Keratohyalin is a protein structure found in granules in the stratum granulosum of the epidermis, which may be involved in keratinization, and in Hassall's corpuscles in the thymus.
In H&E stained sections, they are large deeply stained granules found in the cytoplasm of epithelial cells in keratinized oral mucosa.
Keratohyalin is associated with eleidin, the intermediate between keratohyalin and keratin; histological location is in the stratum lucidum, thick skin.
In the stratum granulosum, the "grainy" third layer of the human epidermis, the protein Keratohyalin forms dense cytoplasmic granules that promote dehydration of the cell as well as aggregation and cross-linking of the keratin fibers. The nuclei and other organelles then disintegrate, and the cells die. Further dehydration creates a tightly interlocked layer of cells that consists of keratin fibers surrounded by keratohyalin.