Is the entire region of interest available for systematic-random sampling?

YES: Use the Cavalieri/point-counting probe for an estimate of cross-sectional area or volume in a homogeneous region, or use the Area Fraction Fractionator probe to estimate the percentage by area or volume of a component of the tissue.

IF NOT: If you only have a portion of the region available, you can’t make an estimate for the whole region.
You need to sample the ENTIRE region systematically and randomly to estimate the volume or cross-sectional area of the region.