Ultrastructural study of osteodentin formation induced by irradiation.

A single dose of 16,000 rads was delivered to a 1 X 1.5 cm area of the cheek of 2 day old Wistar rats. The lower first molar region was removed 1, 3, 5 and 8 days postirradiation and processed for light and electron microscopy observation. The most distinct structural feature at optical level was osteodentin production in the dental papilla, which began at the level of the initiation of Hertwig's epithelial sheath and continued until the pulpal area was completely filled by osteodentin. At the ultrastructural level, ameloblasts exhibited large lipidic vacuoles and irregular intracytoplasmic secretory granules. Odontoblasts underwent multiple regressive alterations with vacuolization and numerous myelin figures. Pulpal cells exhibited a capacity for odontoblastic differentiation with formation of one or more odontoblastic processes and dentin matrix secretion. Thus, irradiation resulted in stimulation of a great potential for dentin production in these cells.