Which layer is one to three cells thick and contains keratohyalin granules, which contain a protein that will aggregate the keratin filaments of the next layer?

• A. Stratum Corneum
• B. Stratum Granulosum
• C. Stratum Spinosum
• D. Stratum Basale

Answer: (B)

The layer in question is the granular layer of the epidermis. This layer is only about one to three cells thick and is distinguished by keratohyalin granules inside the keratinocytes. These granules contain profilaggrin, a protein that is later processed to filaggrin. Filaggrin binds and aggregates keratin intermediate filaments, organizing the cytoskeleton and promoting the terminal differentiation of these cells as they move toward the surface. This aggregation helps form the dense, cross-linked structure that, along with formation of the cornified envelope, leads to the protective, waterproof outer stratum. The basal layer is proliferative and lacks keratohyalin granules; the spinosum has desmosomes and a developing keratin network but no keratohyalin granules; the corneum comprises dead, enucleated cells that no longer actively accumulate granules.