The stratum granulosum, also known as the granular layer, is a distinct and transformative zone within the epidermis, the outermost layer of the skin. This layer is where skin cells shift from living, metabolically active units to the tough, protective structures that form the surface barrier. The stratum granulosum typically sits as the third or fourth layer, orchestrating the final steps of cell maturation and creating a waterproof shield against the external environment.
Location and Structure within the Epidermis
The stratum granulosum is situated directly above the stratum spinosum and immediately beneath the stratum corneum. In most areas of the body (thin skin), the granular layer is the second-to-last layer before the surface. However, in thick skin, such as the palms and soles, the stratum granulosum lies deep to the thin stratum lucidum.
The layer is generally thin, consisting of about three to five layers of cells. As keratinocytes migrate into this zone, they flatten out, taking on a diamond or spindle shape. The cells’ nuclei and internal organelles begin to break down, marking the start of their final transformation into the dead, protective cells of the surface layer. This transition zone is where cells lose their ability to divide and prepare to form the physical barrier.
The Unique Cellular Components
The name “granular layer” comes from the abundance of two specific structures found within the cells: keratohyalin granules and lamellar bodies.
Keratohyalin granules are large, dark, irregularly shaped protein aggregates visible under a microscope. These granules contain profilaggrin, a precursor protein rich in amino acids like histidine and cysteine.
Lamellar bodies, also known as Odland bodies, are smaller, specialized secretory organelles. Produced in the upper stratum spinosum, they become prominent here. Lamellar bodies are filled with a mixture of lipids, including glucosylceramides, free fatty acids, and cholesterol, along with hydrolytic enzymes and antimicrobial peptides.
Creating the Body’s Waterproof Barrier
The stratum granulosum processes and releases internal cellular contents to form the two components of the skin barrier. Keratohyalin granules break down, releasing profilaggrin, which is processed into the protein filaggrin. Filaggrin binds and aggregates the keratin filaments within the cell, creating a dense, rigid protein matrix that contributes to the mechanical strength of the outer layer.
Simultaneously, lamellar bodies fuse with the cell membrane, releasing their lipid and protein contents into the extracellular space. These lipids organize into a highly structured, hydrophobic layer that acts like mortar between the cells of the stratum corneum. This intercellular lipid layer prevents transepidermal water loss, effectively waterproofing the skin. The final breakdown of filaggrin also creates natural moisturizing factor, which helps the outermost layer retain hydration.
Relevance to Skin Disease and Health
Disruption within the stratum granulosum leads to an impaired skin barrier, contributing to many common skin conditions. For instance, genetic mutations affecting profilaggrin production can result in a diminished or absent granular layer. This defect is associated with disorders like ichthyosis, characterized by scaly, dry skin, and is a major risk factor for developing atopic dermatitis.
Abnormalities in lamellar body function are also linked to severe barrier dysfunction. In conditions like psoriasis, the accelerated turnover of skin cells means cells pass through the stratum granulosum too quickly. This rapid transit prevents the proper processing of keratohyalin and the full release of lamellar body contents. The result is a defective, less protective barrier. Maintaining the integrity of this layer is a focus for therapies aimed at restoring skin hydration and overall health.