ALBERT

Notes: Abstract The inorganic component of bone and related hard tissues is generally described as sheets of uniform needle- and plate-like crystals. However, cryofixation has become the method of choice for ultrastructural studies of bone mineral when ladder-like arrangements of filaments contained within deformable microspheres about 1 µm in diameter are apparently the prime structural feature and are consistent with the optical image. The same methodology has now been applied to mature human dentine in caries-free juvenile and adult teeth. These were fixed, sliced, stained for mineral and examined optically or were snap frozen, fragmented under liquid nitrogen, freeze-substituted with methanol or acetone and embedded without thawing in Lowicryl K4M for electron microscopy. Others were processed by traditional transmission electron microscopy methods. To obtain maximum resolution, the electron micrographs were photographically printed as negatives and image-enhanced by digitisation using a Polaroid Sprint Scan 45 and laser printer. In both optical and cryopreparations of juvenile and adult dentine, mineral microspheres up to 1 µm in diameter, were present in the dentinal tubules and peritubular dentine. Within these objects, the mineral was primarily in the form of sinuous electron dense filaments, 5 nm thick, which had a characteristic periodicity. In these preparations needle-like and plate-like structures were rare. In contrast, after traditional transmission electron microscopy preparation although similar filamentous structures remained, the mineral more generally had the familiar form of needles measuring approximately 50 nm in the long axis. The cryopreserved calcified filaments were apparently particularly densely distributed in the intertubular dentine where their parallel ladder-like arrays often formed highly orientated struts and stays. It was concluded that early dentine mineral has the form of filamentous microspheres and as in bone (and other calcifying tissues and cells) has no specific association with collagen. It was also concluded that these structures compact and deform with maturity into a sub-structural framework which may relate to powerful biomechanical forces transmitted through the tissue. Needle- or plate-like mineral is probably rare in vivo in dentine, only becoming commonplace after extensive chemical processing.