The main criterion for the division of keratinized epithelia is the presence or absence of nuclei in the keratinized layer viewed under light microscope. In the orthokeratinized epithelium the cell nuclei disappear in the keratinized layer, whereas in the parakeratinized epithelium flattened, highly condensed nuclei remain in the cell cytoplasm of the keratinized layer until cells exfoliate.
Ref: https://www.sciencedirect.com/science/article/abs/pii/S0968432814001486#:~:text=In%20the%20orthokeratinized%20epithelium%20the,the%20keratinized%20layer%20until%20exfoliation.