Epidermal Lamellar Granules

In the mid-1950s and 1960s, transmission electron microscopes became widely available, leading to many studies of the ultrastructure of various tissues including the epidermis. Most of these studies involved tissue fixation with formaldehyde and postfixation with osmium tetroxide. A few studies employed freeze-fracture electron microscopy. One set of these studies identified a small organelle variously called lamellar granules (LGs), lamellar bodies, membrane-coating granules, cementsomes, and Odland bodies. LGs are round to ovoid in shape, with a diameter of about 200 nm. They have a bounding membrane surrounding a stack of internal lipid lamellae. These small organelles are first seen in the spinous layer and accumulate with differentiation in the granular layer. In the uppermost granular cells, the bounding membrane of the LG fuses into the cell plasma membrane, and the internal contents are extruded into the intercellular space. The initially extruded contents of the LG then rearrange to form the intercellular lamellae of the stratum corneum. In this context, LGs serve as the precursor to the permeability barrier of the skin. Various studies have provided evidence that they are derived from the Golgi apparatus, specifically the <i>trans</i>-Golgi. Isolated LGs contain phosphoglycerides, sphingomyelin, and glucosylceramides. The most unusual lipid component is a linoleate-containing glucosylceramide comprising 30- to 34-carbon ω-hydroxy­acids. Isolated granules also contain acid hydrolases including glucocerebrosidase, sphingomyelinase, and phospholipase A. They also contain proteases and antimicrobial peptides. Defective LGs have been associated with a number of skin diseases including ichthyotic conditions and defective barrier function. Recently, studies employing cryo-transmission electron microscopy have called into question the validity of observations on LGs with more conventional electron microscopic techniques. These studies suggest a continuity of the membrane structure from the Golgi through the intercellular lamellae of the stratum corneum.