High resolution optical microscopy of animal tissues by the use of sub-micrometer thick sections and a new stain.

The introduction of 1 micron-thick sections from plastic embedded material represents a great technical improvement for the study of tissues under the optical microscope. However, even sections of this thickness appear too thick when observed with oil immersion objectives of high numerical aperture. The extremely shallow depth of field of these lenses allows them to differentiate several focal planes within a one micron thick section. This in turn results in ghost images being formed from out of focus structures, a problem particularly vexing when photomicrography is attempted. To circumvent this difficulty, we reduced the thickness of the sections down to an optimum of 0.4 micron. These thinner sections do require a very energetic stain to give enough contrast to the cellular structures; Stevenel Blue, a stain recently adapted for plastic sections [del Cerro et al., Microsc. Acta 83, (2), 117--121 (1980)] proved to be the most suitable for this purpose of several stains tested. In summary, submicrometer thick sections stained with Stevenel Blue allow to reach the limits of visibility permitted by the best available objectives and effectively merge the realm of optical microscopy with that of low power electron microscopy.