Myelin-like inclusions in maturing and senescent muscle cells of rat myocardium.

This paper deals with the ultrastructure of two types of cytoplasmic inclusions, "myelin figures" and "lipid complexes", present in muscle cells of ventricular myocardium of the rat heart. The inclusions were numerous in both the first two postnatal weeks and in the senescent stages, being rarely observed in young adult animals. The myelin figures were entirely made up of myelin-like membrane whorls, whereas the lipid complexes exhibited discrete myeloid caps surrounding a central lipid core. The origin and functional significance of such myelin-like structures are discussed on the basis of ultrastructural and biochemical studies on myocardial physiology and age-related evolution.