Tubuloreticular structures in myocardium: An ultrastructural study

Tubuloreticular structures were observed in cardiac tissue obtained either by biopsy or by operative resection in five (3%) of 169 patients with diverse conditions. Two of these five patients had congenital heart disease; one, aortic valvular disease, and two had cardiomyopathy, which was associated with thyrotoxicosis in one patient and with polymyositis and chronic alcoholism in the other. Tubuloreticular structures were composed of curved or undulating tubules that measured from 200 to 300 Å in diameter, branched extensively, and formed net-like masses either within cisterns of endoplasmic reticulum or within the perinuclear cistern. They occurred in endothelial or fibroblast-like cells, but not in cardiac muscle cells. These structures are known to occur in a wide variety of normal and abnormal animal and plant cells. They are formed through a specialized type of deformation of the membranes of the endoplasmic reticulum. Their precise significance is unknown. Criteria are given for the distinction between tubuloreticular structures and other types of morphologically similar structures that have been described in muscle cells, including aggregates of tubules of sarcoplasmic reticulum, tubules derived from the inner nuclear membranes, annulate lamellae, and highly organized arrays of developing T tubules.