What cells of the heart cant be seen by the naked eye?

The cells of the heart that cannot be seen by the naked eye include cardiomyocytes, which are the muscle cells responsible for the heart's contraction, and specialized conducting cells like pacemaker cells (e.g., Sinoatrial node cells) that regulate heart rhythm. Additionally, endothelial cells lining the blood vessels and fibroblasts, which provide structural support and maintain the extracellular matrix, are also microscopic. These cells play crucial roles in heart function, but their small size makes them invisible without a microscope.