SEM study of the development of rat incisor enamel hypoplasia under hypocalcemia induced by thyro-parathyroidectomy

Several clinical studies have reported the presence of enamel hypoplasia in congenital hypoparathyroidism or hypocalcemia. In previous studies we showed that thyroparathyroidectomy (TPTX) induced perturbations of the ameloblast morphology and secretion, of the rod pattern and of the enamel surface at late secretory stage and beginning of maturation, and limited hypoplasia in the erupted enamel of rat incisor. The aim of the present study was to evaluate by SEM, the extent and evolution of the enamel alterations of thyro-parathyroidectomized rats during the maturation stage. Wistar rats were thyro-parathyroidectomized and sacrificed 57 days later. The incisors were dissected out and processed for SEM. The surface of the incisor was observed from the end of secretion/beginning of maturation to its incisal erupted end. Transverse sections were prepared to study the structural defects and the prism pattern at different stages. The results showed that the surface of the TPTX incisors presented large hypoplastic defects at the end of secretion/beginning of maturation and only small defects in the erupted part. Transverse sections showed that, at the transition from secretion to maturation, the enamel defects extended to the mid-thickness of the tissue. At the incisal end the defects were limited to the outer enamel. As it is difficult to understand how the large apical defects could recover to appear as small hypoplasia at the incisal end, these results raise new questions concerning: (1) the effect of a long term calcium deficiency upon the cellular activity of the ameloblasts, and (2) the capacity of the enamel organ to compensate structural abnormalities.