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

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

Answer: (D)

The key idea is the keratinization stage in the epidermis, specifically the layer where keratohyalin granules mark a transition toward corneification. This thin layer sits above the stratum spinosum and is only a few cells thick. Its cells store keratohyalin granules that contain profilaggrin, a protein that is processed into filaggrin. Filaggrin promotes the aggregation and organization of keratin intermediate filaments into dense macrofibrils, which helps the cells become more rigid and prepared for the next step as they move outward. This keratinization process ultimately leads to the formation of the anucleate, dead cells of the outermost layer. The presence of keratohyalin granules is the hallmark of this layer, distinguishing it from the deeper strata that lack these granules.