Multinucleate ameloblasts in the rat incisor

Cytological examination of the rat incisor enamel organ with the light and electron microscope revealed a small number of ameloblasts which contained two and sometimes three or more nuclei per cell. A multinucleate ameloblast usually contained two vertically apposed nuclei situated near the base of the cell. A narrow cytoplasmic band was interposed between adjacent nuclear envelopes. The apical nucleus was often the more elongated of the two nuclei and it fitted a convexity or a concavity within the more basally positioned nucleus. In serial sections examined with the electron microscope no connections were observed between the nuclei. In animals injected with 3H-thymidine instances of multinucleate ameloblasts were found within the advancing front of labeling where only one of the nuclei contained label. Finally, quantitative analysis by nuclear counting established that multinucleate ameloblasts were 60 times more frequent within the maturation zone as in the secretory zone of amelogenesis. As well, the numbers of multinucleate ameloblasts increased progressively in the course of the maturation stage. It was concluded that multinucleate ameloblasts increase with cell age and likely arise by the process of cell fusion.