Images of a thick fluorescent specimen from a confocal and a conventional microscope. The sample is a chick embryo stained with propidium iodide and antibody against the carboxy-terminal glutamic-acid form of α-tubulin (fluorescein isothiocyanate [FITC] label). (Top left) Low-magnification, wide-field phase-contrast image of the entire embryo. The sample is ∼0.5-mm thick and contains a high density of refractile globules that scatter light efficiently. (Top right) Phase-contrast image at the same magnification as the fluorescence image. (Middle row) Conventional epifluorescence images showing (left) propidium iodide and (right) glu-tubulin distribution. The large amount of out-of-focus light severely reduces contrast. (Bottom left) Optical section obtained by confocal microscopy of exactly the same field and focal plane as the middle row. (Bottom right) Higher-magnification confocal view of a portion of the same field. Mitotic nuclei with condensed chromatin can be readily identified. (Dotted white ellipse) Bundles of tubulin are also seen. The mitotic spindle in these cells is formed predominantly with the tyrosinylated form of α-tubulin and hence is not seen. (Sample kindly provided by Dr. Camille DiLullo, Philadelphia College of Osteopathic Medicine.)