A New Perspective on the Formation of Stratum Corneum Intercellular Space

The past 40 years have seen a huge increase in knowledge about the function of the stratum corneum (SC) and its homeostasis. Put simply, the SC can be visualized as a 2-compartment model, with protein-rich corneocytes and an intercellular space filled with lipids. It has become evident that the SC not only provides a diffusion barrier, but also plays an important role in immune defence, especially in the innate immune response. Anti-microbial peptides, pre-cursors for interleukin-1, as well as lipases and proteases that process components of the intercellular space, can be found here. These molecules are delivered to the intercellular space during the transition from stratum granulosum to SC. The question is: How is this done?

Back in 1960 (1) George F. Odland described submicroscopic granular structures of size 200–300 nm (termed Odland bodies) in the upper part of the epidermis. These structures were subsequently found to contain closely packed membranes (2, 3) and were also called membrane-coating granules or lamellar bodies. They are thought to act as a “multifunctional secretory organelle”, delivering their contents to the intercellular space of the SC through exocytosis (2, 4). The ultrastructural concept of this organelle is based mainly on 2-dimensional sections prepared for electron microscopy.

Lars Norlén and co-workers have, in a series of studies, applied a sophisticated freeze fixation technique, thin freeze sectioning and serial sectioning for electron microscopy (5, 6). In this issue (7) they present results that provide a new perspective on the ultrastructure of Odland bodies. They propose that these structures are not discrete granules, but have a continuous membrane structure. This implies a new interpretation of the dynamics of stratum corneum formation; instead of exocytosis the intercellular space of the SC is formed through a process of membrane unfolding. This idea opens up new perspectives in our view of SC function and homeostasis.

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