Histology, Keratohyalin Granules

The epidermis is comprised of 4 or 5 layers, depending on the location of the skin sampled. These layers from deep to superficial are stratum basale, stratum spinosum, stratum granulosum, stratum lucidum, and stratum corneum. The stratum lucidum is typically only present in the thick skin found in areas such as the palm or soles. The basal layer of the epidermis contains keratinocytes in their least differentiated form wherein they undergo mitosis and proliferate. As the keratinocytes mature and differentiate they migrate superficially, produce keratin, and eventually lose their nuclei and other cellular organelles; the latter cells form stratum corneum or the horny layer of the skin. Keratin, the basic constituent protein of the skin and its appendages, is arranged in the form of keratin filament bundles. Filaggrin is one of the major intermediate filament-associated proteins that interact with keratin filaments in the skin. Its precursor form called profilaggrin accumulates in cytoplasmic granules of the granular layer in the form of structures called keratohyalin granules (KHG).

Keratohyalin granules primarily exist within the stratum granulosum, with some present in the stratum spinosum. These granules are insoluble in water and located within the cytoplasm where they promote dehydration of the cell. Their chief function seems to be cross-linking of keratin filaments which creates the tight barrier that is the epidermis, providing the body with an impermeable layer that protects from invasion by foreign particles. This process of cornification of the epidermis effected by KHG is known as keratinization.