The development and fate of the dental lamina of the mandibular first molar tooth in the rat.

The lamina of the first mandibular molar teeth of rats, age range 13 d intrauterine (i.u.) to 16 d postnatal (p.n.), was examined by light and transmission electron microscopy to establish histological baselines of its development and fate. All material was obtained from animals anaesthetised with ether, killed by cervical dislocation and prepared by routine methods for both types of examination. Contrary to earlier reports that the lamina remains intact throughout development, mesenchymal elements disrupt the lamina. These were seen first at 19 d i.u., as collagen-filled bays in the basal epithelial layers, associated with partial loss of related basal lamina. In the early stages, collagen deposition was limited and it was not obviously preceded by epithelial cell death or transformation, even though many bay-related cells showed lipid and glycogen accumulations. Later disruption of the lamina showed more mesenchymal cells as well as collagen in deeper spaces. After the onset of tooth eruption, mesenchymal cells external to and within the lamina contained lysosomal bodies and these plus evidence of related epithelial cell death and capillaries in the laminar spaces became more and more apparent. Similar collagen deposits were observed in a successional tooth primordium, which appeared at term but eventually aborted between days 5 and 10 p.n. Thus disruption of the lamina by connective tissue began earlier than has been reported previously and progressed as the tooth erupted towards the oral cavity. The evidence suggests that this disruption is initiated and sustained by mesenchymal cell activity rather than by programmed cell death or transformation of the epithelium.