Figure 145

Schematic diagram of the epidermal water barrier. The heterogeneous mixture of glycosphingolipids, phospholipids, and ceramides makes up the lamellae of the lamellar bodies. The lamellar bodies, produced within the Golgi apparatus, are secreted by exocytosis into the intercellular spaces between the stratum granulosum and stratum corneum, where they form the lipid envelope. The lamellar arrangement of lipid molecules is depicted in the intercellular space just below the thickened plasma membrane and forms the cell envelope of the keratinized keratinocyte. The innermost part of the cell envelope consists primarily of loricrin molecules (pink spheres) that are cross-linked by small proline-rich (SPR) proteins and elafin. The layer adjacent to the cytoplasmic surface of the plasma membrane consists of the two tightly packed proteins involucrin and cystatin a. Keratin filaments (tonofilaments) bound by filaggrin are anchored into the cell envelope.

in cell signaling and are partially responsible for inducing cell differentiation, triggering apoptosis, and reducing cell proliferation. As the cells continue to move toward the free surface, the barrier is constantly maintained by keratinocytes entering the process of terminal differentiation. Lamellae may remain as recognizable disks in the intercellular space or may fuse into broad sheets or layers.

Experiments have shown that the epidermis of animals with induced essential fatty acid deficiency (EFAD) is more permeable than normal to water. The membrane-coating granules also have fewer lamellae than normal. Destruc-

lamellar body plasma membrane filaggrin keratin filaments loricrin

SPR elafin involucrin

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