“…5, 8], Takuma [4] found by using TEM on the replicas that the inner organic sheath contained collagen fibrils with regular 64-nm striations, and Isokawa et al [9] showed tubular membranous structures of a fibrous feltwork in it by scanning electron microscopy (SEM). Reliable evidence on the fibrillar outer organic sheath, however, has not been reported [1.5], although the outer organic sheath which did not show fibrils was faintly observed in the half-decalcified ultrathin sections [ 10] as well as in the replicas previously reported [2,7], For morphological studies of the PM in human dentine, various acids have been used, for example formic acid, for mic acid-sodium citrate, hydrochloric acid, nitric acid and phosphotungstic acid, and as a result the organic structures within the PM showed fine fibrils [2,7], a feltwork of deli cate fibrils [5], a low density of amorphous substance [8] and matrix fibres [9] by TEM or SEM.…”