That depends on which stereological probe you are using. Fractionator probes, such as the Optical Fractionator or Space Balls require relatively thick tissue. For example, If the tissue under study has only one or two focal planes—even with a high numerical aperture—it may be too thin. However, other probes, such as the Cavalieri Estimator for area and volume do not require you to focus up or down through the tissue. With probes such as that, you only need know the sectioning thickness. Decide on which probe or probes to use before preparing the tissue. For details please start with the probes page.
This FAQ comes from stereology.info, our curated collection of information and references regarding the use of stereology in the biological sciences. Feel free to review the other FAQs there as well. We hope this thread will spark some in-depth discussion on stereology!