From outside to inside the scales excist of three layers:
- The cosmine layer, a thin layer of enamel and a layer of dentin.
- The vascular layer with big canals and pores.
- The isopedine lamilar layer with growth zones
The normal photomicrographs do not show much of the individual growth zones but when looked through a polarising microscope with the help of crossed nicols the growth zones show as light and dark bands.
With crosssed nicols and a retardation plate the different growth zones get a color and the enamel layer, the thin layer on top of the scale, depending on the direction of the slide under the crossed nicols becomes orange or blue.