Cellular mechanisms of normal growth in the mammalian heart. I. Qualitative and quantitative features of ventricular architecture in the dog from birth to five months of age.

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Abstract
This paper describes the qualitative and quantitative composition of dog myocardium over the first 5 months of life. The quantitative composition of dog right and left ventricle over this period does not vary. A stereological analysis of electron micrographs representing 32,000 micron2 of tissue surface revealed that 79% of the heart is made up of myofibers, whereas 21% is extracellular space. Twenty-eight percent of the extracellular compartment by volume is vasculature (tissue was preserved by immersion rather than vascular perfusion); 72% is occupied by nonvascular elements and "empty" space. In contrast to the remarkable constancy of quantitative composition of the whole myocardium, myocyte shape and dimensions and the arrangement of intercellular connections vary dramatically over the age period studied. In early postnatal life, the morphology of blood vessels, many of which have completely partitioned lumina, also changes significantly.