Confocal optical sections of thick tissue. The developing heart in a chick embryo, labeled with fluorescent antibody against cardiac myosin, was imaged with a 60× (NA = 1.4) oil-immersion objective. (Left) The first optical section of a 3D stack of 160 sections spaced at 0.5-µm increments. The first section was ∼20 µm below the coverslip. (Middle) Section number 30; (right) section number 130. Spherical aberration and scatter increasingly degrade the resolution, so that the 2.2-µm myofibrillar striations, clearly visible in section number 1, are barely resolved in section number 130. (Sample kindly provided by Howard Holtzer, University of Pennsylvania.)