[Quantitative and qualitative changes in the heart muscle of the left ventricle in male rats during development and maturation in the first 6 months of life].

In order to characterise the postnatal development and maturation of heart muscle cells the musculature of the left ventricle of male rats has been investigated electronmicroscopically at birth, at the 1st, 2nd, 3rd, 4th, 5th, 6th, 7th, 14th, and 21st day of life, as well as after the 1st, 2nd, 3rd, 4th, 5th, 6th month. On the basis of these investigations quantitative and qualitative data have been obtained. The development of the heart muscle cells is characterised by 2 processes. 1. Processes of synthesis and development - The ribosomes/polysomes which are to be found in great numbers are a characteristic of the synthesis of the myofilaments which plays an important role in the 1st phase of postnatal development. This synthesis of myofilaments takes place in the sarcoplasm by the formation of new individual myofilaments, in the interior and at the border of existing myofibrils, and at the level of the Z-lines. Mitochondria are developed in particular by the germination of small mitochondria the percentage of which amounts to about 16% at birth. The sarcoplasmic reticulum is developing in its longitudinal as well as transversal forms only during the 1st month. The disci intercalares, however, have been formed in the main already at birth. Cells are furthermore developing by mitotic processes during the first days. 2. Degradation processes - 2.1. The degradation is characterised by focal autophagocytotic processes in mitochondria concerning the cristae as well as the matrix and the outer membrane. During these processes autophagocytotic vacuoles are developing partly with myelin membranes. In addition, whole mitochondria are transformed into autophagocytotic vacuoles. Later on, mitochondria are also condensed and transformed into mitochondrial lipofuscin. 2.2. The autophagocytotic degradation of glycogen together with the sequestration and formation of vacuoles and myelin membranes are particularly striking. Lipofuscin, however, is to be observed in larger quantities only from the 4th month onwards. The investigations have shown that decisive phases of development can be determined: the day of birth, the 3rd, 4th, 5th, and 7th, and 14th day, as well as after the 1st, 3rd, and 6th month. This determination of the specific developmental phases of the heart muscle makes it possible to carry out investigations in a more economical and purposeful way.