The stratum granulosum, sometimes known as the granular layer, is one of the layers in the epidermis, or outer part of the skin. There are five layers in the epidermis altogether, and the stratum granulosum is the one in the middle. Underneath the stratum granulosum are two layers, with the lowest one consisting of actively dividing cells, while above are two layers of flattened inactive cells with the top, outer layer constantly being shed. The stratum granulosum creates a waterproof barrier between these outer layers and the inner, live cells. In medicine, the study of the skin and the conditions affecting it is called dermatology.
Skin anatomy consists of an outer layer, the epidermis, and an underlying layer called the dermis which contains nerves and blood vessels, with fatty tissue beneath. Cells in the epidermis are known as keratinocytes because they make a type of protein called keratin which serves to strengthen the skin. Within the epidermis, the lowest, or basal, layer constantly produces new cells, with older cells moving up to form the stratum spinosum, or prickle layer. In the prickle layer cells are flatter and full of keratin. Older cells in the prickle layer are in turn replaced and move up to form the granular layer, where they begin to lose their nuclei and, by the time they have moved up through the next two layers to reach the top of the epidermis, they have become flattened dead scales.
As the stratum granulosum is impermeable to water and water-soluble substances, it forms a barrier between the active cells of the lower epidermis and the outer dead cells. The granular cells are thought to contain special lamellar granules, which secrete sheets of fatty substances. These are deposited into the spaces between the cells of the stratum granulosum, forming a kind of waterproof sealant, which creates the barrier protecting the lower layers of skin.
The stratum granulosum is around three to five cells thick; along with lamellar granules, these cells also contain what are called keratohyalin granules. Keratohyalin granules are found to be decreased in number, or absent, in people who have a disease known as ichthyosis vulgaris, where the skin becomes dry and scaly. A protein called filaggrin is manufactured by the cells of the granular layer and is normally the main substance found inside keratohyalin granules. Filaggrin is thought to play an important part in the structure of the outer layer of the skin, and also helps it to retain moisture. Ichthyosis vulgaris may be treated by hydrating the skin and adding an ointment on top to stop moisture escaping.