The myofibre, 4–6 m thick, contains one or a few myofibrils. Each myosin filament is surrounded by six actin filaments. The sarcomere banding pattern includes the Z-, A-, I-, M-, N-, and H-band. End-to-end attachments between myofibres are composed of alternating desmosomes and fasciae adhaerentes. Desmosomes and nexuses occur between longitudinally oriented cell surfaces. The sarcoplasmic reticulum is poorly developed but well defined. Peripheral coupling-like structures are common, T-tubules are absent. Membrane bound dense bodies occur in all regions. Areas with ribosomes and single myosin filaments are often seen.

The epicardial cells have a regular hexagonal surface and are much thicker than the endocardial cells. Numerous short and a few longer cytoplasmic extensions face the pericardial cavity.

The fiat endocardial cells contain a large nucleus and small amounts of cytoplasm.