Granulosum

Granulosum, or the stratum granulosum, is a sublayer of the epidermis, the outermost layer of the skin. It forms part of the epidermal differentiation program that leads to the formation of the stratum corneum. The layer lies between the stratum spinosum and the stratum corneum; in thick skin, the stratum lucidum lies between the granulosum and the corneum.

Cellular features: The keratinocytes in this layer are more flattened than those in the spinosum and contain keratohyalin granules, which participate in keratin filament aggregation. They also possess lamellar bodies that release lipids to establish the epidermal water barrier. The granules stain darkly with histological stains and help define the layer.

Function: The granulosum marks the transition from living cells to the dead, flattened corneocytes of the stratum corneum. Keratinization progresses here, with nuclei degrading as cells move toward the surface; lipid secretion contributes to the permeability barrier that protects underlying tissues.

Thickness: It typically comprises a few cell layers, commonly 1–5, varying by body site and skin type.

Clinical relevance: The integrity of the granulosum and its keratinization process contributes to the skin’s barrier function. Disruptions in epidermal differentiation can affect this layer and are associated with disorders of the epidermal barrier.

Etymology: The name derives from Latin granulosus, meaning granular, in reference to the keratohyalin granules that accumulate in this region.