Scanning electron microscopic, light microscopic and microradiographic study of giant tubules in bovine dentin

Giant tubules (GT) in the axiomesiodistal plane of the incisal dentin of 21 unerupted permanent incisors of 1/2-2-yr-old calves were studied by scanning electron microscopy, light microscopy and contact microradiography. A technique is described by which the incisal dentin was split along the axiomesiodistal plane exposing the GT lumina longitudinally for SEM study. The luminal organic content was carefully removed mechanically or chemically. The GT originated 50-250 microns underneath the incisal dentino-enamel junction and converged pulpally, terminating in the incisal edge of the pulp. A confluence of adjacent tubules towards the pulp was seen. Each GT had a relatively constant diameter along its length, in the range of 10-50 micron. The dentinal tubules circumvented the GT origins and pursued a straight course pulpally, parallel to the characteristic wall zone surrounding each GT. Very few dentinal tubules entered the GT lumen at any level. The luminal lining along the larger part of the GT consisted of collagen fibrils forming a criss-cross pattern, while the most pulpal part was lined by a smooth organic sheath. The GT walls at any transversal level within one tooth had a lower mineral content than the circumpulpal dentin between them. It was concluded that the GT formation is not caused by crowding of the odontoblasts.