Electron microscopic structure of the two layers of carious dentin.

Two layers of carious dentin from extracted human teeth were observed with an electron microscope. The first layer, which is superficial and fuchsin-stainable, showed degenerated collagen fibers and granular or leaflike inorganic crystals irregularly scattered. The second layer, which is profound and fuchsinunstainable, showed expanded odontoblastic processes, sound collagen fibers, and apatite crystals bound to the fibers like fringes.