A light microscopic study of the development of the roots of premolar teeth in dogs

Abstract

The histological development of the roots of mandibular premolar teeth of eight dogs was studied by light microscopy. The most coronal part of the root was covered by a fibrous fringe of odontoblastic rather than follicular origin. This fringe appeared to provide the means of attachment for the fibres of the developing periodontal membrane. It did not mineralise, nor was any cementum formation evident in relation to it up to the time of root completion. The first cementum formed was of the cellular type and began to be laid down nearer the apex about the time of tooth eruption. Its formation appeared to be related to morphological changes in Hertwig's root sheath, suggesting a possible inductive relationship between the sheath and follicular cells leading to the differentiation of cementoblasts. As the apices approached completion, the rate of formation of cementum appeared to increase, while that of tubular dentine decreased and finally ceased. However, the peripheral pulp cells, which more coronally formed dentine, now gave rise to a bone-like tissue, which contributed to the mass of the highly cellular root apex. This change in pulpal cell activity is probably linked to premature disruption and disappearance of Hertwig's root sheath before root completion.