Image analysis of maize root caps--estimating cell numbers from 2-D longitudinal sections

The cap of the primary root of maize produces several thousand border cells that are shed from the outside of the cap each day. Border cell production is important in the penetration of soil by roots, and in influencing the activity of both beneficial and pathogenic organisms in the rhizosphere. To improve understanding of the dynamics of border cell production, it is desirable to know the number of cells in different parts of the root cap. An image analysis procedure was used to quantify cell dimensions and locations in the median longitudinal section of maize (Zea mays L.) root caps. Calculations based on root symmetry were then used to estimate the number of cells in 3-dimensions. Our estimation procedure was tested initially using regular arrays of identical square and hexagonal shapes to represent cells. It was then tested using two different tissues showing analogous arrays: a transverse section through the maize root cap junction, and a transverse section through a barley root. Good linear correlations were obtained between the number of cells estimated and the number of cells actually counted in the microscope. The numbers of cells in the whole maize root cap (8870 +/- 390) were then estimated from longitudinal sections. These numbers of cap cells agreed with values that had been estimated for maize by other methods. In the first tier of the cap meristem, ten-times more meristematic cells were located in the cap flanks (> 500 cells) than were present in the columella portion. Similarly, only 7% of cells in the outermost layer of the root were associated with the columella region of the cap, a fraction which compared well with previous measurements of sloughed cells extracted from rhizosphere sand. This present technique can be applied to estimate the numbers of cells in any cylindrically symmetrical tissue from two-dimensional sections.