtained from commercial sources and a group of rats obtained from a colony maintained at the National Institute of Aging. 3. At 24 months, the negative force-frequency response was considerably greater in the former group of animals than in the latter. 4. The sensitivity to the calcium concentration in the tissue bathing solution was significantly increased in aged heart preparations. Increasing the calcium concentration reduced the negative inotropy and the decline in the velocities of contraction and relaxation. The enhancement by calcium was directly proportional to the concentration of the metal in the bathing solution. 5. When aged animals were pretreated with pharmacological doses of dexamethasone, the age-induced alterations in the mechanical responses were reversed. The aged, dexamethasone-treated myocardium also became refractory to calcium concentrations above 2.7 mM in the bathing solution. 6. It is suggested that aging induces multifocal defects and that steroid hormones play a role in the maintenance of integrity of the myocardium. The action of the steroids is on the sarcolemma, the contractile proteins and the sarcoplasmic reticulum.

The development of Purkinje fibres and ordinary myocytes in the bovine fetal heart. An ultrastructural study

A comparative ultrastructural study of bovine Purkinje fibres and ordinary myocytes during fetal development has been undertaken. Differences between the two cell types with respect to the intercalated disc, amount of myofibrils, arrangement of mitochondria, amount of glycogen and formation of T-tubules became apparent gradually. In all stages studied an abundance of intermediate filaments was typical for the Purkinje fibres. Myofibrillar M-bands developed at an earlier stage in Purkinje fibres than in ordinary myocytes. Myofilament-polyribosome complexes typical of adult cow Purkinje fibres were not observed in the fetal hearts. Only in late fetal stages were leptofibrils observed in both cell types. We conclude that in the bovine heart Purkinje fibres develop along a different pathway from ordinary myocytes.