The Granular Layer (Stratum Granulosum of the Epidermis)

• Keratinocytes of the Granular Layer (Stratum Granulosum) contain keratohyalin granules, which are filled with histidine- and cysteine-rich proteins that appear to bind the keratin filaments together.
  • Therefore, the main function of keratohyalin granules is to bind intermediate keratin filaments.
  • At the transition between this layer and the stratum corneum, cells secrete lamellar bodies (containing lipids and proteins) into the extracellular space. This results in the formation of the hydrophobic lipid envelope responsible for the skin’s barrier properties. Concomitantly, cells lose their nuclei and organelles causing the granular cells to become non-viable corneocytes in the stratum corneum.
• Due to loss of differentiation, this layer is absent in Psoriasis* (replaced by parakeratosis = the keratinocytes retain their nucleus). On the other hand this layer is thickened in Lichen Planus.
• Under the microscope, a prominent granular layer is called hypergranulosis.

References: The Cell: A Very Short Introduction. Allen T. Oxford University Press