Previously, a slit-shaped enamel defect had been observed in a premolar with enamel hypoplasia in the innermost enamel adjacent to the DEJ, where an abnormal step-shaped appearance was seen. In these observations, we happened to find the abnormal tubule- or elongated club-shaped defects in the ground enamel sections of 2 human permanent teeth containing an upper incisor and a third molar when observed the ground sections of 252 human teeth. The defects arose as the thicker club-edges from the innermost enamel adjacent to the dentin-enamel junction (DEJ) and the club-tips gradually thinner towards the enamel surfaces. The maximum length was about 500 μm and the thick club-edge defects were up to about 50 μm in width and contained minute granular deposits, which might be identified as whitlockite. The similar defects which showed many circular defects of the innermost enamel running along the DEJ were also observed in a third molar. Their enamel defects might be caused by the entire disappearance of the initially formed ameloblast groups or the partial break in the ameloblast activity during short or longer time of enamel formation.
We examined the origins of the remarkable perikymata on the human permanent enamel surface mainly by using the 252 ground sections obtained from 252 teeth, although several investigators have reported their relationship in enamel hypoplasia. In these observations, the origins were roughly classified into 5 types (1-5). Some sigmoid or curbed lines of Retzius formed the wide or relatively narrow perikymata associated with the irregular dentin-enamel junction (DEJ) (type 1). One clear straight line of Retzius without the irregular DEJ formed the shallow perikymata followed with the clear or somewhat unclear lines of Retzius that converged towards the perikymata (type 2). The appositional enamel formation was concerned with the 2 sub-origins including the remarkable perikymata on the initial points of the appositional regular (type 3-1) and irregular enamel deposited on the resting lines of the already formed enamel (type 3-2). In mild enamel hypoplasia, the abnormal formation of the DEJ was concerned with the 2 sub-origins including the remarkable wave- or scallop-shaped DEJ which induced the disorder of enamel prism directions so that the irregular perikymata was formed (type 4-1) and the extremely step-shaped appearance of the DEJ which induced the abnormal perikymata (type 4-2). No incremental lines of Retzius concerning with the remarkable perikymata as well as the type 4 were also observed in some teeth, probably partial enamel hypoplasia (type 5). Some of their origins (types 1-5) were complexes.