Ultrastructural identification of human fetal Purkinje fibres-a comparative immunocytochemical and electron microscopic study of composition and structure of myofibrillar M-regions

Human fetal Purkinje fibres have in the present study for the first time been identified at the ultrastructural level. This has been accomplished by a comparison of light microscopic criteria for identification (high content of the intermediate filament protein skeletin and high cholinesterase activity) with immunohistochemical and ultrastructural observations of the myofibrillar M-band. In sections incubated with antibodies against the M-line protein MM-creatine kinase the Purkinje fibres showed a cross-striated fluorescence pattern, while the ordinary ventricular myocytes did not. Ultrastructurally a subendocardial cell population in the ventricles was distinguished from ordinary myocytes by containing electron dense M-bands. Since MM-creatine kinase is considered to make an essential contribution to the electron density of the M-band we conclude that the ultrastructurally distinguished cell population represents the Purkinje fibres. In some areas of the atria the myofibrils were also labelled by MM-creatine kinase antibodies and contained electron dense M-bands. This indicates that myofibrillar differentiation proceeds independently in different cardiac tissues.