Book of Abstracts - New Frontiers 2022

Abstracts of poster presentations

GROWTH-RELATED ACTIVITIES AT THE PLASMALEMMA IN NEONATAL CARDIAC MYOCYTES

M. Novotová, A. Zahradníková Jr., I. Zahradník

Institute of Experimental Endocrinology, BMC SAS, Bratislava, Slovakia

Cytoarchitecture of cardiac myocytes develops extensively during the early postnatal days when embryonal myocytes react to increased demand on pumping power of the myocardium by increasing the cell volume but also by optimizing the excitation-contraction machinery. The need for a better understanding of these processes is driven by the missing understanding of the diseased and adapting adult myocardium. We studied the morphology of cardiomyocytes in heart ventricles of 3- to 9-day old rats by transmission electron microscopy. The hearts were chemically fixed and samples of ventricles were processed for electron microscopy. A few days after birth, the rat myocardium comprises a relatively heterogeneous population of cardiomyocytes differing in the degree of myotube maturation. On day 4 postpartum, the myotube volume was rich in cytosol and mitochondria, with few and weak myofibrils in a single layer between nuclei and lateral plasmalemma. Cytoplasmic loci of newly formed myofibrils were present under plasmalemma together with tubular or vesicular profiles (proto-tubules) originating from plasmalemma and distinct from intracellular vesicles originating from the Golgi or endoplasmic reticulum. The plasmalemmal evaginations assumed a ring shape with a narrowing rim perimeter that closed and merged and thus produced intracellular vesicles of plasmalemma encapsulating extracellular fluid. These vesicles kept position near the z-disc where they originated and thus could give rise to proto-tubules in transversal direction by growing and merging together. The lateral plasmalemmal protrusions curved along the outer surface. At a distance of a few hundred nanometers, the protrusions merged at their tips back to the plasmalemma and thus encapsulated the extracellular fluid into flat vesicles. These vesicles were oriented under an angle relative to the plane of the lateral plasmalemma and thus could give rise to longitudinally oriented proto-tubules. Proto-tubules of either origin were often structurally and functionally coupled with the terminal cisternae of the sarcoplasmic reticulum with ryanodine receptors, thus forming the dyads. In the cytoplasm at the axilla of proto-tubules and plasmalemma, a dark material similar to z-disc accumulated and made the basis for attachment of actin myofilaments, the latter developing to the first sarcomeres, orientated parallel between the plasmalemma and the myofibril next to the nucleus. The plasmalemmal protrusions at the apical but also at the lateral sides of myotubes contacted with protrusions of neighboring myocytes, and created or extended intercalated discs with desmosomes and attached actin filaments. The morphological analysis revealed the sequence of steps leading to structural maturation of cardiac myocytes and gave clear evidence for proportional growth of myocyte structures. These findings also indicate the reason behind the appearance of the embryonic phenotype in myocytes of diseased myocardium.

Keywords: neonatal myocardium, cardiac myocytes, plasmalemma, development, morphology

Funding:VEGA 2/0091/19

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